Exercise Physiology in Men and Women

C H A P T E R

36 Exercise Physiology in Men and Women Anne-Marie Lundsgaard*, Andreas M. Fritzen* and Bente Kiens University of Copenhagen, Copenhagen, Denmark

O U T L I N E 36.1 Introduction

525

36.2 Body Composition

526

36.3 Cardiovascular Differences and Maximal Oxygen Uptake

527

36.4 Muscle Fiber Type Composition

528

36.5 Anaerobic and Aerobic Exercise 528 36.5.1 Anaerobic Potential 528 36.5.2 Aerobic Carbohydrate and Lipid Oxidation During Exercise 529 36.6 Substrate Metabolism During Exercise 530 36.6.1 Amino Acid Oxidation During Exercise 530 36.6.2 Lipid Energy Sources Utilized During Exercise 530 36.6.3 Glucose Metabolism 533 36.6.4 Glycogen Stores and Glycolytic Capacity 533 36.7 ATP Resynthesizes in Skeletal Muscle

36.8 Estrogen and its Impact on Metabolism

535

36.9 Gender Differences in Metabolism During Recovery From Exercise

536

36.10 Nutritional Implications in Relation to Exercise 536 36.10.1 Energy Availability in Athletes 536 36.10.2 Dietary Macronutrient Composition 537 36.11 Concluding Highlights

538

References 538

534

36.1 INTRODUCTION Women and men exhibit many gender-specific anthropometric and physiologic characteristics, which may impact the response when the female or male body is subjected to increased metabolic stress in response to physical activity. In terms of aerobic exercise performance, it has been proposed that women may perform similarly or slightly *Funded by the Danish Diabetes Academy, supported by the Novo Nordisk Foundation Principles of Gender-Specific Medicine. DOI: http://dx.doi.org/10.1016/B978-0-12-803506-1.00017-6

36.7.1 Mitochondrial Fatty Acid Transport and Beta-Oxidation 534 36.7.2 Tricarboxylic Acid Cycle and Oxidative Phosphorylation 534 36.7.3 Finetuning of Acetyl-coA Input to the Tricarboxylic Acid Cycle 535

better during long-term endurance exercise than men. Hence, it has been reported that the gender difference in running speed disappears at long running distances,1 and hence women have a reduced completion time and higher mean relative intensity during 90 km running, when men and women of comparable 42.2 km running performance were compared.2 Furthermore, when data from 14 US marathons were obtained from ~92,000 men and women, there was a greater slowing in the running pace of men than women during the second half of the marathon.3 These observations indicate an inherent gender difference in adaptations to endurance exercise.

525

© 2017 Elsevier Inc. All rights reserved.

526

36.  Exercise Physiology in Men and Women

The area of substrate utilization and muscle metabolism during exercise has been the subject of genderspecific research in particular, and an increasing body of evidence suggests that there is a distinct gender dimorphism in the metabolic properties of skeletal muscle. Notably, when the human vastus lateralis muscle was subjected to microarray analysis, gender was reported to have a stronger influence on metabolic gene expression than age and even training status.4 This further suggests that the metabolic responses to exercise vary in women and men. Sex differences can be ascribed to both sex chromosome and sex hormone exposure, and it is clear that gender-specific features of exercise physiology result from both of these factors. Investigating primary gender differences in exercise physiology and substrate metabolism is difficult, as confounding variables like adiposity, fat distribution, hormonal fluctuations, and aerobic fitness level might complicate interpretations. Thus, proper matching of men and women is crucial to determine the effect of gender per se. The available evidence presented in this chapter is derived mainly from studies in healthy men and women, including untrained as well as already trained subjects. As the menopausal transition is proposed to have an impact on metabolism, conclusions are derived from studies in premenopausal women and age-matched men, while the effect of menopause on exercise physiology is covered in a separate section.

36.2  BODY COMPOSITION There is an obvious gender difference in muscle mass and adiposity. On average, women have around twothirds of the skeletal muscle mass of their male counterparts, as measured by MRI-scanning in ~470 men and women, and the gender difference in muscle mass seems to be greater in the upper body than the lower body.5 Notably, the gender difference in muscle mass remains after adjusting for body weight and height. This observation implies that women have a greater body fat mass than men. Actually, the body fat percentage for normalweight women is similar to that of men classified obese.6 The gender difference in fat mass is present already at birth7 and becomes more marked during puberty.8 Furthermore, varying within different age groups, a 6–12% higher body fat was observed in women, when a large cohort of 16,000 12–80 years old men and women was analyzed by bioelectrical impedance.9 Importantly, body fat is also distributed differently in men and women. Men have a higher amount of intra-abdominal (visceral) adipose tissue, whereas women have more subcutaneous fat, particularly in the gluteo-femoral region, as measured by computed

tomography (CT) scanning and magnetic resonance imaging (MRI).10,11 This sex difference in fat distribution, known as the android and gynoid distribution pattern, becomes prominent during puberty and ceases after menopause and has therefore been suggested to be sex-hormone dependent. It should be kept in mind though that despite variation within each gender, the subcutaneous fat depot comprises the majority of the total body fat, corresponding to ~80%. It is possible that an increased anabolic response to exercise may contribute to the observation of greater muscle mass in men than women. In the literature, it is apparent that men and women experience similar relative strength gains to resistance-type exercise training, but there appears to be less muscle hypertrophy with strength improvement in women when compared to men. Several studies have compared the rate of muscle protein synthesis in men and women by using a primed, constant labeled amino acid infusion technique to calculate the fractional turnover rate of muscle, by measuring the incorporation of tracer into muscle protein. Gender differences in protein turnover can then be investigated in both the postabsorptive basal state and in the response to anabolic stimuli, such as exercise. In the basal state, there does not appear to be a difference between men and women in muscle protein fractional synthesis rate.12–14 Only a few studies have evaluated basal muscle protein breakdown rate, and they reported that it is also similar between men and women.13,15 Thus, there does not seem to be any detectable gender difference in basal protein turnover in skeletal muscle. This is evident when expressed per unit of muscle mass, suggesting that the intracellular protein turnover is the same. However, it is obvious that total protein synthesis is higher in men on a whole body level, due to a greater total lean body mass (LBM). Thus when men and women ingested a standard diet with 0.86 g protein/kg body mass (BM)/day and nitrogen balance was assessed over 3 days, total protein turnover was greater in men than women.16 In response to resistance exercise, muscle protein synthesis rate has been investigated by infusion of a phenylalanine tracer during recovery in the fed state, and the increase in protein synthesis was reported to be similar between men and women, despite a 45-fold greater exercise-induced increase in testosterone during exercise in men.17 A similar increase in exercise-induced protein synthesis is also confirmed by another study showing that postexercise muscle protein synthesis rate increased similarly in men and women after resistance exercise, together with a similar increase in the mammalian target of rapamycin (mTOR) and p70S6 kinase (S6K1) phosphorylation,12 indicating that anabolic signaling in skeletal muscle was not subject to gender differences. Thus, the obvious gender difference in muscle

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

36.3 Cardiovascular Differences and Maximal Oxygen Uptake

mass does not seem to result from differences in protein turnover rates in the period following exercise. Rather, there is a genetic influence, which indeed may involve the male-specific Y chromosome, though it is difficult to separate direct genetic effects from hormonal effects in adult humans. Therefore, differences in the hormonal milieu (i.e., testosterone levels) are hypothesized to be important for the regulation of a greater total muscle mass in men, besides the influence of physical activity patterns in men and women that over time may contribute to gender differences in lean mass.

36.3  CARDIOVASCULAR DIFFERENCES AND MAXIMAL OXYGEN UPTAKE During exercise, several physiological adjustments are made by the cardiovascular system to supply skeletal muscle with oxygen and energy substrates. The components of the cardiovascular system have several sexually dimorphic characteristics. The total blood volume is ~70 mL/kg BM in adult women and thus a little lower than in adult men, who have ~80 mL/kg BM. Exercise training is followed by an increase in blood volume of up to 20–25% compared to sedentary subjects,18 an adaptation which is evident in both genders. During the initial weeks of training there is an expansion of plasma volume, after which the greater blood volume is accounted for by an equal increase in plasma and red cell volume. In general, the greater body size of men is associated with a larger heart. Therefore, when comparing the hearts of men and women, investigators have tried to correct the measurements for body surface area. The majority of studies have observed a greater left ventricular mass in men than women, and a higher resultant stroke volume at rest in men than women.19 The stroke volume at rest is ~55 mL and ~65 mL in sedentary women and men, respectively. Some, but not all, studies report that resting heart rate is similar in women and men, with some studies showing that resting heart rate is higher in women. The inconsistent observations in regard to heart rate might be related to improper control of physical activity level, differences in emotional arousal, and subject age, which make accurate conclusions in this area rather difficult to obtain. As the cardiac output is given by the product of stroke volume and the heart rate, it follows that cardiac output is lower in women than men, with resting values of ~3.5 and ~5–6 L/min, respectively. This difference applies for both sedentary and active individuals. During dynamic exercise, cardiac output increases in direct proportion to the increase in oxygen uptake. The maximal heart rate is reported to be similar20 or slightly higher in men than women.21,22 The maximal stroke volume increases with

527

exercise training in both women and men, but is lower in women than men at all exercise intensities.20 A lower maximal stroke volume combined with a similar or slightly lower maximal heart rate in women implies that maximal cardiac output is lower in women than men. In addition to this, women have a lower blood volume than men and a ~12% lower hemoglobin concentration.23 This further implies that the oxygen binding capacity is lower per unit of blood volume in women. The maximal oxygen uptake is determined by the maximal cardiac output and the maximal oxygen extraction by the tissues. The capacity for oxygen extraction in skeletal muscle will be dependent on microvascular blood flow in skeletal muscle and the ability to extract oxygen from the capillaries. The maximal oxygen uptake in L/min (hereafter referred to as VO2peak) is used as an assessment of physical fitness level. A ~30–40% higher VO2peak is observed in men compared with equally trained women, with reports of VO2peak values ~6 L/ min in elite male endurance athletes. The higher VO2peak in men is primarily related to their larger O2 transport organs, given the greater body size of men. Accordingly, when expressed per kg BM, VO2peak is ~10–20% lower in women than men.24 The persisting difference is mainly due to the lower muscle mass and also lower hemoglobin concentration, and hence hematocrit values, in women compared to men. Hence, a more proper way to express maximal oxygen uptake when comparing men and women is in mL/kg LBM/min. Thus, when maximal oxygen uptake is expressed relative to LBM and women and men are carefully matched in regard to training status and activity level, the gender difference in VO2peak becomes small and nonsignificant.25 Interestingly, most of the evidence on gender differences in the cardiovascular response has been documented at rest and during maximal exercise. However, in a recent study trained men and women were studied during submaximal exercise at 40% and 75% of peak workload.26 During the same relative workloads (and corrected for differences in BM) women demonstrated lower cardiac output as a result of lower stroke volume, as heart rate was almost similar between genders. When corrected for wattage (as exercise intensities were set at the same % of peak workload), cardiac output, heart rate, and stroke volume were higher in women than men, indicating that greater cardiac work is needed in women to meet the same physical work demand. Interestingly, as a compensatory mechanism for the lower stroke volume and hence cardiac performance, peripheral oxygen extraction (i.e., arteria-venous O2 difference) was higher in women. It has been suggested that the lower stroke volume in women during submaximal exercise could be the result of a blunted sympathetic response and a higher basal vasodilation, evidenced by lower catecholamine levels in women during submaximal exercise.26,27

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

528

36.  Exercise Physiology in Men and Women

Taken together, the observation of lower VO2peak values in women than men seems to be related to a smaller ventricular mass and thereby stroke volume. This will in turn lead to a lower peak cardiac output during maximal exercise in women. This is coupled with lower blood volume and hemoglobin concentration, which will reduce the oxygen binding capacity. In combination with a higher ratio of fat to muscle mass, this may diminish women's ability to extract oxygen. However, this might to some extent be accounted for by different mechanisms in terms of peripheral oxygen extraction in skeletal muscle.

36.4  MUSCLE FIBER TYPE COMPOSITION Skeletal muscle in humans comprises three major fiber types: type I, IIA, and IIX. The morphology and enzymatic properties of the specific muscle fiber types depend on their myosin heavy chain (MHC) expression. The relative proportion of type I, IIA, and IIX fibers will thereby affect the total muscular capacity for oxidative versus glycolytic energy turnover and substrate storage. Gender differences in the morphology of skeletal muscle have mainly been investigated in the vastus lateralis muscle by the needle biopsy technique. The most extensively used technique is histochemical staining of a cross-section of the muscle biopsy by use of myosin adenosine-triphosphatase (ATP-ase) staining, by which type I, IIA, and IIX can be differentiated in human muscle. The technique also enables determination of the cross-sectional area (CSA) of each muscle fiber type, and hence its relative contribution to the total muscle area. Using this technique, gender differences in muscle morphology have been well documented. An early study, in which the fiber type distribution was investigated in m. biceps brachii and m. vastus lateralis, showed a higher type I/II fiber ratio in women than men.28 They also detected a larger size of type II fibers in men. Later, a higher number of type I muscle fibers was consistently observed in the vastus lateralis muscle in women compared to matched men. When expressed relative to area, the proportion of type I fibers has been described to be 27–35% greater in women, while the proportion of type IIA,29,30 or both IIA and IIX is reported to be greater in men.25 Hence, a greater muscle area is covered by type I fibers in women of untrained, moderately-, and endurance trained matched men and women.25 Furthermore, a larger individual CSA of type IIA,29 and both type IIA and IIX fibers,30 has been observed in men. Others have confirmed these observations of a greater size of type II fibers in men compared to women and hence a greater ratio of type II to I fibers in men, also using myofibrillar ATP-ase staining.31–34 Notably, these immunohistochemically findings are also reflected at the transcriptional

level of the MHC, as MHCI mRNA content is reported to be lower in the vastus lateralis muscle of men than women,35 while MHCIIA and -IIX mRNA levels are higher in men.36 It could be questioned whether the greater proportion of type II fibers in men makes them able to generate more tension during maximal contractions compared to women. At the whole muscle level men are able to generate a greater absolute force than women, but when maximal voluntary concentric strength is related to muscle CSA, there is no significant difference between genders, as reported for elbow flexion, knee extension, and knee flexion.33,37 This suggests that total muscle area, rather than muscle fiber composition, is primary for the greater maximal strength observed in men. Of note, the total muscle fiber number was estimated from biopsies and shown to be similar between genders in these studies, despite men having greater total muscle areas than women. This suggests that the greater absolute force production in men is due to greater fiber sizes. When single fibers were dissected from male and female skeletal muscle, and electrically stimulated to contract, the maximal active tension was greater in both type I and II fibers from male muscle, a gender difference that was eliminated when expressed per fiber CSA.38 The number of capillaries surrounding each muscle fiber is found to be similar in men and women, but due to a lower total amount of type II fibers and a smaller individual area of these, a greater capillary density per given muscle area is observed in women.29,30 This may have implications for the nutritive flow to the muscle fibers, i.e., oxygen and substrate delivery, and the implications of this will be discussed in the context of substrate utilization in Section 36.5.2.2.

36.5  ANAEROBIC AND AEROBIC EXERCISE During exercise, ATP is continuously degraded to ADP in skeletal muscle. The ensuing ATP resynthesis is generated anaerobically from glucose and glycogen and aerobically by the oxidation of lipids and carbohydrates.

36.5.1  Anaerobic Potential It has been documented that there is a greater anaerobic capacity in men than women during maximal intensity exercise, both when absolute and BM corrected estimates of anaerobic capacity are applied. The 30 s sprint test on a bicycle (the so-called Wingate test or modifications thereof) is designed to estimate anaerobic power and capacity. It has been reported that during a single 30 s sprint, there is a higher proportion of ATP regeneration via anaerobic metabolism in

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

36.5  Anaerobic and Aerobic Exercise

men than women, suggesting a higher anaerobic component in men during maximal exercise.39 Furthermore, a greater anaerobic potential in men has been evidenced by greater postexercise disturbances of the acid–base balance and a greater increase in blood lactate concentrations in the blood of men than women during maximal exercise tests.40 In another observation from an incremental cycling maximal exercise test, the lactate threshold was similar between genders, but after this was reached women accumulated plasma lactate at a slower rate than men, with men having greater plasma lactate concentrations at exhaustion.41 These findings are further supported by observations of a lower increase in blood lactate accumulation in women than men after a single42 or several 30 s sprints on a cycle ergometer.43 The greater lactate accumulation in the circulation of men during intense exercise may reflect a greater glycolytic activity in muscle. Interestingly, the anaerobic performance during 3 × 30 s Wingate tests has been described to be highly correlated with the proportion of type II fibers and the activity of the enzyme phosphofructokinase (PFK) in both men and women.44 This goes well along with the greater proportion of type II fibers in men, and reports of increased capacity of the glycolytic enzymes in male muscle, which will be discussed in a subsequent paragraph.

529

During exercise carbohydrate and lipids are the main energy sources. The respective contribution of lipid versus carbohydrates to fuel energy consumption can be measured by indirect calorimetry, which reflects whole-body substrate oxidation. Skeletal muscle substrate utilization can also be estimated more specifically by calculation of leg respiratory quotient (RQ), which reflects the arterialvenous O2 and CO2 exchange in blood obtained from an artery and vein across the muscle tissue. In practice, this method is difficult and requires proper handling of the blood samples. However, during steady-state whole-body exercise, it has been shown that the respiratory exchange ratio (RER) reflects substrate utilization well in skeletal muscle. Therefore, RER measurements are usually used for determination of substrate utilization during exercise.

the subjects prior to an exercise bout greatly influences substrate utilization during exercise, which is why it is important to standardize the diet before the test. A final factor to consider when comparing the metabolism between women and men during exercise is the possible effect of the female sex hormones on substrate utilization during exercise. It has become clear that 17-β estradiol (hereafter referred to as estrogen) has a wide spectrum of actions and that estrogen is implicated in the regulation of metabolism in skeletal muscle. In premenopausal women, the levels of female sex hormones undergo profound fluctuations during each menstrual cycle phase, with plasma estrogen concentrations varying from 10 to 300 pg/mL. In the early follicular phase, estrogen and progesterone concentrations in the blood are at their lowest, while estrogen concentration is peaking at the end of the follicular phase. Despite these changes in circulating estrogen, differences have not been reported for resting whole-body metabolic rate and RER,45 fasting plasma glucose and insulin,46 or fatty acid (FA) concentrations47 when the follicular and luteal phase are compared. This suggests that substrate metabolism at rest is not significantly affected by the menstrual cycle. There are conflicting findings with regard to substrate metabolism during aerobic exercise, concerning whether the time point in the menstrual cycle has an impact on substrate oxidation during exercise. Several studies do not find any change in substrate oxidation during exercise throughout the menstrual cycle,46,48,49 while others have suggested that women have higher lipid utilization in the luteal phase compared to the follicular phase.50–53 Thus, it is difficult to conclude whether there is an actual effect of menstrual cycle phase on substrate choice during exercise, which may be related to improper dietary control in the studies and differences in training status. In many gender-comparative studies, it has become the norm that women are subjected to experiments in the follicular phase (day 7–11), due to the lower levels of circulating female sex hormones during this period compared with the luteal phase. This minimizes the differences in the level of sex hormones between genders. However, there are also many gender-comparative studies, which do not consider menstrual cycle phase, and furthermore do not control for habitual activity level or diet.

36.5.2.1  Matching of Women and Men for Comparison of Substrate Utilization During Exercise When comparing the substrate utilization during exercise between women and men it is important to match the genders in accordance with cardiorespiratory fitness and training history. In addition, the workload during the exercise bout should be matched according to VO2peak relative to LBM. The dietary status of

36.5.2.2  Respiratory Exchange Ratio During Exercise Several studies, but not all, have shown that the relative fat oxidation during exercise is higher in women than in men. However, to do proper gender-comparative studies during exercise, the above mentioned requirements for matching of men and women must be considered. All these criteria were indeed fulfilled in study by Roepstorff et al.27 First, women and men did not differ

36.5.2  Aerobic Carbohydrate and Lipid Oxidation During Exercise

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

530

36.  Exercise Physiology in Men and Women

in VO2peak per kg LBM, training history, and physical activity level, suggesting that women and men had similar training status. Also, the relative exercise intensity during 90 min of cycling exercise did not differ between women and men (60% of VO2peak), heart rate was identical, and so was the fiber type recruitment pattern during the exercise bout. In this well controlled study, fat oxidation was significantly higher during the submaximal exercise bout in women than in the matched men.29 In addition, when calorimetric data from 25 studies comparing substrate oxidation in men and women during endurance exercise (>60 min) were summarized in a systematic review, mean RER data indicate a greater relative fat oxidation in women compared to men (RER 0.87 vs 0.90).54 Lower RER values in women than men are observed for both untrained and trained subjects, and are maintained when untrained women and men complete a similar training regimen. In general, the relative fat utilization during exercise depends on exercise intensity. From RER measurements, the fat oxidation rate can be calculated from O2 and CO2 by use of stoichiometric calculations.55 During submaximal exercise, the absolute FA oxidation rate increases from low to moderate exercise intensities, whereafter it declines with increasing exercise intensities, thereby forming a bell-like curve. The top of the curve represents the maximal rate of fat oxidation and has been referred to as FATmax.56 In a large number of subjects (300 men and women) it was demonstrated by indirect calorimetry that the maximal fat oxidation rate was higher in women (8.3 ± 0.2 mg/kg fat free mass [FFM] ∙ per min) than in men (7.4 ± 0.2 mg/kg FFM∙per min) during submaximal incremental exercise tests on a treadmill. The intensity eliciting the maximal fat oxidation rate was 52% in women versus 45% of VO2peak in men, and hence higher in women.57 The findings indicate that men have an earlier shift in using carbohydrate as the predominant fuel at increasing exercise intensity. A later study, applying indirect calorimetry during submaximal incremental test on cycle ergometer, has confirmed that the intensity that elicits the maximal fat oxidation rate is higher in women than men (58% vs 50% of VO2peak).58 Thus the curve seems to be right-shifted in women. The higher fat oxidation in women than in men during submaximal exercise might be due to a better maintenance of cellular energy balance in skeletal muscle by women. Support for this notion are the findings of an increase in the AMP/ATP ratio in men, but not in women, during prolonged submaximal exercise at the same relative workload29 and a smaller ATP reduction in women than in men during repeated bouts of high intensity exercise.43 A better maintenance of muscle cellular energy balance during exercise in women than in men appears to be due to the sex-specific muscle morphology. As previously described, the proportion of the oxidative

type I muscle fibers is higher in women than in men, women have a smaller muscle fiber CSA, in particular of type II fibers, and women have a higher capillary density compared with men. Together, these intrinsic factors of female skeletal muscle favor an increased potential for improved oxidative substrate utilization, and thus the potential for enhanced FA oxidation. That the higher fat oxidation during exercise in women appears to be due to gender-specific muscle morphology is supported by the findings of a significant correlation between fat oxidation, the proportion of type I fibers, and capillary density.29

36.6  SUBSTRATE METABOLISM DURING EXERCISE 36.6.1  Amino Acid Oxidation During Exercise The relative contribution of protein to the energy delivery during aerobic exercise is small, and therefore often not considered when nonprotein RER values are calculated from gaseous exchange. Estimation of protein utilization is possible using amino acid tracers, and it has been estimated that protein oxidation comprises around 1–5% of total aerobic exercise energy expenditure, but will increase during prolonged exercise or if muscle glycogen stores are low before exercise.59 During exercise, the branched chain amino acids leucine, isoleucine, and valine are preferentially oxidized in skeletal muscle.60 As indirect evidence for amino acid oxidation during exercise, it has been shown that urinary urea excretion is greater in men than women on a day with exercise (15.1 km run) under conditions of a controlled diet.61 This observation of a lower increase in urea excretion in women than men in response to exercise has later been verified by several studies. When amino acid oxidation was directly measured by use of stable isotopes, leucine oxidation during submaximal endurance exercise was described to be ~70% greater in endurance trained men than women,16 and this higher leucine oxidation in men has also been confirmed by other research groups studying endurance exercise (50–60% of VO2peak) for 60–90 min.62 Thus, it is a consistent finding that men oxidize a greater amount of amino acids and lower amounts of lipids during exercise compared to women.

36.6.2  Lipid Energy Sources Utilized During Exercise Lipids utilized during exercise originate from three different sources: FA liberated from adipose tissue, FA liberated from hydrolysis of circulating triacylglycerol (TG), and FA liberated from intramyocellular triacylglycerol (IMTG), and it appears that there are gender differences in the utilization of these energy sources.

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

36.6 Substrate Metabolism During Exercise

36.6.2.1  Increased Sensitivity to Lipolytic stimuli in Female Adipose Tissue The concentration of plasma FA is of importance for lipid availability to skeletal muscle. Investigators have determined that women have a higher postprandial plasma FA concentration than men, which may be consistent with their larger relative fat mass. Indeed, in a large systematic review including 43 studies with reports of overnight-fasted plasma FA concentrations it was concluded that plasma FA concentration was higher in women than men (median 517  μ mol/L in women vs 434  μ mol/Lin men).63 This gender difference was confirmed when plasma FA kinetics were further investigated by applying 2.2-2H2-palmitate or U-13C-palmitate tracers, as a higher basal FA rate of appearance (Ra) was shown in women compared to men.64–66 Together, these findings demonstrate that women have higher fasting FA concentrations at rest, and thereby a higher FA availability per unit of their LBM. Exercise induces an increase in circulating catecholamines, which among other things stimulates lipolysis in adipose tissue through β1-, β2-, and β3-adrenoceptors and thereby enhances the plasma FA concentration to accommodate the increased FA use by the exercising muscles. When exercise was performed at the same relative intensity, it has been shown that men have higher circulating epinephrine and norepinephrine concentrations than women, irrespective of training status.27,67,68 Notably, women exhibit a higher lipolytic sensitivity to catecholamine as shown by a higher increase in plasma glycerol concentration than in men when infused to a similar plasma concentration of epinephrine and norepinephrine.69 This is likely related to a greater stimulation of lipolysis in subcutaneous adipose tissue, as the contribution of visceral adipose tissue on measures of whole body lipolysis is relatively small. In this context, it has been shown in vitro that epinephrine stimulates lipolysis in subcutaneous adipocytes from men to a lower extent than in those from women.70 In female adipocytes, a greater β-adrenergic efficiency was coupled with lower α2-adrenergic receptor activation (antilipolytic role). Thus, the greater sensitivity to lipolytic stimuli in women is reported to be partly related to lower adipose tissue antilipolytic α2 activation in women than men in response to epinephrine, which has recently been confirmed in an in vivo study.71 Despite lower epinephrine concentrations during exercise in women than in men, higher plasma concentrations of glycerol and Ra of glycerol have been observed in women compared with men during the same relative exercise intensity. This has been observed in untrained and moderately trained subjects during acute 60–90  min of submaximal exercise and after 7 weeks of endurance training.27,68,72 Importantly it

531

should be noted that indices of greater lipolysis in women than men are still observed when absolute body fat mass is matched in women and men.73 The higher plasma glycerol concentration observed in untrained and moderately trained individuals was also followed by a higher arterial plasma FA concentration in females than in males when submaximal exercise was performed.72 On the other hand, in endurance trained women and men subjected to 90 min submaximal exercise at the same relative workload, the arterial glycerol concentration was similar between genders, as was the arterial plasma FA concentration and rate of appearance of FA expressed relative to LBM.74 The discrepancy between these findings appears attributable to differences in training status of the subjects. Notably, when uptake and oxidation of plasma FA was measured across the exercising leg in endurance trained individuals, no gender differences in the uptake and oxidation of plasma FA were obtained when exercise was performed at low (25% of VO2peak), moderate (60–65% of VO2peak), or high intensities (85% of VO2peak).74,75 Hence, the uptake of plasma FA during exercise does not appear to be greater in endurance trained women, despite a greater stimulation of lipolysis. 36.6.2.2  Intramyocellular Triacylglycerol Utilization The FA taken up into skeletal muscle can be esterified with glycerol to form triglycerides (TG), which is stored in lipid droplets. When cellular energy demands increase, IMTG can be hydrolyzed by lipolysis to yield FA available for oxidation. The Kiens group was the first to demonstrate that IMTG content is ~25–30% higher in women compared to men,25,74 and this finding was later supported by themselves and others.27,30,76,77 Gender differences in the concentration of TG in muscle have mainly been evaluated in the vastus lateralis muscle by biochemical analyses and histochemical Oil red O staining, but it has also been demonstrated by use of two-dimensional magnetic resonance spectroscopy (MRS) that women have higher content of intramyocellular lipid in the soleus muscle.78 It is well known that type I muscle fibers contain more IMTG than type II fibers,79 and since women usually have more type I fibers than matched men, this could partly explain the higher IMTG content on the level of whole muscle in women compared with men. However, it has also been documented that both type I and type II muscle fibers from women contain more TG compared with men. In the literature, IMTG concentrations have often been described to be negatively associated with whole-body insulin sensitivity. However, endurance trained athletes, having a high level of lipid oxidation, exhibit both a high content of IMTG and enhanced insulin sensitivity.80 The same scenario is present in trained women, who express

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

532

36.  Exercise Physiology in Men and Women

high levels of IMTG and at the same time have higher insulin sensitivity compared to men.81 The observations of higher IMTG concentrations in female skeletal muscle in women could also in part be attributed to the higher basal plasma FA concentration in women than men, which increases the availability of FA to skeletal muscle. In skeletal muscle, uptake of FA is mediated by lipid binding proteins and passive diffusion.82 The fatty acid translocase CD36 (FAT/CD36) is the most studied FA transporter in skeletal muscle; others studied include the membrane bound FA binding protein (FABPpm), FA transport protein 1 (FATP1), and FA transport protein 4 (FATP4), all of which are involved in plasma membrane transport and handling of FA. In particular, FAT/CD36 has been suggested to be important for FA uptake into skeletal muscle during exercise.83–85 Notably, a higher mRNA and protein content of FAT/CD36 in skeletal muscle have been reported in women compared to men, irrespectively of training status.86 Although the mRNA content of the other FA transporters are observed to be higher in women than men, as demonstrated for FABPpm,86 FATP1,87 and also the cytosolic fatty acid binding protein (FABPc),54,88 there have to our knowledge not been any reports that the protein content of these are higher in women. Thus, only FAT/CD36 is confirmed to be higher at the protein level in women than men, an observation that indeed implies that women may have a greater capacity to increase FA transport into skeletal muscle. Hence, the greater IMTG concentrations in women may be linked to a more efficient FA uptake mediated by FAT/CD36, in combination with their higher plasma FA concentrations. Interestingly, during 60–90 min submaximal exercise at same relative workload women utilize IMTG to a larger extent than men, irrespective of training status.25,29,74 Thus, in these studies it was reported that IMTG content was reduced by ~25–35% in women, while breakdown of IMTG in matched men was barely detectable. Triacylglycerol breakdown in skeletal muscle (as in adipose tissue) is carried out by the adipose triglyceride lipase (ATGL), which hydrolyzes the first ester bond thereby releasing FA and forming diacylglycerol (DAG). DAG is hydrolyzed by hormone sensitive lipase (HSL), generating monoacylglycerol and another FA. In the last step monoacylglycerol is hydrolyzed by monoacylglycerol lipase (MGL).89 When total TG hydrolase activity was measured in skeletal muscle homogenates obtained at rest in the fasting state, a twofold higher activity was reported in women compared to men, with no differences in DAG hydrolase activity.90 These findings suggest that there is a higher maximal capacity for ATGL-mediated lipolysis in women than men. This is not linked to gender differences in the protein content of ATGL and its activator CGI-58, which is similar in matched men and women (Kiens, unpublished

observations). Regarding HSL, 90 min submaximal exercise increased HSL activity to a similar extent in women and men, despite a higher total protein content of HSL in women than men.27 However, the increased IMTG breakdown during submaximal exercise in women was not coupled with increased HSL activity. These findings point to a gender-specific regulation of ATGL phosphorylation and activity during exercise, which awaits further gender-comparative studies. It has been found by use of electron microscopy that IMTG in women is localized in a higher number of smaller lipid droplets compared to men.77 This morphologic characteristic might increase accessibility of lipases and proteins associated with the lipid droplets, and thereby increase the turnover of IMTG. Interestingly, lipid droplets in women are located closer to mitochondria after an exercise bout,76 a location which may increase susceptibility to oxidation. The phospholipid surface of lipid droplets is covered with a number of proteins involved in lipid metabolism and trafficking of the lipid droplets. In untrained men and women, matched for VO2peak/kg LBM, skeletal muscle protein expression of perilipin 2, 3, 4, and 5 (also known as ADRP, TIP47, S3-12, and OXPAT, respectively) was 1.5- to 2-fold higher in women.91 Of these, perilipin 3 may be considered important for lipid droplet lipolysis,92 and perilipin 5 has been described to interact with lipolytic key proteins as ATGL and its activator CGI-58.93 Furthermore, recent work also indicates that perilipin 5 mediates an interaction between lipid droplets and mitochondria.94 Taken together, smaller and more abundant lipid droplets and increased expression of perilipins in women are likely to increase association with lipases and eventually mitochondria during exercise, thereby increasing lipolytic turnover of IMTG in women compared with men. Whether the higher expressions of the perilipins are simply due to a higher content of lipid droplets in women remains to be elucidated. Studies have consistently shown that IMTG contributes to a larger extent as energy fuel in women. On the other hand, likely due to their lower intramuscular lipid levels, men may be more dependent on plasma lipids, which includes FA from circulating very-low density lipoprotein-triacylglycerol (VLDL-TG). A greater utilization of FA from VLDL-TG could be due to enhanced hydrolysis in the capillary bed of skeletal muscle, and here muscle lipoprotein lipase (LPL) is an important player. It is a well-known observation that trained subjects have a higher LPL activity in muscle in the resting fasted state compared to untrained subjects. This is evident in both men and women, with no gender difference in basal LPL activity.86 Muscle LPL activity in men may be greater than that of women during exercise. Hence, when LPL activity was measured immediately after 90 min exercise (85% of lactate threshold),

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

36.6 Substrate Metabolism During Exercise

there was an exercise-induced increase of 56% in men, while mLPL activity was similar to preexercise values in women.95 In support of this, it has also been shown in another study that muscle LPL activity is increased during exercise in men,96 though not compared to women in this study. Thus, it seems possible that men activate LPL in skeletal muscle more during submaximal exercise. An increase in muscle LPL-activity will increase lipolysis of circulating VLDL-TG and thereby release FA to be taken up by the surrounding tissue. In accordance, VLDL-TG might be a useful energy substrate in men during exercise.74

36.6.3  Glucose Metabolism When skeletal muscle contracts during exercise, intracellular signaling events lead to an increased translocation of glucose transporter 4 (GLUT4) to the plasma membrane, in order to increase glucose uptake. A similar total protein content of GLUT4 in skeletal muscle of men and women has been observed,30 despite reports of higher GLUT4 mRNA content in women compared to men.54,97 Hexokinase II (HKII) is another key protein involved in glucose uptake, catalyzing the phosphorylation of glucose to glucose-6-phosphate (G6P) after entry into the skeletal muscle cells. In this way, HKII maintains the concentration gradient that facilitates the transport of glucose. Also, the addition of the phosphate group ensures that glucose is trapped within the muscle cells. For HKII, a 56% higher protein content has been demonstrated in women compared to men,98 which agrees with the finding of 2.4-fold higher HKII mRNA in female skeletal muscle.54 It can be hypothesized that an increased capacity for intracellular phosphorylation of glucose will facilitate its uptake and thereby contributes to an increased capacity for glucose uptake in women. During exercise, however, women have not been shown to rely more on plasma glucose than men, and thus it appears that women may not benefit from their increased hexokinase capacity, at least during submaximal aerobic exercise. In this regard, it should also be noted that when maximal HKII activity is evaluated in vitro in skeletal muscle biopsies of men and women, the activity was found to be similar.99 During 90 min submaximal aerobic exercise, glucose Ra and glucose rate of disappearance (Rd) were similar in untrained men and women both before and after a training period.72 Likewise, in endurance trained individuals no significant gender differences were observed during 90 min of submaximal exercise (60% of VO2peak) in glucose uptake across the exercising leg when expressed per kg lean leg mass.74 Even during more intense exercise (88% of VO2peak), glucose Rd expressed relative to LBM was similar between women and men.100

533

36.6.4  Glycogen Stores and Glycolytic Capacity Glucose can be stored as glycogen in skeletal muscle. Depending on training status and carbohydrate intake, the total amount of glycogen in muscles can comprise ~350–500 g. In the resting fasting state, skeletal muscle glycogen content is not different between untrained and trained men and women, matched for training status and during conditions of a controlled diet.29,61 The similarity of the glycogen stores is confirmed by observations of a similar activity of the rate-limiting enzyme of glycogen synthesis, glycogen synthase (GS), when evaluated in vitro in skeletal muscle homogenates obtained at rest from matched men and women (Kiens, unpublished observations). During exercise, the greater catecholamine concentrations in men than women during submaximal as well as maximal intensity exercise may be speculated to increase glycogenolysis to a greater extent in men. There have, however, been divergent findings in regard to whether men use more or less muscle glycogen during exercise compared to women. Considering the studies that have directly assessed glycogen breakdown in skeletal muscle biopsies, there are some reports of men using more skeletal muscle glycogen than women. Hence, after submaximal treadmill running for 90 min at 65% of VO2peak, a 25% greater glycogen breakdown was observed in well-trained men compared to women,101 while submaximal bicycle exercise seemed to induce a similar glycogen breakdown in men and women.61,74 When it comes to more intense exercise, like 30 s bicycle sprinting, glycogen depletion was described to be 50% less in type I muscle fibers in women than in men.102 The discrepancies may be related to the type of exercise and fiber type recruitment, with the latter study demonstrating that the gender difference may be more pronounced during intense exercise. In support of a greater capacity for glycogenolysis in men, a higher maximal activity of glycogen phosphorylase (GP) has been reported in muscle homogenates from untrained men compared to women.103 In the muscle cells, G6P derived from either glycogen breakdown or glucose taken up from plasma is substrate for glycolysis, of which the end-product pyruvate can be converted to acetyl-coA which is a fuel for the tricarboxylic acid (TCA) cycle. Notably, the glycolytic capacity appears to be greater in men, due to several reports of higher maximal activity of enzymes important for glycolysis. Higher activities of PFK (third step in glycolysis), pyruvate kinase (PK) (final step in glycolysis), and lactate dehydrogenase (LDH) (interconversion of pyruvate and lactate) have been demonstrated in muscle homogenates from untrained men compared with women.103 These findings are supported in later studies, demonstrating a higher PFK, LDH, and also

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

534

36.  Exercise Physiology in Men and Women

malate dehydrogenase (MDH) activity in muscles of men compared to women.44,104 The observations were made in homogenates from the vastus lateralis muscle, but in addition to these findings a higher maximal activity of LDH and PFK in men than women was also observed when the tibialis anterior muscle was used for comparison.105 These findings clearly indicate that men have a higher capacity for glycogenolysis and glycolytic flux during exercise. This might imply that men are better able to cope with intense myocellular energy demands. Along these lines, a lower ratio between β-hydroxy acyl-CoA dehydrogenase (HAD) activity and glycolytic enzyme activity in skeletal muscle was observed in men compared to women,103 which could indicate that male muscle has a higher potential for glycolysis rather than beta-oxidation. Considering the fact that type II fibers have a higher glycolytic potential compared to type I fibers,106 it is possible that the difference in glycolytic capacity between men and women is simply related to the higher relative amount of type II fibers in men. This has not been studied, but could be investigated using single-fiber analyses.

36.7  ATP RESYNTHESIZES IN SKELETAL MUSCLE Apparently a gender-specific difference exists in the utilization of energy substrates during exercise. The question is whether there may be gender-specific differences in the molecular machineries that resynthesize ATP from the different energy metabolites in skeletal muscle.

36.7.1  Mitochondrial Fatty Acid Transport and Beta-Oxidation In the cytosol of the skeletal muscle cells, the FA taken up from plasma or liberated from IMTG lipolysis is activated to fatty acyl-CoA by acyl-CoA synthase. Before oxidiation in the mitochondria, fatty acyl-CoA has to be converted to acylcarnitine to cross the outer mitochondrial membrane, a reaction catalyzed by carnitine palmitoyl transferase 1 (CPT1). CPT1 activity is reported to be a key regulator of FA oxidation in skeletal muscle. Free carnitine is required to create acylcarnitine, and it has been proposed that the absolute amount of free carnitine near CPT1 is vital for the regulation of FA oxidation during exercise.107 There is no gender difference in the level of muscle free carnitine at rest,108 while gender differences in muscle free carnitine during exercise have not been investigated. It has been observed that muscle CPT1 mRNA content is higher in women,54 a finding which was also retained

in myotubes obtained from women compared to men.109 However, CPT1 protein content and enzyme activity, measured in intact mitochondria isolated from vastus lateralismuscle biopsies, were reported to be similar in both untrained and trained men and women,110,111 and there does not seem to be a difference in the maximal capacity of CPT1. Within the mitochondria, the fatty acyl-CoA enters the beta-oxidation pathway, in which the acyl-CoA dehydrogenases catalyze the first oxidation reaction. Studies have shown that both long-chain acyl-CoA dehydrogenase (LCAD) mRNA,54 and very long- and medium-chain acyl-CoA dehydrogenase (VLCAD and MCAD) protein content are higher in skeletal muscle of women than men.112 Mitochondrial trifunctional protein α (TFPα) catalyzes the second (hydration) and third (oxidation) reaction for acyl-CoA substrates, while TFPβ catalyzes the fourth thiolysis reaction in the production of acetyl-coA. The mRNA as well as protein content of TFPα is reported to be higher in women,88,112 while TFPβ protein content seems to be similar in men and women.35 The HAD enzyme is part of TFPα and catalyzes the third reaction which leads to NADH. The maximal activity of HAD seems however to be similar in men and women.34,113 Collectively, women have a higher expression of FA oxidation enzymes responsible for FA oxidation compared with men, especially in the initial parts of the beta-oxidation, which together contribute to a higher capacity for generation of acetyl-coA from FA.

36.7.2  Tricarboxylic Acid Cycle and Oxidative Phosphorylation The acetyl-coA from glycolysis and beta-oxidation fuels the TCA cycle, which produces NADH coenzymes to fuel oxidative phosphorylation. The maximal in vitro activity of citrate synthase (CS), the first and rate-limiting enzyme in the TCA cycle, is similar in skeletal muscle from men and women,30,113 and hence there is a similar capacity of the TCA cycle between genders. As maximal CS activity has been found to be the best marker of mitochondrial content measured by electron microscopy in human skeletal muscle,114 these findings may also be indicative of a similar mitochondrial content in skeletal muscle in men and women. Exercise training can increase TCA cycle capacity, as it is well documented that endurance trained subjects have a higher CS activity compared to untrained subjects, and it has been shown that the activity of CS increases to a similar extent in women and men after a period with aerobic training.34 With regard to oxidative phosphorylation, the maximal activity of cytochrome C oxidoreductase (complex III) and cytochrome C oxidase (complex IV) are also reported to be similar in muscles of men and women.34 Along with these observations, when maximal ATP

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

36.8  Estrogen and its Impact on Metabolism

production during oxidative phosphorylation was measured in freshly isolated mitochondria from skeletal muscle, a similar ATP production rate was observed in sedentary men and women.115 Also, when mitochondrial respiration was analyzed in an oxygraph on muscle bundles, and complex I-, II-, III-, and IV-dependent respiration measured individually, no gender differences were observed.116 Together, the capacity for acetyl-coA flux through TCA and ATP production from oxidative phosphorylation appears to be similar in men and women, suggesting an equal capacity for myocellular energy generation from acetyl-coA. The propensity for a higher relative FA utilization in women than in men is therefore not due to a higher mitochondrial capacity in women but rather to an increased ratio of beta-oxidative capacity to glycolytic capacity in women, concomitant with increased cellular availability of FA from IMTG and plasma sources.

36.7.3  Finetuning of Acetyl-coA Input to the Tricarboxylic Acid Cycle Pyruvate dehydrogenase (PDH) is a convergence point in the regulation of the metabolic finetuning between glucose and FA oxidation. Hence, PDH converts pyruvate to acetyl-coA, and thereby increases the influx of acetyl-coA from glycolysis into the TCA cycle. Pyruvate dehydrogenase kinase 4 (PDK4) is a regulator of PDH, as it inhibits PDH activity, which in turn will increase the influx of acetyl-coA from beta-oxidation into the TCA cycle, thereby leading to enhanced FA oxidation and slowing of glycolysis or glycolytic intermediates to alternative metabolic pathways. A role for estradiol in the transcriptional regulation of PDK4 has been documented, suggesting gender-specific regulation of PDK4. Thus, when human primary myotubes obtained from women and men are incubated with 17-β estradiol, PDK4 mRNA content is increased in female myotubes,109 and a study in humans has shown that estrogen treatment during menopause led to an increase in PDK4 mRNA in skeletal muscle.117 The protein content of PDK4 in human skeletal muscle has not been subject to gendercomparative studies. However, it has been observed that PDH-E1α protein content is 25% lower in skeletal muscle of women than men and that PDK4 mRNA is higher in female skeletal muscle (Kiens, unpublished observations). These observations suggest that there is a lower requirement for PDH in female skeletal muscle, perhaps due to a lower glycolytic activity in women. A similar sexual dimorphism has been observed in the rat heart. Thus, the PDH complex was significantly higher expressed in male hearts compared with female rat hearts, with the gene expression of PDK4 being significantly higher in female heart.118 Taken together, these findings suggest that gender differences in PDK4 protein

535

content and the regulation of PDH activity may contribute to explain the difference in the oxidative flux of glucose and FA in men and women in relation to exercise.

36.8  ESTROGEN AND ITS IMPACT ON METABOLISM The described gender differences in substrate metabolism as well as the molecular differences in glycolytic and beta-oxidative capacity are likely in part related to sex hormone effects. The greater relative lipid utilization during exercise in women has been speculated to be partly ascribed to estrogen actions. In support of a role for estrogen, differences in exercise substrate metabolism between women and men are not observed in childhood, but become evident with puberty.119 In addition, fat oxidation of postmenopausal women was 33% lower during same relative exercise at 50% of VO2 peak when compared to premenopausal women.120 It was also observed in a longitudinal study that resting fat oxidation was lowered by 32% in post- compared to premenopausal women, when subjected to 24 h whole room calorimetry.121 The estrogen receptors α (ERα) and β (ERβ) are both expressed in human skeletal muscle.122,123 By immunohistochemistry, it has been shown that ERα and ERβ are localized to the nuclei of the myofibers, with comparable expression levels in women and men.124 A nuclear localization of the receptors suggests a role in transcriptional regulation of muscle enzymes and proteins. ERα appears to be the most important isoform, as it is markedly higher expressed than ERβ in human skeletal muscle.123 Estradiol incubation of human myotubes increases only ERα mRNA, but not ERβ mRNA.125 Interestingly, both ERα and ERβ mRNA were reported to be three- to fivefold higher in endurance trained men compared to moderately active men, suggesting a role of these estrogen receptors in the adaptations to exercise in skeletal muscle.126 The ERα and ERβ receptors mainly function as transcription factors, while some estrogen actions in muscle may also be mediated by nongenomic effects, as extra nuclear estrogen receptors have been identified. In this context, there is evidence for a mitochondrial location of ERα in the C2C12 mouse skeletal muscle cell line, demonstrated by immunostaining of labeled estradiol binding to mitochondrial fractions.127 Hence, a role for estrogen and ERα in regulation of mitochondrial metabolism human muscle is suggested, but still unexplored. Today many women use oral contraceptives (OC) that modify hormonal status. The active estrogen is often ethinyl estradiol, which is reported to be the most potent of the estrogen agonists.128 OC use reduces natural estrogen production, and depending on OC type, three to five

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

536

36.  Exercise Physiology in Men and Women

times more exogenous estrogen is provided compared with normal endogenous estrogen concentrations.129 Considering the role of estrogen in skeletal muscle substrate metabolism, the use of OC may indeed have implications for exercise metabolism. There is, however, a large variation in the type of OC that is administered (i.e., monophasic and triphasic), and the type and dose of estrogen and progesterone are also subject to large differences. These variations make interpretations of the existing studies more difficult, and thus the evidence for the effect of OC use on exercise metabolism in women is not clear. Several interventional or cross-sectional studies have investigated whether OC use has implications for substrate utilization and aerobic capacity. It has been suggested by some studies that inactive women on monophasic OC have greater FA concentrations, and hence greater relative lipid utilization during exercise compared to non-OC users. However, this has not been found in trained women, and several inconsistent findings require that further studies are needed in this area. It has also been reported that triphasic OC may have a negative impact on VO2peak,49 but this is not found with monophasic OC use. The available literature has been reviewed by Burrows and Peters,129 systematically taking into account the OC type. Taken together, wellcontrolled studies with a high sample size are required to gain further insight into this complex area.

36.9  GENDER DIFFERENCES IN METABOLISM DURING RECOVERY FROM EXERCISE While women rely more on lipids during exercise compared to men, the opposite scenario is evident in the period following exercise. Hence, a recent meta-analysis including 18 studies investigating substrate utilization in men and women during 2–22 h of recovery from 60–120 min endurance exercise at 28–75% of VO2peak, has reported a greater FA oxidation in men than women after exercise, when investigated by indirect calorimetry and in the postabsorptive state.130 Furthermore, when tracer analysis was added to the indirect calorimetry and applied during 3 h of recovery from moderate intensity exercise at 45% or 65% of VO2peak, a greater FA oxidation was confirmed in men compared to women.131 Thus, it seems to be well documented that women exhibit a greater oxidative utilization of glucose in the period following exercise than men. This is likely due to the reason that plasma FA are used for replenishment of the IMTG that was broken down during exercise, rather than being used to cover the oxidative needs during postexercise recovery. In men, IMTG stores are not depleted during exercise, and therefore it might be more beneficial to oxidize FA, while preserving glucose to resynthesize

glycogen in skeletal muscle. This reciprocal shift in preferential substrate utilization in recovery compared to the exercise situation may counterbalance the difference between the amount of glucose and FA used in response to exercise. It has been shown by use of 1H-NMR that the exercise-induced decrease (25%) in IMTG is fully replenished after 20 h on an eucaloric medium-fat diet (33 E% fat), but not on a low-fat diet (10 E%) fat, suggesting that sufficient exogenous dietary FA are needed in order to efficiently resynthesize IMTG stores.

36.10  NUTRITIONAL IMPLICATIONS IN RELATION TO EXERCISE 36.10.1  Energy Availability in Athletes For some athletes it is important to pay attention to their total energy intake. However, appetite is not a reliable indicator of energy needs in athletes, as it has been well documented that prolonged or heavy exercise suppresses ad libitum food intake. Therefore, energy intake can sometimes be lower than the athlete’s energy requirements, in particular during periods with a high training volume. The energy deficit produced during exercise does not induce compensatory responses in appetite, which has been described to be due to exercise-induced suppression of circulating acylated ghrelin,132 and concomitant increases in peptide YY (PYY), glucagon-like peptide-1 (GLP-1), and pancreatic polypeptide (PP) levels,133 which together will suppress appetite and hunger sensations. Some recent studies have added to the understanding of the impact of exercise on appetite in a well-controlled gender-comparative setup. When moderately active men and women exercised corresponding to 30% of daily energy expenditure at 70% of VO2peak, no differences in PYY3–36 and acylated ghrelin were observed, and both genders experienced a similar suppression of appetite and ad libitum energy intake. These findings were confirmed in another study, also in moderately active men and women, which exhibited similar appetite, acylated ghrelin, and PYY3–36 responses to an exercise-induced energy deficit from 60 min running at 70% of VO2peak.134 To date, no studies have compared the appetite hormone response to high-intensity exercise in well trained men and women. Despite these reports of a similar regulation of exercise-induced appetite suppression between genders, it has been frequently found that the energy intake of trained women is lower than the corresponding energy requirement. The term energy availability is defined as dietary energy intake minus exercise energy expenditure,135 and it has been reported that low energy availability in female athletes is prevalent during severe training periods and in sports where body weight has

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

36.10  Nutritional Implications in Relation to Exercise

implications for performance or esthetic appeal. Women undertaking heavy exercise training are indeed susceptible to periods of low energy availability, which results in hormonal disturbances and a high prevalence of menstrual disorders. Over time, low energy availability may result in disturbance of the gonadotropin releasing hormone (GnRH) pulse in the hypothalamus, which again disrupts luteinizing hormone pulsatility, with implications for ovarian function and estrogen homeostasis. The term “female athlete triad” encompasses the spectrum of restrained eating (low energy availability), menstrual dysfunction, and poor bone health, which will increase the risk of stress fractures, osteopenia, or osteoporosis later in life. It has been controversial whether the underlying culprit is insufficient body fat stores or the stress of exercise. However, it has become clear that the hormonal disturbances in women undergoing extensive training periods are due to the energy cost of exercise, and hence low energy availability that leads to luteal suppression and anovulation, and not the exercise in itself. In this context, it has been shown that hormonal disturbances occur when energy availability is reduced below a threshold between 20 and 30 kcal/kg LBM∙per day,135 and therefore the minimal energy requirement has been set to 30–45 kcal/kg LBM∙per day, plus the amount of energy needed for physical activity, to preserve normal reproductive function and skeletal health.136 On the other side of the spectrum, it has been proposed that women may be more resistant than men to exercise-induced weight loss, in part because women have a proportionally greater fat mass than men. Hence, this is in contrast to the conditions described above in well trained female athletes, and is more likely to be relevant for obese and untrained subjects. It has been speculated that this greater fat storage capacity is due to estrogenic actions in the hypothalamic regions that regulate food intake, energy expenditure, and white adipose tissue distribution.137 It has been shown by prediction equations that the energy cost associated with body weight loss is greater in women (30–32 MJ/kg BM) than men (21–23 MJ/kg BM),138 but in a recent review investigating exercise-induced weight loss in men and women, it was described that even though many of the studies reported a significantly higher weight loss in men than women, the effect size was very small and not of physiologic significance.139 Therefore, there does not appear to be a need for a gender-specific prescription when exercise-induced weight loss regimens are devised.

36.10.2  Dietary Macronutrient Composition The current guidelines for nutrition in sports have been composed without paying particular attention to whether there is a need of specific guidelines for the

537

female versus male athlete. Here, we offer a short summary of general advice on macronutrient composition, with the addition of a few studies that are relevant to whether or not there is a need for gender-specific nutritional prescriptions. The guidelines for carbohydrate intake are preferentially provided in grams relative to BM rather than as percentage of dietary energy. Currently, a carbohydrate intake range of 5–7 g/kg BM∙per day has been suggested for general training needs and 7–10 g/kg BM∙per day for the increased needs of endurance athletes.140 There does not appear to be any reason to make explicit recommendations for carbohydrate intake in women and men, except when it comes to carbohydrate loading as a performance enhancing strategy before long-term endurance exercise. In this context, the efficacy of carbohydrate loading has been proposed to be different in men and women. In an early study, it was shown that increasing dietary carbohydrate intake from 55 to 75 E% for 4 days increased skeletal muscle glycogen concentration and enhanced cycling performance in endurance trained men, but not in women.141 The lack of an increased glycogen content in muscle of women was attributed to a low total energy intake, which resulted in an absolute carbohydrate intake of less than 6.5 g/kg·per day. Hence, in a later follow-up study it was shown that increasing total energy intake, while maintaining 75 E% carbohydrate, resulted in a carbohydrate intake of 8 g/kg·per day, and increased muscle glycogen content in women.99 In regard to daily protein intake, the current US, Canadian, and Australian RDI state that a protein intake between 0.75 and 0.8 g/kg BM will meet the needs of 98% of the population. It has been proposed that this is an underestimation,142 and it is still debated what the requirements are in aerobically or resistance trained athletes. To determine dietary protein requirements in the endurance or resistance training state, nitrogen balance studies have been applied. On the basis of the existing evidence the American College of Sports Medicine Position Stand recommends that dietary protein requirements range from 1.2  to  2 g/kg BM∙per day in athletes to support metabolic adaptations, repair, and protein turnover. In support of this, it has been found that well trained women and men were both in a negative nitrogen balance, when ingesting 0.8 and 0.94 g/kg BM∙ per day.16 Furthermore, in a study measuring 72 h of nitrogen balance in endurance training women (~10 h/ week), it was estimated that 1.6 g/kg BM∙per day was needed to attain nitrogen balance.143 Most athletes can easily reach these protein requirements from their usual diet. The intake of dietary fat by athletes is in general proposed to be in accordance with the public health guidelines. However, athletes should be discouraged

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

538

36.  Exercise Physiology in Men and Women

from fat intakes <20 E%, as lower fat intakes could reduce the intake of fat-soluble vitamins and essential fatty acids. It has been shown in endurance runners that by increasing dietary fat, the difference between caloric intake and energy expenditure becames gradually less in both women and men.144 Therefore, sufficient dietary fat intake may be a strategy to increase dietary energy density and thereby energy availability in athletes with high training volumes. In particular this could be relevant for premenopausal women, who need to pay attention to their energy availability in order to avoid this to become too low. Therefore, a dietary fat intake of 20–30 E% is recommended.

36.11  CONCLUDING HIGHLIGHTS Women have a lower heart ventricular mass and thereby stroke volume, which implies that maximal cardiac output is lower in women than men. In addition, women are generally smaller, have a lower blood volume and hemoglobin concentration than men. Together, this leads to the observation that women have lower absolute maximal oxygen uptake than men. However, when these differences are viewed relative to LBM the difference is minimized. ● There is a greater proportion of type I fibers per given muscle area in women, together with a greater capillary density. ● Women have greater fat oxidation than men during submaximal exercise at the same relative exercise intensity. In addition, the maximal fat oxidation rate is reported to be higher in women. The greater fat utilization in female skeletal muscle might be related to the greater proportion of type I fibers and higher capillary density. ● There is in general a larger IMTG content in skeletal muscle of women and a greater IMTG utilization during exercise in women compared to men. ● Men have a greater amino acid (i.e., leucine) oxidation during exercise than women, but absolute protein oxidation during exercise is relatively small in both genders. ● Despite the growing evidence of notable gender differences, women are still under-represented compared to men in sport and exercise medicine research.145 Further studies should be emphasized to fully understand the female physiology and metabolism during exercise. ●

References 1. Bam J, Noakes TD, Juritz J, Dennis SC. Could women outrun men in ultramarathon races? Med Sci Sports Exerc. 1997;29(2):244–247.

2. Speechly DP, Taylor SR, Rogers GG. Differences in ultra-endurance exercise in performance-matched male and female runners. Med Sci Sports Exerc. 1996;28(3):359–365. 3. Deaner RO, Carter RE, Joyner MJ, Hunter SK. Men are more likely than women to slow in the marathon. Med Sci Sports Exerc. 2015;47(3):607–616. 4. Roth SM, Ferrell RE, Peters DG, Metter EJ, Hurley BF, Rogers MA. Influence of age, sex, and strength training on human muscle gene expression determined by microarray. Physiol Genomics. 2002;10(3):181–190. 5. Janssen I, Heymsfield SB, Wang ZM, Ross R. Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr. J Appl Physiol (1985). 2000;89(1):81–88. 6. Nielsen S, Guo Z, Johnson CM, Hensrud DD, Jensen MD. Splanchnic lipolysis in human obesity. J Clin Invest. 2004;113(11):1582–1588. 7. Rodriguez G, Samper MP, Olivares JL, Ventura P, Moreno LA, Perez-Gonzalez JM. Skinfold measurements at birth: sex and anthropometric influence. Arch Dis Child Fetal Neonatal Ed. 2005;90(3):F273–F275. 8. Ogle GD, Allen JR, Humphries IR, et  al. Body-composition assessment by dual-energy x-ray absorptiometry in subjects aged 4-26 y. Am J Clin Nutr. 1995;61(4):746–753. 9. Chumlea WC, Guo SS, Kuczmarski RJ, et al. Body composition estimates from NHANES III bioelectrical impedance data. Int J Obes Relat Metab Disord. 2002;26(12):1596–1609. 10. Kvist H, Chowdhury B, Grangard U, Tylen U, Sjostrom L. Total and visceral adipose-tissue volumes derived from measurements with computed tomography in adult men and women: predictive equations. Am J Clin Nutr. 1988;48(6):1351–1361. 11. Lemieux S, Prud'homme D, Bouchard C, Tremblay A, Despres JP. Sex differences in the relation of visceral adipose tissue accumulation to total body fatness. Am J Clin Nutr. 1993;58(4):463–467. 12. Dreyer HC, Fujita S, Glynn EL, Drummond MJ, Volpi E, Rasmussen BB. Resistance exercise increases leg muscle protein synthesis and mTOR signalling independent of sex. Acta Physiol (Oxf). 2010;199(1):71–81. 13. Fujita S, Rasmussen BB, Bell JA, Cadenas JG, Volpi E. Basal muscle intracellular amino acid kinetics in women and men. Am J Physiol Endocrinol Metab. 2007;292(1):E77–E83. 14. Markofski MM, Volpi E. Protein metabolism in women and men: similarities and disparities. Curr Opin Clin Nutr Metab Care. 2011;14(1):93–97. 15. Jahn LA, Barrett EJ, Genco ML, Wei L, Spraggins TA, Fryburg DA. Tissue composition affects measures of postabsorptive human skeletal muscle metabolism: comparison across genders. J Clin Endocrinol Metab. 1999;84(3):1007–1010. 16. Phillips SM, Atkinson SA, Tarnopolsky MA, MacDougall JD. Gender differences in leucine kinetics and nitrogen balance in endurance athletes. J Appl Physiol (1985). 1993;75(5):2134–2141. 17. West DW, Burd NA, Churchward-Venne TA, et al. Sex-based comparisons of myofibrillar protein synthesis after resistance exercise in the fed state. J Appl Physiol (1985). 2012;112(11):1805–1813. 18. Convertino VA. Blood volume response to physical activity and inactivity. Am J Med Sci. 2007;334(1):72–79. 19. Nio AQ, Stohr EJ, Shave R. The female human heart at rest and during exercise: a review. Eur J Sport Sci. 2015;15(4):286–295. 20. Joyner MJ, Casey DP. Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs. Physiol Rev. 2015;95(2):549–601. 21. Loe H, Rognmo O, Saltin B, Wisloff U. Aerobic capacity reference data in 3816 healthy men and women 20-90 years. PLoS One. 2013;8(5):e64319. 22. Sydo N, Abdelmoneim SS, Mulvagh SL, Merkely B, Gulati M, Allison TG. Relationship between exercise heart rate and age in men vs women. Mayo Clin Proc. 2014;89(12):1664–1672.

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

REFERENCES

23. Murphy WG. The sex difference in haemoglobin levels in adults - mechanisms, causes, and consequences. Blood Rev. 2014;28(2):41–47. 24. Charkoudian N, Joyner MJ. Physiologic considerations for exercise performance in women. Clin Chest Med. 2004;25(2):247–255. 25. Steffensen CH, Roepstorff C, Madsen M, Kiens B. Myocellular triacylglycerol breakdown in females but not in males during exercise. Am J Physiol Endocrinol Metab. 2002;282(3):E634–E642. 26. Wheatley CM, Snyder EM, Johnson BD, Olson TP. Sex differences in cardiovascular function during submaximal exercise in humans. Springerplus. 2014;3:445. 27. Roepstorff C, Donsmark M, Thiele M, et  al. Sex differences in hormone-sensitive lipase expression, activity, and phosphorylation in skeletal muscle at rest and during exercise. Am J Physiol Endocrinol Metab. 2006;291(5):E1106–E1114. 28. Brooke MH, Engel WK. The histographic analysis of human muscle biopsies with regard to fiber types. 1. Adult male and female. Neurology. 1969;19(3):221–233. 29. Roepstorff C, Thiele M, Hillig T, et  al. Higher skeletal muscle alpha2AMPK activation and lower energy charge and fat oxidation in men than in women during submaximal exercise. J Physiol. 2006;574(Pt 1):125–138. 30. Hoeg L, Roepstorff C, Thiele M, Richter EA, Wojtaszewski JF, Kiens B. Higher intramuscular triacylglycerol in women does not impair insulin sensitivity and proximal insulin signaling. J Appl Physiol. 2009;107(3):824–831. 31. Staron RS, Hagerman FC, Hikida RS, et al. Fiber type composition of the vastus lateralis muscle of young men and women. J Histochem Cytochem. 2000;48(5):623–629. 32. Simoneau JA, Bouchard C. Human variation in skeletal muscle fiber-type proportion and enzyme activities. Am J Physiol. 1989;257(4 Pt 1):E567–E572. 33. Miller AE, MacDougall JD, Tarnopolsky MA, Sale DG. Gender differences in strength and muscle fiber characteristics. Eur J Appl Physiol Occup Physiol. 1993;66(3):254–262. 34. Carter SL, Rennie CD, Hamilton SJ. Tarnopolsky. Changes in skeletal muscle in males and females following endurance training. Can J Physiol Pharmacol. 2001;79(5):386–392. 35. Maher AC, Fu MH, Isfort RJ, Varbanov AR, Qu XA, Tarnopolsky MA. Sex differences in global mRNA content of human skeletal muscle. PLoS One. 2009;4(7):e6335. 36. Welle S, Tawil R, Thornton CA. Sex-related differences in gene expression in human skeletal muscle. PLoS One. 2008;3(1):e1385. 37. Holmback AM, Porter MM, Downham D, Andersen JL, Lexell J. Structure and function of the ankle dorsiflexor muscles in young and moderately active men and women. J Appl Physiol (1985). 2003;95(6):2416–2424. 38. Yu F, Hedstrom M, Cristea A, Dalen N, Larsson L. Effects of ageing and gender on contractile properties in human skeletal muscle and single fibres. Acta Physiol (Oxf). 2007;190(3):229–241. 39. Hill DW, Smith JC. Gender difference in anaerobic capacity: role of aerobic contribution. Br J Sports Med. 1993;27(1):45–48. 40. Wiecek M, Maciejczyk M, Szymura J, Szygula Z. Changes in oxidative stress and acid-base balance in men and women following maximal-intensity physical exercise. Physiol Res. 2015;64(1):93–102. 41. Sargent C, Scroop GC. Plasma lactate accumulation is reduced during incremental exercise in untrained women compared with untrained men. Eur J Appl Physiol. 2007;101(1):91–96. 42. Gratas-Delamarche A, Le CR, Delamarche P, Monnier M, Koubi H. Lactate and catecholamine responses in male and female sprinters during a Wingate test. Eur J Appl Physiol Occup Physiol. 1994;68(4):362–366. 43. Esbjornsson-Liljedahl M, Bodin K, Jansson E. Smaller muscle ATP reduction in women than in men by repeated bouts of sprint exercise. J Appl Physiol (1985). 2002;93(3):1075–1083.

539

44. Esbjornsson M, Sylven C, Holm I, Jansson E. Fast twitch fibres may predict anaerobic performance in both females and males. Int J Sports Med. 1993;14(5):257–263. 45. Piers LS, Diggavi SN, Rijskamp J, van Raaij JM, Shetty PS, Hautvast JG. Resting metabolic rate and thermic effect of a meal in the follicular and luteal phases of the menstrual cycle in wellnourished Indian women. Am J Clin Nutr. 1995;61(2):296–302. 46. Horton TJ, Miller EK, Glueck D, Tench K. No effect of menstrual cycle phase on glucose kinetics and fuel oxidation during moderate-intensity exercise. Am J Physiol Endocrinol Metab. 2002;282(4):E752–E762. 47. Heiling VJ, Jensen MD. Free fatty acid metabolism in the follicular and luteal phases of the menstrual cycle. J Clin Endocrinol Metab. 1992;74(4):806–810. 48. Suh SH, Casazza GA, Horning MA, Miller BF, Brooks GA. Luteal and follicular glucose fluxes during rest and exercise in 3-h postabsorptive women. J Appl Physiol (1985). 2002;93(1):42–50. 49. Casazza GA, Suh SH, Miller BF, Navazio FM, Brooks GA. Effects of oral contraceptives on peak exercise capacity. J Appl Physiol. 2002;93(5):1698–1702. 50. Nicklas BJ, Hackney AC, Sharp RL. The menstrual cycle and exercise: performance, muscle glycogen, and substrate responses. Int J Sports Med. 1989;10(4):264–269. 51. Campbell SE, Angus DJ, Febbraio MA. Glucose kinetics and exercise performance during phases of the menstrual cycle: effect of glucose ingestion. Am J Physiol Endocrinol Metab. 2001;281(4):E817–E825. 52. Zderic TW, Coggan AR, Ruby BC. Glucose kinetics and substrate oxidation during exercise in the follicular and luteal phases. J Appl Physiol (1985). 2001;90(2):447–453. 53. Devries MC, Hamadeh MJ, Phillips SM, Tarnopolsky MA. Menstrual cycle phase and sex influence muscle glycogen utilization and glucose turnover during moderate-intensity endurance exercise. Am J Physiol Regul Integr Comp Physiol. 2006;291(4):R1120–R1128. 54. Tarnopolsky MA. Sex differences in exercise metabolism and the role of 17-beta estradiol. Med Sci Sports Exerc. 2008;40(4):648–654. 55. Frayn KN. Calculation of substrate oxidation rates in vivo from gaseous exchange. J Appl Physiol Respir Environ Exerc Physiol. 1983;55(2):628–634. 56. Achten J, Gleeson M, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Med Sci Sports Exerc. 2002;34(1):92–97. 57. Venables MC, Achten J, Jeukendrup AE. Determinants of fat oxidation during exercise in healthy men and women: a crosssectional study. J Appl Physiol. 2005;98(1):160–167. 58. Cheneviere X, Borrani F, Sangsue D, Gojanovic B, Malatesta D. Gender differences in whole-body fat oxidation kinetics during exercise. Appl Physiol Nutr Metab. 2011;36(1):88–95. 59. Lemon PW, Yarasheski KE, Dolny DG. The importance of protein for athletes. Sports Med. 1984;1(6):474–484. 60. Tarnopolsky M. Protein requirements for endurance athletes. Nutrition. 2004;20(7-8):662–668. 61. McKenzie S, Phillips SM, Carter SL, Lowther S, Gibala MJ, Tarnopolsky MA. Endurance exercise training attenuates leucine oxidation and BCOAD activation during exercise in humans. Am J Physiol Endocrinol Metab. 2000;278(4):E580–E587. 62. Lamont LS, McCullough AJ, Kalhan SC. Gender differences in leucine, but not lysine, kinetics. J Appl Physiol (1985). 2001;91(1):357–362. 63. Karpe F, Dickmann JR, Frayn KN. Fatty acids, obesity, and insulin resistance: time for a reevaluation. Diabetes. 2011;60(10):2441–2449. 64. Magkos F, Patterson BW, Mohammed BS, Klein S, Mittendorfer B. Women produce fewer but triglyceride-richer very lowdensity lipoproteins than men. J Clin Endocrinol Metab. 2007;92(4):1311–1318.

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

540

36.  Exercise Physiology in Men and Women

65. Nielsen S, Guo Z, Albu JB, Klein S, O'Brien PC, Jensen MD. Energy expenditure, sex, and endogenous fuel availability in humans. J Clin Invest. 2003;111(7):981–988. 66. Mittendorfer B, Magkos F, Fabbrini E, Mohammed BS, Klein S. Relationship between body fat mass and free fatty acid kinetics in men and women. Obesity (Silver Spring). 2009;17(10):1872–1877. 67. Horton TJ, Pagliassotti MJ, Hobbs K, Hill JO. Fuel metabolism in men and women during and after long-duration exercise. J Appl Physiol (1985). 1998;85(5):1823–1832. 68. Davis SN, Galassetti P, Wasserman DH, Tate D. Effects of gender on neuroendocrine and metabolic counterregulatory responses to exercise in normal man. J Clin Endocrinol Metab. 2000;85(1):224–230. 69. Horton TJ, Dow S, Armstrong M, Donahoo WT. Greater systemic lipolysis in women compared with men during moderatedose infusion of epinephrine and/or norepinephrine. J Appl Physiol. 2009;107(1):200–210. 70. Crampes F, Riviere D, Beauville M, Marceron M, Garrigues M. Lipolytic response of adipocytes to epinephrine in sedentary and exercise-trained subjects: sex-related differences. Eur J Appl Physiol Occup Physiol. 1989;59(4):249–255. 71. Schmidt SL, Bessesen DH, Stotz S, Peelor III FF, Miller BF, Horton TJ. Adrenergic control of lipolysis in women compared with men. J Appl Physiol (1985). 2014;117(9):1008–1019. 72. Carter SL, Rennie C, Tarnopolsky MA. Substrate utilization during endurance exercise in men and women after endurance training. Am J Physiol Endocrinol Metab. 2001;280(6):E898–E907. 73. Mittendorfer B, Horowitz JF, Klein S. Effect of gender on lipid kinetics during endurance exercise of moderate intensity in untrained subjects. Am J Physiol Endocrinol Metab. 2002;283(1):E58–E65. 74. Roepstorff C, Steffensen CH, Madsen M, et  al. Gender differences in substrate utilization during submaximal exercise in endurance-trained subjects. Am J Physiol Endocrinol Metab. 2002;282(2):E435–E447. 75. Romijn JA, Coyle EF, Sidossis LS, Rosenblatt J, Wolfe RR. Substrate metabolism during different exercise intensities in endurancetrained women. J Appl Physiol (1985). 2000;88(5):1707–1714. 76. Devries MC, Lowther SA, Glover AW, Hamadeh MJ, Tarnopolsky MA. IMCL area density, but not IMCL utilization, is higher in women during moderate-intensity endurance exercise, compared with men. Am J Physiol Regul Integr Comp Physiol. 2007;293(6):R2336–R2342. 77. Tarnopolsky MA, Rennie CD, Robertshaw HA, FedakTarnopolsky SN, Devries MC, Hamadeh MJ. Influence of endurance exercise training and sex on intramyocellular lipid and mitochondrial ultrastructure, substrate use, and mitochondrial enzyme activity. Am J Physiol Regul Integr Comp Physiol. 2007;292(3):R1271–R1278. 78. Velan SS, Said N, Narasimhan K, et  al. Gender Differences in Musculoskeletal Lipid Metabolism as Assessed by Localized Two-Dimensional Correlation Spectroscopy. Magn Reson Insights. 2008;2008(2):1–6. 79. Essen B, Jansson E, Henriksson J, Taylor AW, Saltin B. Metabolic characteristics of fibre types in human skeletal muscle. Acta Physiol Scand. 1975;95(2):153–165. 80. Goodpaster BH, He J, Watkins S, Kelley DE. Skeletal muscle lipid content and insulin resistance: evidence for a paradox in endurance-trained athletes. J Clin Endocrinol Metab. 2001;86(12):5755–5761. 81. Lundsgaard AM, Kiens B. Gender differences in skeletal muscle substrate metabolism - molecular mechanisms and insulin sensitivity. Front Endocrinol (Lausanne). 2014;5:195. 82. Glatz JF, Luiken JJ, Bonen A. Membrane fatty acid transporters as regulators of lipid metabolism: implications for metabolic disease. Physiol Rev. 2010;90(1):367–417.

83. Jeppesen J, Albers PH, Rose AJ, et  al. Contraction-induced skeletal muscle FAT/CD36 trafficking and FA uptake is AMPK independent. J Lipid Res. 2011;52(4):699–711. 84. Ibrahimi A, Bonen A, Blinn WD, et  al. Muscle-specific overexpression of FAT/CD36 enhances fatty acid oxidation by contracting muscle, reduces plasma triglycerides and fatty acids, and increases plasma glucose and insulin. J Biol Chem. 1999;274(38):26761–26766. 85. McFarlan JT, Yoshida Y, Jain SS, et al. In vivo, fatty acid translocase (CD36) critically regulates skeletal muscle fuel selection, exercise performance, and training-induced adaptation of fatty acid oxidation. J Biol Chem. 2012;287(28):23502–23516. 86. Kiens B, Roepstorff C, Glatz JF, et  al. Lipid-binding proteins and lipoprotein lipase activity in human skeletal muscle: influence of physical activity and gender. J Appl Physiol. 2004;97(4):1209–1218. 87. Binnert C, Koistinen HA, Martin G, et  al. Fatty acid transport protein-1 mRNA expression in skeletal muscle and in adipose tissue in humans. Am J Physiol Endocrinol Metab. 2000;279(5):E1072–E1079. 88. Fu MH, Maher AC, Hamadeh MJ, Ye C. Tarnopolsky MA. Exercise, sex, menstrual cycle phase, and 17beta-estradiol influence metabolism-related genes in human skeletal muscle. Physiol Genomics. 2009;40(1):34–47. 89. Zechner R, Kienesberger PC, Haemmerle G, Zimmermann R, Lass A. Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores. J Lipid Res. 2009;50(1):3–21. 90. Moro C, Galgani JE, Luu L, et al. Influence of gender, obesity, and muscle lipase activity on intramyocellular lipids in sedentary individuals. J Clin Endocrinol Metab. 2009;94(9):3440–3447. 91. Peters SJ, Samjoo IA, Devries MC, Stevic I, Robertshaw HA, Tarnopolsky MA. Perilipin family (PLIN) proteins in human skeletal muscle: the effect of sex, obesity, and endurance training. Appl Physiol Nutr Metab. 2012;37(4):724–735. 92. Covington JD, Galgani JE, Moro C, et al. Skeletal muscle perilipin 3 and coatomer proteins are increased following exercise and are associated with fat oxidation. PLoS One. 2014;9(3):e91675. 93. Granneman JG, Moore HP, Mottillo EP, Zhu Z, Zhou L. Interactions of perilipin-5 (Plin5) with adipose triglyceride lipase. J Biol Chem. 2011;286(7):5126–5135. 94. Wang H, Sreenevasan U, Hu H, et al. Perilipin 5, a lipid dropletassociated protein, provides physical and metabolic linkage to mitochondria. J Lipid Res. 2011;52(12):2159–2168. 95. Perreault L, Lavely JM, Kittelson JM, Horton TJ. Gender differences in lipoprotein lipase activity after acute exercise. Obes Res. 2004;12(2):241–249. 96. Kiens B, Lithell H, Mikines KJ, Richter EA. Effects of insulin and exercise on muscle lipoprotein lipase activity in man and its relation to insulin action. J Clin Invest. 1989;84(4):1124–1129. 97. Rune A, Salehzadeh F, Szekeres F, Kuhn I, Osler ME, Al-Khalili L. Evidence against a sexual dimorphism in glucose and fatty acid metabolism in skeletal muscle cultures from age-matched men and post-menopausal women. Acta Physiol (Oxf). 2009;197(3):207–215. 98. Hoeg LD, Sjoberg KA, Jeppesen J, et al. Lipid-induced insulin resistance affects women less than men and is not accompanied by inflammation or impaired proximal insulin signaling. Diabetes. 2011;60(1):64–73. 99. Tarnopolsky MA, Zawada C, Richmond LB, et al. Gender differences in carbohydrate loading are related to energy intake. J Appl Physiol. 2001;91(1):225–230. 100. Marliss EB, Kreisman SH, Manzon A, Halter JB, Vranic M, Nessim SJ. Gender differences in glucoregulatory responses to intense exercise. J Appl Physiol (1985). 2000;88(2):457–466. 101. Tarnopolsky LJ, MacDougall JD, Atkinson SA, Tarnopolsky MA, Sutton JR. Gender differences in substrate for endurance exercise. J Appl Physiol (1985). 1990;68(1):302–308.

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

REFERENCES

102. Esbjornsson-Liljedahl M, Sundberg CJ, Norman B, Jansson E. Metabolic response in type I and type II muscle fibers during a 30-s cycle sprint in men and women. J Appl Physiol (1985). 1999;87(4):1326–1332. 103. Green HJ, Fraser IG, Ranney DA. Male and female differences in enzyme activities of energy metabolism in vastus lateralis muscle. J Neurol Sci. 1984;65(3):323–331. 104. Simoneau JA, Lortie G, Boulay MR, Thibault MC, Theriault G, Bouchard C. Skeletal muscle histochemical and biochemical characteristics in sedentary male and female subjects. Can J Physiol Pharmacol. 1985;63(1):30–35. 105. Jaworowski A, Porter MM, Holmback AM, Downham D, Lexell J. Enzyme activities in the tibialis anterior muscle of young moderately active men and women: relationship with body composition, muscle cross-sectional area and fibre type composition. Acta Physiol Scand. 2002;176(3):215–225. 106. Borges O, Essen-Gustavsson B. Enzyme activities in type I and II muscle fibres of human skeletal muscle in relation to age and torque development. Acta Physiol Scand. 1989;136(1):29–36. 107. Jeppesen J, Kiens B. Regulation and limitations to fatty acid oxidation during exercise. J Physiol. 2012;590(Pt 5):1059–1068. 108. Lennon DL, Stratman FW, Shrago E, et al. Effects of acute moderate-intensity exercise on carnitine metabolism in men and women. J Appl Physiol. 1983;55(2):489–495. 109. Salehzadeh F, Rune A, Osler M, Al-Khalili L. Testosterone or 17{beta}-estradiol exposure reveals sex-specific effects on glucose and lipid metabolism in human myotubes. J Endocrinol. 2011;210(2):219–229. 110. Berthon PM, Howlett RA, Heigenhauser GJ, Spriet LL. Human skeletal muscle carnitine palmitoyltransferase I activity determined in isolated intact mitochondria. J Appl Physiol. 1998;85(1):148–153. 111. Costill DL, Fink WJ, Getchell LH, Ivy JL, Witzmann FA. Lipid metabolism in skeletal muscle of endurance-trained males and females. J Appl Physiol. 1979;47(4):787–791. 112. Maher AC, Akhtar M, Vockley J, Tarnopolsky MA. Women have higher protein content of beta-oxidation enzymes in skeletal muscle than men. PLoS One. 2010;5(8):e12025. 113. Roepstorff C, Schjerling P, Vistisen B, et  al. Regulation of oxidative enzyme activity and eukaryotic elongation factor 2 in human skeletal muscle: influence of gender and exercise. Acta Physiol Scand. 2005;184(3):215–224. 114. Larsen S, Nielsen J, Hansen CN, et  al. Biomarkers of mitochondrial content in skeletal muscle of healthy young human subjects. J Physiol. 2012;590(14):3349–3360. 115. Karakelides H, Irving BA, Short KR, O'Brien P, Nair KS. Age, obesity, and sex effects on insulin sensitivity and skeletal muscle mitochondrial function. Diabetes. 2010;59(1):89–97. 116. Thompson JR, Swanson SA, Casale GP, et  al. Gastrocnemius mitochondrial respiration: are there any differences between men and women? J Surg Res. 2013;185(1):206–211. 117. Ronkainen PH, Pollanen E, Alen M, et al. Global gene expression profiles in skeletal muscle of monozygotic female twins discordant for hormone replacement therapy. Aging Cell. 2010;9(6):1098–1110. 118. Vijay V, Han T, Moland CL, Kwekel JC, Fuscoe JC, Desai VG. Sexual dimorphism in the expression of mitochondria-related genes in rat heart at different ages. PLoS One. 2015;10(1): e0117047. 119. Aucouturier J, Baker JS, Duche P. Fat and carbohydrate metabolism during submaximal exercise in children. Sports Med. 2008;38(3):213–238. 120. Abildgaard J, Pedersen AT, Green CJ, et al. Menopause is associated with decreased whole body fat oxidation during exercise. Am J Physiol Endocrinol Metab. 2013;304(11):E1227–E1236.

541

121. Lovejoy JC, Champagne CM, de JL, Xie H, Smith SR. Increased visceral fat and decreased energy expenditure during the menopausal transition. Int J Obes (Lond). 2008;32(6):949–958. 122. Lemoine S, Granier P, Tiffoche C, Rannou-Bekono F, Thieulant ML, Delamarche P. Estrogen receptor alpha mRNA in human skeletal muscles. Med Sci Sports Exerc. 2003;35(3):439–443. 123. Wiik A, Glenmark B, Ekman M, et al. Oestrogen receptor beta is expressed in adult human skeletal muscle both at the mRNA and protein level. Acta Physiol Scand. 2003;179(4):381–387. 124. Wiik A, Ekman M, Johansson O, Jansson E, Esbjornsson M. Expression of both oestrogen receptor alpha and beta in human skeletal muscle tissue. Histochem Cell Biol. 2009;131(2):181–189. 125. Dieli-Conwright CM, Spektor TM, Rice JC. Todd SE. Oestradiol and SERM treatments influence oestrogen receptor coregulator gene expression in human skeletal muscle cells. Acta Physiol (Oxf). 2009;197(3):187–196. 126. Wiik A, Gustafsson T, Esbjornsson M, et al. Expression of oestrogen receptor alpha and beta is higher in skeletal muscle of highly endurance-trained than of moderately active men. Acta Physiol Scand. 2005;184(2):105–112. 127. Milanesi L, Russo de BA, Boland R. Expression and localization of estrogen receptor alpha in the C2C12 murine skeletal muscle cell line. J Cell Biochem. 2008;104(4):1254–1273. 128. Bennink HJ. Reprint of Are all estrogens the same? Maturitas. 2008;61(1-2):195–201. 129. Burrows M, Peters CE. The influence of oral contraceptives on athletic performance in female athletes. Sports Med. 2007;37(7):557–574. 130. Henderson GC, Alderman BL. Determinants of resting lipid oxidation in response to a prior bout of endurance exercise. J Appl Physiol (1985). 2014;116(1):95–103. 131. Henderson GC, Fattor JA, Horning MA, et  al. Lipolysis and fatty acid metabolism in men and women during the postexercise recovery period. J Physiol. 2007;584(Pt 3):963–981. 132. King JA, Miyashita M, Wasse LK, Stensel DJ. Influence of prolonged treadmill running on appetite, energy intake and circulating concentrations of acylated ghrelin. Appetite. 2010;54(3):492–498. 133. Schubert MM, Sabapathy S, Leveritt M, Desbrow B. Acute exercise and hormones related to appetite regulation: a metaanalysis. Sports Med. 2014;44(3):387–403. 134. Alajmi N, Deighton K, King JA, et  al. Appetite and energy intake responses to acute energy deficits in females versus males. Med Sci Sports Exerc. 2016;48(3):412–420. 135. Loucks AB, Thuma JR. Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. J Clin Endocrinol Metab. 2003;88(1):297–311. 136. Loucks AB, Kiens B, Wright HH. Energy availability in athletes. J Sports Sci. 2011;29(Suppl 1)) S7-15. 137. Mauvais-Jarvis F, Clegg DJ, Hevener AL. The role of estrogens in control of energy balance and glucose homeostasis. Endocr Rev. 2013;34(3):309–338. 138. Pietrobelli A, Allison DB, Heshka S, et  al. Sexual dimorphism in the energy content of weight change. Int J Obes Relat Metab Disord. 2002;26(10):1339–1348. 139. Williams RL, Wood LG, Collins CE, Callister R. Effectiveness of weight loss interventions--is there a difference between men and women: a systematic review. Obes Rev. 2015;16(2): 171–186. 140. Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29 (Suppl 1)):S17–S27. 141. Tarnopolsky MA, Atkinson SA, Phillips SM, MacDougall JD. Carbohydrate loading and metabolism during exercise in men and women. J Appl Physiol. 1995;78(4):1360–1368.

PRINCIPLES OF GENDER-SPECIFIC MEDICINE

542

36.  Exercise Physiology in Men and Women

142. Elango R, Humayun MA, Ball RO, Pencharz PB. Evidence that protein requirements have been significantly underestimated. Curr Opin Clin Nutr Metab Care. 2010;13(1):52–57. 143. Houltham SD, Rowlands DS. A snapshot of nitrogen balance in endurance-trained women. Appl Physiol Nutr Metab. 2014;39(2):219–225.

144. Horvath PJ, Eagen CK, Ryer-Calvin SD, Pendergast DR. The effects of varying dietary fat on the nutrient intake in male and female runners. J Am Coll Nutr. 2000;19(1):42–51. 145. Costello JT, Bieuzen F, Bleakley CM. Where are all the female participants in Sports and Exercise Medicine research? Eur J Sport Sci. 2014;14(8):847–851.

PRINCIPLES OF GENDER-SPECIFIC MEDICINE