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Position of The American Dietetic Association and The Canadian Dietetic Association: Nutrition for physical fitness and athletic performance for adults
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ADA REPORTS
Position of The American Dietetic Association and The Canadian Dietetic Association: Nutrition for physical fitness and athletic performance for adults ealthy People 2000 outlines specific health-related goals for the United States in the areas of nutrition and physical fitness (1).1,2 As North Americans begin to make changes in these areas and professionals work to promote positive behaviors as part of a new life-style, they need accurate and appropriate nutrition information. In addition, there is a need for continued research on the role of nutrition in achieving optimal athletic performance; these findings should be disseminated to physically active persons and athletes . The registered dietitian is the best professional to provide nutrition information and the dietitian with advanced training in sports nutrition is the professional most qualified to provide advice to athletes.
BODY COMPOSITION Optimal body weights for health and competition should be determined individually, and are influenced by age, gender, physical activity or sport, somatotype, genetics, and individual variation. It is well established that health risks increase for those who are more than 20% over optimal weight (5,6). Health risks increase as proportion of bodyfat increases. Thus, the importance of maintaining a healthy body composition extends beyond the aesthetic and athletic goals to which an individual may aspire and lends credibility to the idea that body composition is a preferred measure of health and fitness over body weight alone. A variety of indexes and tools may be used to predict optimal body weight ranges for health and athletic performance. Body mass index (BMI), although of limited usefulness with competiPOSITION STATEMENT tive athletes, is an indicator of body composition and may also be It is thepositionof TheAmericanDieteticAssociationand The related to health outcomes. Desirable BMI ranges are 21.9 to 22.4 CanadianDietetic Association to support access to accurate for men and 21.3 to 22.1 for women. Values greater than 27.8 for and appropriateinformation thatexplains the interrelation- men and 27.3 for women are related to excess weight and ships between exercise and nutrition, reinforces the impor- increased risk of cardiovascular disease, high blood pressure, and tant role of nutrition, and encourages appropriatefood diabetes (6,7). 3 choices to achieve optimal physicalfitness and athletic perWeight-for-height charts are useful for tracking growthvelocity formance. foryouthand maybe used to predict desirable body weight ranges for the average population (8,9). However, these charts do not North Americans have become more physically active and partici- provide information on body composition and the distribution of pate in exercise programs for health benefits and recreation. lean body mass and adipose tissue. The overweight person is not People of all ages are physically active and their level of participa- necessarily overfat, and the underweight person is not necessarily tionvaries from recreationalwalking to competitive sports. Proper lean. This limitation becomes evident when these tables are used nutrition plays an important role in maximizing everyone's ability with certain athletes, especially those involved in power sports, to maintain higher levels of physical activity, regardless of age and who appear to be overweight according to these charts but type of activity. Hard physical training has the potential to affect actually have a high lean-to-fat ratio. health negatively by increasing the risk of dehydration and inMost athletes require a high strength-to-weight ratio to achieve creasing nutrient needs. Therefore, it is important to clarify and optimal athletic performance. Because adipose tissue adds to quantify the specific stressors that varying levels of physical body weight without contributing to strength, it is important to activity introduce on people of different ages, genders, and health be able to discriminate between these components of weight. status. These stressors, as they apply to different populations, Percent body fat values for athletes typically range from 5% to should be a major thrust in the development of nutrition educa12% in men and 10% to 20% in women and vary between sports tion materials. and between positions in a specific sport. For the general populaPhysical activity is beneficial to everyone's health, regardless of tion, optimal body fat percentage ranges tend to increase with age. their initial level of fitness. The most significant benefits are Generally, body fat levels of 15% to 18% for men and 20% to 25% experienced by those at the lowest fitness level who become more for women are considered acceptable. Body fat levels greater than active, even in very-low-level activities (2). For those who wish to 26% for men and 29% for women are considered a potential risk begin a new exercise program, it is important to consider the factor for the development of chronic diseases. Body fat levels guidelines for beginning and maintaining exercise programs pro- greater than 25% for men and 30% for women are considered vided by the American College of Sports Medicine (3,4). clinical indicators of obesity and are considered a risk factor in the Although the benefits of diet and exercise are well established, development of chronic diseases (10,11). At the other end of the individuals with certain clinical conditions, including diabetes, spectrum, body fat levels below 4% and 10% for men and women, cardiovascular disease, and metabolic disorders, should consult a respectively, are possible indicators of eating disorders. physician before initiating any exercise program. Although a Body composition assessment provides data that can be used to moderate exercise program (40% to 60% Vo2max) can be started calculate a desirable body weight range, based on lean body by most individuals of any age, men over age 40 and women over weight, that is most suited for the individual's health or perforage 50 who plan to begin a vigorous exercise program (> 60% mance goals. Techniques commonly used to measure body comVo2max) should first consult a physician (5). position include hydrostatic weighing, anthropometric measures (including skinfolds), bioelectrical impedance analysis (BIA), and dual energy x-ray absorptiometry (DEXA) (12). 'Editor's note: Because this is a joint Position of The American Dietetic Association and The Canadian Dietetic Association, where appropriate, 3 footnotes give specific references for practitioners in Canada. Canadian values for BMI: healthy range for men and women is20 to 25; 2 Health andWelfare Canada.ActionTowardsHealthyEating.Canada's measures >27 are related to health risk. From:Health and Welfare Guidelinesfor Healthy Eating and Recommended Strategiesfor Canada. Promoting Healthy Weights: A Discussion Paper. Ottawa: Implementation. Ottawa: Supply and Services Canada; 1990. Supply and Services Canada; 1988. JOURNAL OF THE AMERICAN DIETETIC ASSOCIATION / 691
ADA REPORTS Hydrostatic (or underwater) weighing is the gold standard for assessing body composition, but it requires substantial training, a large investment in equipment, and a high level of subject cooperation. Although this method is useful in research and clinical settings, its limitations make it impractical for general use. Skinfold measurements, taken with calipers at specific body sites, are among the most practical and cost-effective methods for estimating body composition. Numerous combinations of skinfold measurements taken at different sites and applied to regression equations may be used to predict body composition. A populationspecific equation that reflects such considerations as age, gender, race, somatotype, and type of physical activitymaybe selected, or ageneralized equation maybe more appropriate. Although skinfold measurements may be of limited use to predict body composition accurately in extremely lean or obese individuals, they are nonetheless useful with all populations to monitor changes over a period. The standardized technique for obtaining skinfold measurements is described in the literature and should be the basis for all practitioner training to reduce measurement error (13). BIA, which is based on the principle that electrical current applied to the body will meet greater resistance with more fat, is an accurate, noninvasive method for estimating body composition. Lean body tissue has a greater water and electrolyte level, lower electrical resistance, and greater conductivity than adipose tissue. In principle, the lower the electrical impedance, the greater an individual's lean body mass. Normal hydration status is essential; underhydration, which commonly occurs in athletes, will lead to inaccurate results. Because of normal fluctuations in body water, hydration status must be assessed before BIA is performed (12). DEXA is a precise method for quantifying the skeletal and soft tissue components of body mass. DEXA provides a direct measurement of body composition, whereas other common methods make predictions based on variables indirectly related to body composition (eg, water content of tissue). Scan times are low, ranging from 10 to 20 minutes, and patient radiation dosage is 800 to 2,000 times lowerthan a typical chest x-ray (0.02 to 0.05 mRem, compared with a typical chest x-ray at 40 mRem) (14). DEXA is not useful in the field because the equipment is expensive and is not portable. It is currently used principally for laboratory research and in clinical settings.
ENERGY Energy needs, which change throughout the life cycle, are high during periods of growth and progressively decrease after age 30. A high activity level in combination with rapid growth dramatically increases the energy requirement. The decreased energy needs associated with aging often accompany life-style changes. Decreased physical activity, loss of lean body mass, and increased food intake result in a higher percentage of body fat, a positive caloric balance, and a resulting weight gain. To prevent weight gain and achieve optimal health, it has been suggested that approximately 2,000 kcal/week should be expended in physical activity (15). Energy needs are based on basal metabolic rate plus the intensity, duration, frequency, and type of exercise performed. The energy equivalents of different activities are available (16). The goal of a physical activity program for healthy weight loss - for athletes or nonathletes--is to reduce body fat and maintain lean body tissue. Rapid weight loss promotes loss of lean body tissue and muscle, compromises nutrient intake, and increases the likelihood of regaining weight. Rapid weight loss may also compromise endurance and impair both cardiac function and body temperature regulation. For the general population, a program of regular aerobic exercise (20- to 30-minute sessions, at least 3 times per week), combined with a hypocaloric, low-fat diet (< 30% energy from fat) 692/ JUNE 1993 VOLUME 93 NUMBER 6
is most effective for achieving desirable changes in body composition and bodyweight. Using such a program, muscle mass can be better maintained while body fat is reduced. During the initial stages of a weight reduction program, weight loss may be minimal, even though arm, leg, and trunk circumference measurements decrease. This occurs because of an increase inmuscle mass at the same time as body fat is lost. After several weeks, an individual will continue to lose body fat, as muscle gain levels off, resulting in a loss of body weight. For sports with competitive seasons, it is important that athletes enter their season at an optimal level of conditioning. The competitive season is not a time for weight loss or weight gain. The off-seasontraining program should address their needs for adjustments in body composition by combining the appropriate dietary and exercise regimen to achieve the desired results. Many athletes train and compete throughout the year, so a coordinated plan of gradual change should be implemented in a way that will not negatively affect performance. Dietary changes should be introduced as an integral part of an overall training program, rather than as restrictions of a "diet," to avoid the negative connotations that this may bring. Individuals engaged in vigorous physical training programs have varied energy needs, ranging from 2,000 to 6,000 kcal/day or more (17,18). The needs of a 200-lb (91-kg) bodybuilder will differ from those of an 80-lb (36-kg) gymnast. A well-planned individualized dietary program is necessary to help athletes consume relatively large quantities of food, which is difficult with the combined constraints of training, sleep, school, and work. The lower caloric density of the preferred high-carbohydrate/low-fat diet makes it even more difficult to provide sufficient food to meet high energy needs. Multiple strategies, including more frequent eating, planned snacks, and consumption of high-carbohydrate meals should be considered where feasible. For many athletes, gaining weight or maintaining a high weight is important for optimal athletic performance. An individualized eating plan that provides frequent feedings or "mini-meals" that will complement their training schedules must be designed for these athletes. Liquid meal supplements may be helpful as they offer a means for providing a high-carbohydrate meal in an easily delivered, ready-to-consume package. Weight fluctuations may occur cyclically in some athletes; their competitive weight is usually lower than their weight during the off-season. The resulting weight cycle (commonly described as a "yo-yo" cycle) is similar to that experienced by individuals who participate in rapid weight loss programs (19). Ideally, weight loss should be gradual (0.5 to 1 lb or 0.23 to 0.45 kg per week) and should be achieved wellbefore the start of the competitive season to assure maximal strength. Attention should be given to compulsive eating behavior and wide swings in body weight because these can be signs of eating behavior disorders (19). Some weight loss techniques cause diuresis and a resultant decrease in plasma volume and reduced blood flow to the kidneys, skin, and muscles. This combination of conditions lowers the body's ability to sweat and regulate body temperature, increases susceptibility to heat illness, and reduces exercise endurance (20). Evaluation of eating practices should be a standard component of the nutrition assessment of athletes to determine the potential risk to health and athletic performance.
ENERGY-NUTRIENT INTAKE DISTRIBUTION Ideal distribution of carbohydrate, protein, and fat for athletes is similarto recommendations in theDietaryGuidelinesforAmericans (21) and the Canadian Nutrition Recommendations.4 4
HealthandWelfare Canada.NutritionRecommendations.TheReport of the Scientific Review Committee. Ottawa: Supply and Services Canada;1990.
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Quantitive differences are realized, however, because of the increased energy requirements of athletes. Carbohydrate In general, it is recommended that 60% to 65% of total energy should come from carbohydrate. Athletes who train exhaustively on successive days or who compete in prolonged endurance events should consume a diet that provides 65% to 70% of energy from carbohydrates A threshold of 500 to 800 g (2,000 to 3,200 kcal) carbohydrate per day, regardless of the total daily energy intake, may be necessary to maintain maximal muscle glycogen stores in athletes (22-24). Maximizing muscle glycogen stores provides greater energy reserve for aerobic and anaerobic activities, resulting in greater endurance and delayed fatigue (25). To maximize muscle glycogen storage, preparation for a competition of long duration (> 90 minutes of continuous activity) should include at least 3 days of intake that includes 800 g carbohydrate per day (or 6 tolO g carbohydrate per kilogrambody weight per day) and a week of tapered workouts that ends in complete rest the day before competition. This dietary protocol, or "modified carbohydrate loading" plan, maintains high glycogen stores and may be followed numerous times throughout the year without health risk (26). This modified plan has been accepted since the mid-1980s when it was recognized that the original carbohydrate-loading scheme, which involved a carbohydrate depletion phase, resulted in a diminished abilityto train before the competitive event and introduced potential health risks. Protein Athletes may require more protein than the Recommended Dietary Allowance (RDA) (27) of 0.8 g/kg body weight (28). The increased requirement for protein appears to be small, however, and not at a level of 2 to 2.5 times greater than the RDA, or Recommended Nutrient Intake (RNI)6 in Canada, as suggested in the lay literature. Recent data confirm the protein-sparing effect of carbohydrate. In addition, research suggests that the quantity of dietary protein needed to achieve maximal protein deposition is 1.5 g/kg body weight, and that the limiting factor for muscle protein deposition is energy intake, not protein (29,30). Therefore, athletes who wish to increase muscle mass should meet their energy requirements first, through an adequate intake of carbohydrate, and then check that they have met their protein needs. Protein needs can be calculated both as a percentage of total energy and on a per kilogram body weight basis. For athletes with exceptionally high energy intakes, providing 12% to 15% of total energy from protein may be excessive. When energy intake is low, as typically observed for many young women or low body weight athletes, protein needs calculated as a percentage of energy may be inadequate. In both of these cases, 1.0 to 1.5 g protein per kilogram body weight may be a more appropriate guide for intake than protein as a percent of total energy. The use of amino acid and protein supplements by athletes is common, despite the lack of well-controlled clinical studies to justify their use. Even the highest protein requirements can be met easily with a balanced diet that includes a variety of foods. Therefore, excessive protein intake, either through consumption of high-protein foods or protein/amino acid supplements, is unnecessary, does not contribute to athletic performance or increase muscle mass, and actually may be detrimental to health and athletic performance. Increased intake of amino acids does 5
Nutrition Recommendations for Canadians: General population: 55% total energy derived from carbohydrate; athletes: 60% to 66%. 'The Canadian Recommended Nutrient Intake for protein is 0.86 g/kg. Health and Welfare Canada. Nutrition Recommendations. The Report of the Scientijic Roview Committee. Ottawa: Supply and Services Canada: 1990.
not result in an increased utilization or increased muscle protein synthesis in the body. Providing dietary protein at a level that exceeds the requirement results in amino acids being stored as energy (fat) or oxidized. Neither situation is desirable; amino acid oxidation increases the risk of dehydration because of the need to remove protein metabolic end products via the urine, and the storage of amino acid carbon as fat is metabolically inefficient. Fat Fat should contribute no more than 30% of total energy to the diet. Total fat intake is likely to be higher in athletes who must consume very large quantities of energy (> 4,000 kcal) in order to maintain body mass. This is typical of linemen on football teams and in other sports where a large muscle mass is important for athletic success and for those who participate in extensive training programs of long duration on a daily basis. Carbohydrate and protein needs should first be calculated for the individual. Fat can then be used to provide additional energy, but only to a maximum level of 30% of total energy. Energy needs above this level should be derived from high-carbohydrate/low-fat food sources. VITAMINS AND MINERALS Vitamins and minerals play an important role in the metabolism of protein, carbohydrate, and lipids and in muscle function. Although physical activity increases the need for some vitamins and minerals, this increased requirement typically can be met by consuming abalanced high-carbohydrate, moderate-protein, lowfat diet. Individuals at risk for low vitarnin/mineral intake are those who consume a low-calorie diet. Athletes commonly consume substances, including vitamin and mineral supplements, that they believe have ergogenic properties. Although vitamin and mineral supplementation may improve the nutritional status of individuals consuming marginal amounts of nutrients from food and may improve performance in those with deficiencies, there is no scientific evidence to support the general use of supplements to improve athletic performance (31,32). The increased energy intake of athletes should provide the additional vitamins and minerals necessary if a wide variety of foods is included in the diet. Iron and calcium are two minerals of concern, especially for young athletes and women of all ages involved in physical activity. Iron, as a component of hemoglobin and myoglobin, is essential for oxidative metabolism to occur. Iron depletion (low iron stores) is seen in both male and female athletes but does not appear to affect athletic performance directly. If the condition progresses to iron deficiency (low hemoglobin level), then athletic performance is negatively affected (33). The incidence of iron-deficiency anemia is rare and is similar among physically trained individuals and the general population (34). When it occurs, the cause of anemia in athletes is probably multifactorial. Low caloric intakes often do not supply sufficient iron to maintain adequate iron stores. Diets that provide little or no meat limit the most available dietary sources of iron. Exerciseassociated factors include hemolysis of erythrocyteNs, alterations in iron metabolism, hematuria, increased erythroc e osmotic fragility resulting in decreased red blood cell survival time, and a possible shift in the oxygen dissociation curve (35). Athletes should be screened periodically to assess iron status. Typically, changes in storage and transport iron will be noted before decreases in functional iron (hemoglobin and hematocrit levels). Thus, early screening for serum ferritin levels is recommended. If iron deficiency is found, nutrition guidance should be provided. A transient, nonclinical "sports anemia" (hemodiltltional iron deficiency) is sometimes experienced by athletes as a result; of an increase in plasma volume that is associated with the initiation of training. However, this form of anemia has no apparent effect on performance (36). J()OU[RNAL OF THE AMERICAN DIETETIC ASS()('IATION /693
ADA REPORTS
Osteoporosis is a major health concern for women in North America and is related to calcium intake, estrogen level, alcohol and caffeine intake, family history, and the amount and type of physical activity. The emphasis for prevention of osteoporosis should be to maximize the body's stores of calcium early in life, maintain that level, and minimize any loss (37). A calcium intake of 800 to 1,200 mg/day is recommended (1,27) to protect against the development of osteoporosis. 7 Achieving this recommended daily intake, in conjunction with performing regular weight-bearing activities, will promote the deposition of calcium in bone and thereby reduce the risk of developing osteoporosis (38). Nutrition guidance should be given to low-body-weight, amenorrheic women who appear to be at high risk of early osteoporosis. The amenorrhea commonly associated with low levels of body fat, high physical activity (especially gymnastics, swimming, and long-distance running), and eating behavior disorders, may hinder bone development at a time when bone should be forming at its maximum rate (39). When amenorrhea is present, it appears that consumption of 120% of the calcium RDA or RNI helps bones to develop properly and maintain density (40). This amenorrhea should not be accepted and ignored by the athlete; side effects, such as an increased incidence of stress fractures, could have a serious effect on athletic performance. HYDRATION Increased muscular activity leads to an increase in heat production in the body; this is dissipated, in part, through the production of sweat. To prevent dehydration, water must be replaced at a faster rate. Dehydration has an adverse effect on muscle strength, endurance, and coordination and increases the risk of cramps, heat exhaustion, and life-threatening heat stroke (41-43). Some individuals may lose up to 6 to 8 lb (2.7 to 3.6 kg) of sweat per hour during strenuous activity. To ensure that lost fluid is replaced, body weight should be measured before and after training sessions; each pound (0.45 kg) lost should be replaced with 16 oz (1 pint or 474 mL) fluid. If weight is not within 1 to 2 lb (0.45 to 0.9 kg) of the previous day's preexercise weight, additional fluids should be consumed before exercising. Ideally, athletes should learn to consume adequate fluids during activity so that body weight remains relatively constant before and after exercise. Guidelines for fluid consumption include consuming at least 2 cups (16 to 20 oz or 474 to 592 mL) fluid about 2 hours before exertion, followed by another 2 cups (474 mL) fluid approximately 15 to 20 minutes before endurance exercise. In hot and humid environments, frequent small servings (4 to 6 oz or 118 to 178 mL every 15 minutes) of plain cool (40 to 50°F or 5 to 10°C) water or other rehydration beverage are recommended throughout exertion (44). Vigorous exercise may delay the thirst mechanism, making it difficult to replace fluid loss without a plan for periodic consumption. Athletes should become accustomed to consuming fluid at regular intervals (with or without thirst) during training sessions so that they do not experience discomfort during competition. For most persons exercising for 1 hour or less in moderate temperature conditions, an appropriate fluid replacement is cool water (43). The typical North American diet provides ample sodium, potassium, chloride, and magnesium to replace sweat losses in most cases. Longer exercise ( >1 hour) and/or exercise in extreme enviromental conditions, such as a high temperature/ humidity index, may warrant consumption of a sports beverage as part of the hydration-replacement process. Commercial sport 7
Canadian Recommended Nutrient Intake for calcium is 800 mg/day.
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beverages are available that contain electrolytes to enhance absorption and supply energy through various carbohydrate sources. The athlete should choose the specific sports beverage that best meets his or her needs. Beverages of choice should taste good to promote consumption, not cause gastric distress, provide a carbohydrate source for enhanced performance during endurance activities, and optimize rate of fluid absorption. Research shows that 6% to 8% glucose, glucose polymer, or sucrose with a small amount of sodium in a hypotonic or isotonic solution are all absorbed quickly and help maintain blood glucose levels during exercise (45). Beverages that use fructose as the principle carbohydrate source may cause gastric distress in some athletes and require more time before they can be used by muscles as a fuel because of the necessary conversion of fructose to glucose (46). Guidelines for amount of intake should adhere to those listed for hydration with water. ERGOGENIC AIDS Compounds such as bee pollen, caffeine, glycine, carnitine, lecithin, brewer's yeast, and gelatin are claimed, through anecdotal evidence, to improve strength or endurance. Although popular with some athletes because of this perceived ergogenic benefit, scientific research has failed to substantiate the claims for these products or inadequate research has been undertaken with these supplements. In some cases, as with caffeine (a restricted substance per the International Olympic Committee), the research results are equivocal (47). However, because of the placebo effect, some athletes are convinced that certain foods, dietary regimens, or supplements improve performance. These substances may provide certain psychological benefits rather than proven physiological benefits. However, when use of these substances replaces a sound nutrition program, health and performance may be compromised, resulting in serious consequences. GENERAL RECOMMENDATIONS Nutrition concerns and recommendations for ensuring that nutrition does not become a limiting factor in athletic performance were presented in the previous sections. The underlying principle that unites all of these recommendations is that athletes must consume a well-balanced and varied diet on a daily basis to support their training efforts. Preexercise/Competition What an athlete eats immediately before exercise or competition makes a difference, both physically and psychologically. Dietary guidelines for before exercise mainly involve the provision of an adequate carbohydrate and fluid intake to support exercise. Two to 6 hours before an endurance event, depending on individual preference, a meal providing 85 to 200 g carbohydrate should be consumed (48). Food choices should provide mainly complex carbohydrates and should be low in fat and protein. Athletes should choose foods that are familiar and comforting, have been consumed before training sessions, and are known not to cause gastric distress. Athletes should not experiment with new foods or beverages on competition day. A liquid meal supplement that provides a balance of fluid and nutrients may be helpful for athletes with precompetition jitters and gastric distress. Five to 10 minutes before exercise or competition, fluids containing a small amount of carbohydrate may be consumed to maintain blood glucose level. There does not appear to be a significant insulin rebound effect from this practice, although some athletes do not tolerate this comfortably (43,48). During Exercise/Competition Small carbohydrate feedings during endurance events of more than 1 hour may delay fatigue. Feedings should be at regular
intervals to provide about 24 g carbohydrate every 30 minutes (49). Athletes should become comfortable with these feedings during their practice sessions. Athletes often find that consumption of carbohydrate-containing sport beverages is better tolerated than solid foods during activity. Athletes should consume fluid on a fixed time schedule during activity, regardless of thirst. After Exercise/Competition Carbohydrate consumption after exercise ensures repletion of muscle glycogen. Research shows that muscle will replete glycogen stores to a higher degree when up to 600 g easily digestible carbohydrate is consumed within the first several hours after exercise. The athlete should begin eating high- carbohydrate foods as soon as possible after physical exertion. Blood glucose, insulin, and glycogen synthetase levels will remain elevated to promote glycogen synthesis and replete the muscle reserves (50). SPECIAL CONSIDERATIONS Pregnancy Active women who become pregnant may continue to exercise, but only after gaining approval from a physician. Women who initiate an exercise program during pregnancy must do so only with low-level activities, and only with the supervision of a physician. Guidelines have been established to determine who may exercise safely. In addition, there are various recommendations for exercise intensities, hyperthermia precautions, types of exercise, safe body movements, and contraindicated exercises (5,51). Several studies have demonstrated that exercise during pregnancy exerts several benefits, including improved self-esteem, lower maternal weight gain, improved maternal aerobic fitness, and potentially improved labor outcomes (52-56). It also has been demonstrated that excessive physical activity and inadequate energy intakes can lead to suboptimal maternal weight gain and fetal growth retardation (52,53). Lactation Recent evidence suggests that lactating women who exercise have significantly lower body fat and higher energy expenditures than nonexercising women. In addition, exercising women produce a slightly greater volume of milk (57). Lactating women who exercise should consume sufficient fluids to avoid dehydration and sufficient energy to compensate for that expended during exercise. The increased intake should include the recommended additional 500 kcal required for lactating women. Childhood Diet arv considerations for children who exercise regularly differ only in the need for special attention to energy requirements. Adequate caloric intake is important to achieve optimal growth velocities and maintain health status. Children have a greater surface area and lower sweating capacitythan adults and, as a result, are more susceptible tohyperthermia than are adults. This problem is exacerbated by the fact that children produce more heat per unit body weight than adults and are less capable of transferring this heat from the muscles to the skin (58). The differences in thermoregulation between children and adults strongly suggest that children should not be exposed to the same exercise intensities as adults, regardless of environmental conditions. In addition, they appear to be more prone to overuse injuries. Any exercise training programthat is initiated by a child should progress slowly, allowing ample time for acclimatization, conditioning, and skill development to improve athletic performance, An ()pinion Statement by the American College of Sports Medicine provides guidance for the development of a training program for children (59).
Adolescence Adolescent athletes require accurate information on establishing safe weight and body composition goals. This is particularly important for athletes striving for extremely low body-fat levels, such as wrestlers, gyrrnasts, rowers, dancers, and swimners. Studies suggest that some adolescent athletes, particularly womel, consume diets that are low in energy and nutrients (60). The combination of high nutrient and energy requirements necessary to support growth and training and dietary self-restrictior justifies monitoring these weight-conscious athletes for signs of dietary deficiencies and related health problems. These athletes should be provided with information regarding healthful fod choices, meal planning, the role of snacks, finding time for eating, and realistic goal setting. Vegetarianism As demonstrated by the numerous elite athletes who are vegetarians, foods of animal origin are not essential to ensure optimal athletic performance. However, foods of animal origin are excellent sources of protein, iron, zinc, calcium and vit.anill 3-12. These nutrients are not easily obtained on vegetarian intakes in amounts needed without intensive dietary planning. The diets of vegetarian athletes should be evaluated to ensure an intake of a wide variety of foods and food combinations that provide all required nutrients in adequate anmoout:s. Detraining Nutrition information and counseling should be available to those individuals who make a significant reduction in their level of physical activity: fromthe college athlete who stops training to the elite athlete who retires. Such former athletes may require 'retraining" to develop new eating behaviors, achieve a healthy nonathletic body weight and body fat composition, altd help t hem understand how to make food choices to promote goo(I health. SUMMARY The importance of diet and healthful food choices il optillizing health status, fitness levels, and athletic performance has been recognized by both participants and professionals. There continues to be a need for the interpretation of new research findings in this fast-growing discipline and for the dissemination of nutrition information and training techniques for a broad spectrum of individuals involved in various forms of physical activity. The registered dietitian who has specialized in exercise physiology and sports nutrition has the knowledge and counseling skills to act as the provider of this nutrition information . Additional i fornnation may be obtained in the Sports Nutritio. Ma.uall, 2nd edition, published by The American Dietetic Association and the Sports and Cardiovascular Nutrition dietetic practice group as well as in Sport NutritionfortheAthletes (f Co (rnada,published by the Sport NutritionAdvisory Committee of the Sports Nle(icine and Science Council of Canada. References 1. Healthy People 2000: National Health Pro,noltiou od Disease Prevention Objectives. Washington, DC: I!S Dept Health anl Hllmall Services; 1990. 2. Blair SN, Kohl HW, Paffenbarger RS, Clark D)(, Cooper KH, Gibbons LW. Physical fitness and all-cause mortality: a prospective study of healthy men and women. JAMA. 1989; 262: 2395-2401. 3. American College of Sports Medicine. Position statement: the recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness in healthy adults. Meed Sci Sports Ever. 1990; 22: 265-274. 4. American College of Sports Medicine. ACS.i F'itue.,s Book Champaign, Ill: Leisure Press; 1992. 5. American College of Sports Medicine. Gl idelt.es Jfor F rercisc O()tURNAL,OF THE AMERICAN I)IETETI( ASS()(IATON / 695
Testing and Prescription.Philadelphia, Pa: Lea &Febiger; 1991. 6. Consensus Panel addresses obesity question. JAMA. 1985; 254:1878. 7. National Research Council. Diet and Health. Implications for Reducing ChronicDiseaseRisk. Washington, DC: National Academy Press; 1989. 8. 1983 Metropolitan height and weight tables. Stat Bull MetropInsur Co. 1983: 64:3. 9. Weigley ES. Average? Ideal? Desirable? A brief overview of heightweight tables in the United States. JAm Diet Assoc. 1984; 84:417-423. 10. Doxey GE, Fairbanks B, Housh TH, Johnson GO, Datch F, Lohman TG. Body composition roundtable: Part 1. Scientific considerations. Natl Strength ConditionJ 1987; 9:12-15. 11. Position of The American Dietetic Association: optimal weight as a health promotion strategy. JAm Diet Assoc. 1989; 89:1814-1817. 12. Jensen MD. Research techniques for body composition assessment. JAm Diet Assoc. 1992; 92: 454-460. 13. Lohman TG, Roche AF, Martorell R, eds. Anthropometric Standardization Reference Manual. Champaign, Ill: Human Kinetics; 1988. 14. Mazess RB, Barden HS, Bisek JP, Hanson J. Dual-energy x-ray absorptiometryfor total-body and regional bone-mineral and soft-tissue composition. Am J Clin Nutr. 1990; 51:1106-1112. 15. Paffenbarger RS, Hyde RT, Wing AL. Physical activity, all-cause mortality, and longevity of college alumni. NEngl JMed. 1986; 314: 605. 16. Karch Fl, McArdle WD. Nutrition, Weight Controland Exercise. Philadelphia, Pa: Lea &Febiger; 1983. 17. Horton E. Metabolic fuels, utilization, and exercise. Am, J Clin Nutr. 1989; 49:931-932. 18. Durnin J. The energy cost of exercise. ProcNutrSoc. 1985; 44:273. 19. Wilmore JH. Body weight standards and athletic performance. In: Brownell KD, ed. Eating, Body Weight and PerformanceinAthletes: Disorders of Modern Society. Philadelphia, Pa: Lea &Febiger; 1991. 20. Sawka MN, Pandolf KB. Effects of body water loss on physiological function and exercise performance. In: Gisolfi CV, Lamb DR, eds. Perspectives in Exercise Science and Sports Medicine: Fluid Homeostasis During Exercise. Indianapolis, Ind: Benchmark Press; 1990:31-38. 21. Dietary GuidelinesforAmericans. 3rd ed. Washington, DC: US Depts of Agriculture and Health and Human Services; 1990. Home and Garden Bulletin No. 232. 22. Sherman WM, Lamb DR. Nutrition and Prolonged Exercise. In: Lamb DR, Murray R, eds. Perspectivesin Exercise Science and Sports Medicine: Prolonged Exercise. Indianapolis, Ind: Benchmark Press; 1988: 213-280. 23. Ivy J. Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. JAppl Physiol. 1988; 64:1480-1485 24. Zachwieja J. Influence of muscle glycogen depletion on the rate of resynthesis. Med Sci Sports Exer. 1991; 23:44-48. 25. Costill DL, Sherman WM, Fink WJ, Maresh C, Witten M, Miller JM. The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. Am J Clin Nutr 1981; 34: 1831-1836. 26. Sherman WM, Costill DL, Fink WJ, Miller JM. The effect of exercise and diet manipulation on muscle glycogen and its subsequent use during performance. Int J Sports Med. 1981; 2: 114-118. 27. Food and Nutrition Board. Recommended Dietary Allowances 10th ed. Washington, DC: National Academy of Sciences; 1989. 28. Lenon PWR. Effect of exercise on protein requirements. JSports Sci. 1991; 9:53-70. 29. Paul GL. Dietary protein requirements for physically active individuals. Sports Med. 1987; 8:154-176. 30. Butterfield G, Cady C, Moynihan S. Effect of increasing protein intake on nitrogen balance in recreational weight lifters. Med Sci Sports Exer. 1992; 24:S-71. Abstract. 31. Roe DA. Vitamin requirements for increased physical activity. In: Horton ED, Terjung RL, eds. Exercise, Nutrition, and Energy Metabolism. New York, NY: Macmillan Publishing Co; 1989: 172-179. 32. Singh A, Moses FM, Deuster PA. Chronic multivitamin-mineral supplementation does not enhance physical performance. Med Sci Sports Exer 1992; 24: 726-732. 33. Sherman AR, Kramer B. Iron nutrition and exercise. In: Hickson JE, Wolinsky I, eds. Nutrition in Exercise Sport. Boca Raton, Fla: CRC Press; 1989: 291-30(1. 34. Haymes, EM. Vitamin and mineral supplementation to athletes. Int J SportsNutr. 1991; 1: 146-169. 35. Clement DB, Sawchuk LL. Iron status and sports performance. 696 / JUNE 1993 VOLUME 93 NUMBER 6
Sports Med. 1984; 1:65. 36. Eichner ER. The anemias of athletes. Phys Sportsmed. 1986; 14(9):122-130. 37. Halioua L, Anderson JB. Lifetime calcium intake and physical activity habits: independent and combined effects on the radial bone of healthy premenopausal Caucasian women. Am .1 Cliv Notsr. 1989; 49:534-541. 38. Dalsky GP. The role of exercise in the prevention of osteoporosis. Comp Thr. 1989; 15:30-37. 39. DrinkwaterBL, Nilson K, et al. Bone mineral content of amenorrheic and eumenorrheic athletes. NEngl J Med. 1984; 311:277. 40. Myburgh KH, Hutchins J, Fataar AB, Hough SF, Hoakes TI). Low bone density is an etiologic factor for stress fracture in athletes. Ann Intern Med. 1990; 113:754-759. 41. Tipton CM. Consequences of rapid weight loss. In: Haskell W, Scala J, Whitman J, eds. Nutrition and Athletic Performance. Paleo Alto, Calif: Bull Publishing Co; 1982: 176-199. 42. Ahlman V, Karoven M. Weight reduction by sweating in wrestlers and its effect on physical fitness. J Sports Med Phys Fit. 1962; 7:58. 43. Costill DL, Saltin B. Factors limiting gastric emptying during rest and exercise. JAppI Physiol 1974; 37:679. 44. Williams MH. NutritioobrFitness aed Sport. 3rd ed. hlubuque, Iowa: Wm C Brown Publishers; 1992. 45. Murray R. The effects of consuming carbohydrate-electrolyte beverages on gastric emptying and fluid absorption during and following exercise. Sports Med. 1987; 4: 322-351. 46. Fruth JM, Gisolfi CV. Effects of carbohydrate consumption on endurance performance: fructose versus glucose. In: Fox EL, ed. Nutrient UtilizationDuringFxercise.C Columbus: Ross Laboratories; 1983. 47. Slavin JL, Joensen DJ. Caffeine and sports performance. Phys Sportsmed. 1985; 13:191. 48. Sherman WM. Carbohydrate feedings before and after exercise. In: Lamb DR, Williams MH, eds. Perspectives in Exercise Science and ,SportsMedicine. Vol 4. Ergonomics: Enhancement ofPerfor~7nnce il Exercise and Sport. Indianapolis, Ind: Brown-Benchmark; 1991. 50. Coyle EF, Hagberg JM, Hurley BF, Martin WH, Ehsani AA, Holloszy 10O.Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. JAppl Physiol. 1983; 55:230. 51. Ivy JL, Katz ST,, Cutler (L, Sherman WM, Coyle EF. Muscle glycogen synthesis after exercise: effects of time of carbohydrate ingestion. J Appl Physiol. 1988; 64:1480-1485. 52. Exercise During Pregolawcy and Postnatal Period: (ACOG) Home Exercise Prograims. Washington, DC: American College of Obstetricians and Gynecologists; 1985. 53. .Jarski RS, Trippet DL. The risks and benefits of exercise during pregnancy. J Fam Pract. 1990; 30:185-189. 54. Wolfe LA, Hall P, Webb KA, et al. Prescription for aerobic exercise during pregnancy. Sports Med. 1989; 8: 273-301. 55. Sady SP, Carpenter MW. Aerobic exercise during pregnancy. Spo rts Med. 1989; 7:357-375. 56. Fishbein EG, Phillips M. How safe is exercise during pregnancy? J Obstet Gynecol Neonatal Nuors. 1990; 19:45-49. 57. Huch R, Erkkola R. Pregnancy and exercise--a short review. LrJ Obstet Gynecol. 1990; 97:2()8-214. 58. Lovelady CA, Lonnerdal B, Dewey KG. Lactation performance of exercising women. Am J Clin N7ltr. 1990;52:103-109. 59. Saris WHM, Browns F. Nutritional concerns for the young athlete. In: Childrenand Exercise XII. Champaign, Ill: Human Kinetics; 1986. 60. American College of Sports Medicine. Opinion statement on physical fitness in children and youth. Med Sci Sports F:xer 1988; 20:422. 61. Calabrese LH. Nutritional and medical aspects of gymnastics. In: Weider, ed. Clin Sports Med 1985; 4(1): 23-30 * ADA Position adopted by the House of Delegates on October26, 1986, reaffirmed on October 24, 1991, and updated for simultaneous publication in the Jou7ral of The American Dietetic Association and the Joura.l of The CanadianAssociation. * Recognition is given to the following for their contributions: A lthors: Maureen Smith Plomden, MS, RD; Dan Benardot, PhD, RD Reviewers: The Canadian Dietetic Association (Lynn Gates, MSc, RPDt; Donna Nadolny, MSc, RPDt; Marilyn Booth, MSc:, RPDt; Pamela Lynch, PDt; Barbara Marriage, MSc, RD; Marielle Ledoux, PhD, dtp); Sports and Cardiovascular Nutritionists dietetic practice group; Kristine Larson Clark, PhD, RD; Kristin .. Reimers, MS, RD; Penny M. Kris Etherton, PhD, RD; Keith Wheeler, PhD
ADA REPORTS
Position of The American Dietetic Association and The Canadian Dietetic Association: Nutrition for physical fitness and athletic performance for adults ealthy People 2000 outlines specific health-related goals for the United States in the areas of nutrition and physical fitness (1).1,2 As North Americans begin to make changes in these areas and professionals work to promote positive behaviors as part of a new life-style, they need accurate and appropriate nutrition information. In addition, there is a need for continued research on the role of nutrition in achieving optimal athletic performance; these findings should be disseminated to physically active persons and athletes . The registered dietitian is the best professional to provide nutrition information and the dietitian with advanced training in sports nutrition is the professional most qualified to provide advice to athletes.
BODY COMPOSITION Optimal body weights for health and competition should be determined individually, and are influenced by age, gender, physical activity or sport, somatotype, genetics, and individual variation. It is well established that health risks increase for those who are more than 20% over optimal weight (5,6). Health risks increase as proportion of bodyfat increases. Thus, the importance of maintaining a healthy body composition extends beyond the aesthetic and athletic goals to which an individual may aspire and lends credibility to the idea that body composition is a preferred measure of health and fitness over body weight alone. A variety of indexes and tools may be used to predict optimal body weight ranges for health and athletic performance. Body mass index (BMI), although of limited usefulness with competiPOSITION STATEMENT tive athletes, is an indicator of body composition and may also be It is thepositionof TheAmericanDieteticAssociationand The related to health outcomes. Desirable BMI ranges are 21.9 to 22.4 CanadianDietetic Association to support access to accurate for men and 21.3 to 22.1 for women. Values greater than 27.8 for and appropriateinformation thatexplains the interrelation- men and 27.3 for women are related to excess weight and ships between exercise and nutrition, reinforces the impor- increased risk of cardiovascular disease, high blood pressure, and tant role of nutrition, and encourages appropriatefood diabetes (6,7). 3 choices to achieve optimal physicalfitness and athletic perWeight-for-height charts are useful for tracking growthvelocity formance. foryouthand maybe used to predict desirable body weight ranges for the average population (8,9). However, these charts do not North Americans have become more physically active and partici- provide information on body composition and the distribution of pate in exercise programs for health benefits and recreation. lean body mass and adipose tissue. The overweight person is not People of all ages are physically active and their level of participa- necessarily overfat, and the underweight person is not necessarily tionvaries from recreationalwalking to competitive sports. Proper lean. This limitation becomes evident when these tables are used nutrition plays an important role in maximizing everyone's ability with certain athletes, especially those involved in power sports, to maintain higher levels of physical activity, regardless of age and who appear to be overweight according to these charts but type of activity. Hard physical training has the potential to affect actually have a high lean-to-fat ratio. health negatively by increasing the risk of dehydration and inMost athletes require a high strength-to-weight ratio to achieve creasing nutrient needs. Therefore, it is important to clarify and optimal athletic performance. Because adipose tissue adds to quantify the specific stressors that varying levels of physical body weight without contributing to strength, it is important to activity introduce on people of different ages, genders, and health be able to discriminate between these components of weight. status. These stressors, as they apply to different populations, Percent body fat values for athletes typically range from 5% to should be a major thrust in the development of nutrition educa12% in men and 10% to 20% in women and vary between sports tion materials. and between positions in a specific sport. For the general populaPhysical activity is beneficial to everyone's health, regardless of tion, optimal body fat percentage ranges tend to increase with age. their initial level of fitness. The most significant benefits are Generally, body fat levels of 15% to 18% for men and 20% to 25% experienced by those at the lowest fitness level who become more for women are considered acceptable. Body fat levels greater than active, even in very-low-level activities (2). For those who wish to 26% for men and 29% for women are considered a potential risk begin a new exercise program, it is important to consider the factor for the development of chronic diseases. Body fat levels guidelines for beginning and maintaining exercise programs pro- greater than 25% for men and 30% for women are considered vided by the American College of Sports Medicine (3,4). clinical indicators of obesity and are considered a risk factor in the Although the benefits of diet and exercise are well established, development of chronic diseases (10,11). At the other end of the individuals with certain clinical conditions, including diabetes, spectrum, body fat levels below 4% and 10% for men and women, cardiovascular disease, and metabolic disorders, should consult a respectively, are possible indicators of eating disorders. physician before initiating any exercise program. Although a Body composition assessment provides data that can be used to moderate exercise program (40% to 60% Vo2max) can be started calculate a desirable body weight range, based on lean body by most individuals of any age, men over age 40 and women over weight, that is most suited for the individual's health or perforage 50 who plan to begin a vigorous exercise program (> 60% mance goals. Techniques commonly used to measure body comVo2max) should first consult a physician (5). position include hydrostatic weighing, anthropometric measures (including skinfolds), bioelectrical impedance analysis (BIA), and dual energy x-ray absorptiometry (DEXA) (12). 'Editor's note: Because this is a joint Position of The American Dietetic Association and The Canadian Dietetic Association, where appropriate, 3 footnotes give specific references for practitioners in Canada. Canadian values for BMI: healthy range for men and women is20 to 25; 2 Health andWelfare Canada.ActionTowardsHealthyEating.Canada's measures >27 are related to health risk. From:Health and Welfare Guidelinesfor Healthy Eating and Recommended Strategiesfor Canada. Promoting Healthy Weights: A Discussion Paper. Ottawa: Implementation. Ottawa: Supply and Services Canada; 1990. Supply and Services Canada; 1988. JOURNAL OF THE AMERICAN DIETETIC ASSOCIATION / 691
ADA REPORTS Hydrostatic (or underwater) weighing is the gold standard for assessing body composition, but it requires substantial training, a large investment in equipment, and a high level of subject cooperation. Although this method is useful in research and clinical settings, its limitations make it impractical for general use. Skinfold measurements, taken with calipers at specific body sites, are among the most practical and cost-effective methods for estimating body composition. Numerous combinations of skinfold measurements taken at different sites and applied to regression equations may be used to predict body composition. A populationspecific equation that reflects such considerations as age, gender, race, somatotype, and type of physical activitymaybe selected, or ageneralized equation maybe more appropriate. Although skinfold measurements may be of limited use to predict body composition accurately in extremely lean or obese individuals, they are nonetheless useful with all populations to monitor changes over a period. The standardized technique for obtaining skinfold measurements is described in the literature and should be the basis for all practitioner training to reduce measurement error (13). BIA, which is based on the principle that electrical current applied to the body will meet greater resistance with more fat, is an accurate, noninvasive method for estimating body composition. Lean body tissue has a greater water and electrolyte level, lower electrical resistance, and greater conductivity than adipose tissue. In principle, the lower the electrical impedance, the greater an individual's lean body mass. Normal hydration status is essential; underhydration, which commonly occurs in athletes, will lead to inaccurate results. Because of normal fluctuations in body water, hydration status must be assessed before BIA is performed (12). DEXA is a precise method for quantifying the skeletal and soft tissue components of body mass. DEXA provides a direct measurement of body composition, whereas other common methods make predictions based on variables indirectly related to body composition (eg, water content of tissue). Scan times are low, ranging from 10 to 20 minutes, and patient radiation dosage is 800 to 2,000 times lowerthan a typical chest x-ray (0.02 to 0.05 mRem, compared with a typical chest x-ray at 40 mRem) (14). DEXA is not useful in the field because the equipment is expensive and is not portable. It is currently used principally for laboratory research and in clinical settings.
ENERGY Energy needs, which change throughout the life cycle, are high during periods of growth and progressively decrease after age 30. A high activity level in combination with rapid growth dramatically increases the energy requirement. The decreased energy needs associated with aging often accompany life-style changes. Decreased physical activity, loss of lean body mass, and increased food intake result in a higher percentage of body fat, a positive caloric balance, and a resulting weight gain. To prevent weight gain and achieve optimal health, it has been suggested that approximately 2,000 kcal/week should be expended in physical activity (15). Energy needs are based on basal metabolic rate plus the intensity, duration, frequency, and type of exercise performed. The energy equivalents of different activities are available (16). The goal of a physical activity program for healthy weight loss - for athletes or nonathletes--is to reduce body fat and maintain lean body tissue. Rapid weight loss promotes loss of lean body tissue and muscle, compromises nutrient intake, and increases the likelihood of regaining weight. Rapid weight loss may also compromise endurance and impair both cardiac function and body temperature regulation. For the general population, a program of regular aerobic exercise (20- to 30-minute sessions, at least 3 times per week), combined with a hypocaloric, low-fat diet (< 30% energy from fat) 692/ JUNE 1993 VOLUME 93 NUMBER 6
is most effective for achieving desirable changes in body composition and bodyweight. Using such a program, muscle mass can be better maintained while body fat is reduced. During the initial stages of a weight reduction program, weight loss may be minimal, even though arm, leg, and trunk circumference measurements decrease. This occurs because of an increase inmuscle mass at the same time as body fat is lost. After several weeks, an individual will continue to lose body fat, as muscle gain levels off, resulting in a loss of body weight. For sports with competitive seasons, it is important that athletes enter their season at an optimal level of conditioning. The competitive season is not a time for weight loss or weight gain. The off-seasontraining program should address their needs for adjustments in body composition by combining the appropriate dietary and exercise regimen to achieve the desired results. Many athletes train and compete throughout the year, so a coordinated plan of gradual change should be implemented in a way that will not negatively affect performance. Dietary changes should be introduced as an integral part of an overall training program, rather than as restrictions of a "diet," to avoid the negative connotations that this may bring. Individuals engaged in vigorous physical training programs have varied energy needs, ranging from 2,000 to 6,000 kcal/day or more (17,18). The needs of a 200-lb (91-kg) bodybuilder will differ from those of an 80-lb (36-kg) gymnast. A well-planned individualized dietary program is necessary to help athletes consume relatively large quantities of food, which is difficult with the combined constraints of training, sleep, school, and work. The lower caloric density of the preferred high-carbohydrate/low-fat diet makes it even more difficult to provide sufficient food to meet high energy needs. Multiple strategies, including more frequent eating, planned snacks, and consumption of high-carbohydrate meals should be considered where feasible. For many athletes, gaining weight or maintaining a high weight is important for optimal athletic performance. An individualized eating plan that provides frequent feedings or "mini-meals" that will complement their training schedules must be designed for these athletes. Liquid meal supplements may be helpful as they offer a means for providing a high-carbohydrate meal in an easily delivered, ready-to-consume package. Weight fluctuations may occur cyclically in some athletes; their competitive weight is usually lower than their weight during the off-season. The resulting weight cycle (commonly described as a "yo-yo" cycle) is similar to that experienced by individuals who participate in rapid weight loss programs (19). Ideally, weight loss should be gradual (0.5 to 1 lb or 0.23 to 0.45 kg per week) and should be achieved wellbefore the start of the competitive season to assure maximal strength. Attention should be given to compulsive eating behavior and wide swings in body weight because these can be signs of eating behavior disorders (19). Some weight loss techniques cause diuresis and a resultant decrease in plasma volume and reduced blood flow to the kidneys, skin, and muscles. This combination of conditions lowers the body's ability to sweat and regulate body temperature, increases susceptibility to heat illness, and reduces exercise endurance (20). Evaluation of eating practices should be a standard component of the nutrition assessment of athletes to determine the potential risk to health and athletic performance.
ENERGY-NUTRIENT INTAKE DISTRIBUTION Ideal distribution of carbohydrate, protein, and fat for athletes is similarto recommendations in theDietaryGuidelinesforAmericans (21) and the Canadian Nutrition Recommendations.4 4
HealthandWelfare Canada.NutritionRecommendations.TheReport of the Scientific Review Committee. Ottawa: Supply and Services Canada;1990.
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Quantitive differences are realized, however, because of the increased energy requirements of athletes. Carbohydrate In general, it is recommended that 60% to 65% of total energy should come from carbohydrate. Athletes who train exhaustively on successive days or who compete in prolonged endurance events should consume a diet that provides 65% to 70% of energy from carbohydrates A threshold of 500 to 800 g (2,000 to 3,200 kcal) carbohydrate per day, regardless of the total daily energy intake, may be necessary to maintain maximal muscle glycogen stores in athletes (22-24). Maximizing muscle glycogen stores provides greater energy reserve for aerobic and anaerobic activities, resulting in greater endurance and delayed fatigue (25). To maximize muscle glycogen storage, preparation for a competition of long duration (> 90 minutes of continuous activity) should include at least 3 days of intake that includes 800 g carbohydrate per day (or 6 tolO g carbohydrate per kilogrambody weight per day) and a week of tapered workouts that ends in complete rest the day before competition. This dietary protocol, or "modified carbohydrate loading" plan, maintains high glycogen stores and may be followed numerous times throughout the year without health risk (26). This modified plan has been accepted since the mid-1980s when it was recognized that the original carbohydrate-loading scheme, which involved a carbohydrate depletion phase, resulted in a diminished abilityto train before the competitive event and introduced potential health risks. Protein Athletes may require more protein than the Recommended Dietary Allowance (RDA) (27) of 0.8 g/kg body weight (28). The increased requirement for protein appears to be small, however, and not at a level of 2 to 2.5 times greater than the RDA, or Recommended Nutrient Intake (RNI)6 in Canada, as suggested in the lay literature. Recent data confirm the protein-sparing effect of carbohydrate. In addition, research suggests that the quantity of dietary protein needed to achieve maximal protein deposition is 1.5 g/kg body weight, and that the limiting factor for muscle protein deposition is energy intake, not protein (29,30). Therefore, athletes who wish to increase muscle mass should meet their energy requirements first, through an adequate intake of carbohydrate, and then check that they have met their protein needs. Protein needs can be calculated both as a percentage of total energy and on a per kilogram body weight basis. For athletes with exceptionally high energy intakes, providing 12% to 15% of total energy from protein may be excessive. When energy intake is low, as typically observed for many young women or low body weight athletes, protein needs calculated as a percentage of energy may be inadequate. In both of these cases, 1.0 to 1.5 g protein per kilogram body weight may be a more appropriate guide for intake than protein as a percent of total energy. The use of amino acid and protein supplements by athletes is common, despite the lack of well-controlled clinical studies to justify their use. Even the highest protein requirements can be met easily with a balanced diet that includes a variety of foods. Therefore, excessive protein intake, either through consumption of high-protein foods or protein/amino acid supplements, is unnecessary, does not contribute to athletic performance or increase muscle mass, and actually may be detrimental to health and athletic performance. Increased intake of amino acids does 5
Nutrition Recommendations for Canadians: General population: 55% total energy derived from carbohydrate; athletes: 60% to 66%. 'The Canadian Recommended Nutrient Intake for protein is 0.86 g/kg. Health and Welfare Canada. Nutrition Recommendations. The Report of the Scientijic Roview Committee. Ottawa: Supply and Services Canada: 1990.
not result in an increased utilization or increased muscle protein synthesis in the body. Providing dietary protein at a level that exceeds the requirement results in amino acids being stored as energy (fat) or oxidized. Neither situation is desirable; amino acid oxidation increases the risk of dehydration because of the need to remove protein metabolic end products via the urine, and the storage of amino acid carbon as fat is metabolically inefficient. Fat Fat should contribute no more than 30% of total energy to the diet. Total fat intake is likely to be higher in athletes who must consume very large quantities of energy (> 4,000 kcal) in order to maintain body mass. This is typical of linemen on football teams and in other sports where a large muscle mass is important for athletic success and for those who participate in extensive training programs of long duration on a daily basis. Carbohydrate and protein needs should first be calculated for the individual. Fat can then be used to provide additional energy, but only to a maximum level of 30% of total energy. Energy needs above this level should be derived from high-carbohydrate/low-fat food sources. VITAMINS AND MINERALS Vitamins and minerals play an important role in the metabolism of protein, carbohydrate, and lipids and in muscle function. Although physical activity increases the need for some vitamins and minerals, this increased requirement typically can be met by consuming abalanced high-carbohydrate, moderate-protein, lowfat diet. Individuals at risk for low vitarnin/mineral intake are those who consume a low-calorie diet. Athletes commonly consume substances, including vitamin and mineral supplements, that they believe have ergogenic properties. Although vitamin and mineral supplementation may improve the nutritional status of individuals consuming marginal amounts of nutrients from food and may improve performance in those with deficiencies, there is no scientific evidence to support the general use of supplements to improve athletic performance (31,32). The increased energy intake of athletes should provide the additional vitamins and minerals necessary if a wide variety of foods is included in the diet. Iron and calcium are two minerals of concern, especially for young athletes and women of all ages involved in physical activity. Iron, as a component of hemoglobin and myoglobin, is essential for oxidative metabolism to occur. Iron depletion (low iron stores) is seen in both male and female athletes but does not appear to affect athletic performance directly. If the condition progresses to iron deficiency (low hemoglobin level), then athletic performance is negatively affected (33). The incidence of iron-deficiency anemia is rare and is similar among physically trained individuals and the general population (34). When it occurs, the cause of anemia in athletes is probably multifactorial. Low caloric intakes often do not supply sufficient iron to maintain adequate iron stores. Diets that provide little or no meat limit the most available dietary sources of iron. Exerciseassociated factors include hemolysis of erythrocyteNs, alterations in iron metabolism, hematuria, increased erythroc e osmotic fragility resulting in decreased red blood cell survival time, and a possible shift in the oxygen dissociation curve (35). Athletes should be screened periodically to assess iron status. Typically, changes in storage and transport iron will be noted before decreases in functional iron (hemoglobin and hematocrit levels). Thus, early screening for serum ferritin levels is recommended. If iron deficiency is found, nutrition guidance should be provided. A transient, nonclinical "sports anemia" (hemodiltltional iron deficiency) is sometimes experienced by athletes as a result; of an increase in plasma volume that is associated with the initiation of training. However, this form of anemia has no apparent effect on performance (36). J()OU[RNAL OF THE AMERICAN DIETETIC ASS()('IATION /693
ADA REPORTS
Osteoporosis is a major health concern for women in North America and is related to calcium intake, estrogen level, alcohol and caffeine intake, family history, and the amount and type of physical activity. The emphasis for prevention of osteoporosis should be to maximize the body's stores of calcium early in life, maintain that level, and minimize any loss (37). A calcium intake of 800 to 1,200 mg/day is recommended (1,27) to protect against the development of osteoporosis. 7 Achieving this recommended daily intake, in conjunction with performing regular weight-bearing activities, will promote the deposition of calcium in bone and thereby reduce the risk of developing osteoporosis (38). Nutrition guidance should be given to low-body-weight, amenorrheic women who appear to be at high risk of early osteoporosis. The amenorrhea commonly associated with low levels of body fat, high physical activity (especially gymnastics, swimming, and long-distance running), and eating behavior disorders, may hinder bone development at a time when bone should be forming at its maximum rate (39). When amenorrhea is present, it appears that consumption of 120% of the calcium RDA or RNI helps bones to develop properly and maintain density (40). This amenorrhea should not be accepted and ignored by the athlete; side effects, such as an increased incidence of stress fractures, could have a serious effect on athletic performance. HYDRATION Increased muscular activity leads to an increase in heat production in the body; this is dissipated, in part, through the production of sweat. To prevent dehydration, water must be replaced at a faster rate. Dehydration has an adverse effect on muscle strength, endurance, and coordination and increases the risk of cramps, heat exhaustion, and life-threatening heat stroke (41-43). Some individuals may lose up to 6 to 8 lb (2.7 to 3.6 kg) of sweat per hour during strenuous activity. To ensure that lost fluid is replaced, body weight should be measured before and after training sessions; each pound (0.45 kg) lost should be replaced with 16 oz (1 pint or 474 mL) fluid. If weight is not within 1 to 2 lb (0.45 to 0.9 kg) of the previous day's preexercise weight, additional fluids should be consumed before exercising. Ideally, athletes should learn to consume adequate fluids during activity so that body weight remains relatively constant before and after exercise. Guidelines for fluid consumption include consuming at least 2 cups (16 to 20 oz or 474 to 592 mL) fluid about 2 hours before exertion, followed by another 2 cups (474 mL) fluid approximately 15 to 20 minutes before endurance exercise. In hot and humid environments, frequent small servings (4 to 6 oz or 118 to 178 mL every 15 minutes) of plain cool (40 to 50°F or 5 to 10°C) water or other rehydration beverage are recommended throughout exertion (44). Vigorous exercise may delay the thirst mechanism, making it difficult to replace fluid loss without a plan for periodic consumption. Athletes should become accustomed to consuming fluid at regular intervals (with or without thirst) during training sessions so that they do not experience discomfort during competition. For most persons exercising for 1 hour or less in moderate temperature conditions, an appropriate fluid replacement is cool water (43). The typical North American diet provides ample sodium, potassium, chloride, and magnesium to replace sweat losses in most cases. Longer exercise ( >1 hour) and/or exercise in extreme enviromental conditions, such as a high temperature/ humidity index, may warrant consumption of a sports beverage as part of the hydration-replacement process. Commercial sport 7
Canadian Recommended Nutrient Intake for calcium is 800 mg/day.
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beverages are available that contain electrolytes to enhance absorption and supply energy through various carbohydrate sources. The athlete should choose the specific sports beverage that best meets his or her needs. Beverages of choice should taste good to promote consumption, not cause gastric distress, provide a carbohydrate source for enhanced performance during endurance activities, and optimize rate of fluid absorption. Research shows that 6% to 8% glucose, glucose polymer, or sucrose with a small amount of sodium in a hypotonic or isotonic solution are all absorbed quickly and help maintain blood glucose levels during exercise (45). Beverages that use fructose as the principle carbohydrate source may cause gastric distress in some athletes and require more time before they can be used by muscles as a fuel because of the necessary conversion of fructose to glucose (46). Guidelines for amount of intake should adhere to those listed for hydration with water. ERGOGENIC AIDS Compounds such as bee pollen, caffeine, glycine, carnitine, lecithin, brewer's yeast, and gelatin are claimed, through anecdotal evidence, to improve strength or endurance. Although popular with some athletes because of this perceived ergogenic benefit, scientific research has failed to substantiate the claims for these products or inadequate research has been undertaken with these supplements. In some cases, as with caffeine (a restricted substance per the International Olympic Committee), the research results are equivocal (47). However, because of the placebo effect, some athletes are convinced that certain foods, dietary regimens, or supplements improve performance. These substances may provide certain psychological benefits rather than proven physiological benefits. However, when use of these substances replaces a sound nutrition program, health and performance may be compromised, resulting in serious consequences. GENERAL RECOMMENDATIONS Nutrition concerns and recommendations for ensuring that nutrition does not become a limiting factor in athletic performance were presented in the previous sections. The underlying principle that unites all of these recommendations is that athletes must consume a well-balanced and varied diet on a daily basis to support their training efforts. Preexercise/Competition What an athlete eats immediately before exercise or competition makes a difference, both physically and psychologically. Dietary guidelines for before exercise mainly involve the provision of an adequate carbohydrate and fluid intake to support exercise. Two to 6 hours before an endurance event, depending on individual preference, a meal providing 85 to 200 g carbohydrate should be consumed (48). Food choices should provide mainly complex carbohydrates and should be low in fat and protein. Athletes should choose foods that are familiar and comforting, have been consumed before training sessions, and are known not to cause gastric distress. Athletes should not experiment with new foods or beverages on competition day. A liquid meal supplement that provides a balance of fluid and nutrients may be helpful for athletes with precompetition jitters and gastric distress. Five to 10 minutes before exercise or competition, fluids containing a small amount of carbohydrate may be consumed to maintain blood glucose level. There does not appear to be a significant insulin rebound effect from this practice, although some athletes do not tolerate this comfortably (43,48). During Exercise/Competition Small carbohydrate feedings during endurance events of more than 1 hour may delay fatigue. Feedings should be at regular
intervals to provide about 24 g carbohydrate every 30 minutes (49). Athletes should become comfortable with these feedings during their practice sessions. Athletes often find that consumption of carbohydrate-containing sport beverages is better tolerated than solid foods during activity. Athletes should consume fluid on a fixed time schedule during activity, regardless of thirst. After Exercise/Competition Carbohydrate consumption after exercise ensures repletion of muscle glycogen. Research shows that muscle will replete glycogen stores to a higher degree when up to 600 g easily digestible carbohydrate is consumed within the first several hours after exercise. The athlete should begin eating high- carbohydrate foods as soon as possible after physical exertion. Blood glucose, insulin, and glycogen synthetase levels will remain elevated to promote glycogen synthesis and replete the muscle reserves (50). SPECIAL CONSIDERATIONS Pregnancy Active women who become pregnant may continue to exercise, but only after gaining approval from a physician. Women who initiate an exercise program during pregnancy must do so only with low-level activities, and only with the supervision of a physician. Guidelines have been established to determine who may exercise safely. In addition, there are various recommendations for exercise intensities, hyperthermia precautions, types of exercise, safe body movements, and contraindicated exercises (5,51). Several studies have demonstrated that exercise during pregnancy exerts several benefits, including improved self-esteem, lower maternal weight gain, improved maternal aerobic fitness, and potentially improved labor outcomes (52-56). It also has been demonstrated that excessive physical activity and inadequate energy intakes can lead to suboptimal maternal weight gain and fetal growth retardation (52,53). Lactation Recent evidence suggests that lactating women who exercise have significantly lower body fat and higher energy expenditures than nonexercising women. In addition, exercising women produce a slightly greater volume of milk (57). Lactating women who exercise should consume sufficient fluids to avoid dehydration and sufficient energy to compensate for that expended during exercise. The increased intake should include the recommended additional 500 kcal required for lactating women. Childhood Diet arv considerations for children who exercise regularly differ only in the need for special attention to energy requirements. Adequate caloric intake is important to achieve optimal growth velocities and maintain health status. Children have a greater surface area and lower sweating capacitythan adults and, as a result, are more susceptible tohyperthermia than are adults. This problem is exacerbated by the fact that children produce more heat per unit body weight than adults and are less capable of transferring this heat from the muscles to the skin (58). The differences in thermoregulation between children and adults strongly suggest that children should not be exposed to the same exercise intensities as adults, regardless of environmental conditions. In addition, they appear to be more prone to overuse injuries. Any exercise training programthat is initiated by a child should progress slowly, allowing ample time for acclimatization, conditioning, and skill development to improve athletic performance, An ()pinion Statement by the American College of Sports Medicine provides guidance for the development of a training program for children (59).
Adolescence Adolescent athletes require accurate information on establishing safe weight and body composition goals. This is particularly important for athletes striving for extremely low body-fat levels, such as wrestlers, gyrrnasts, rowers, dancers, and swimners. Studies suggest that some adolescent athletes, particularly womel, consume diets that are low in energy and nutrients (60). The combination of high nutrient and energy requirements necessary to support growth and training and dietary self-restrictior justifies monitoring these weight-conscious athletes for signs of dietary deficiencies and related health problems. These athletes should be provided with information regarding healthful fod choices, meal planning, the role of snacks, finding time for eating, and realistic goal setting. Vegetarianism As demonstrated by the numerous elite athletes who are vegetarians, foods of animal origin are not essential to ensure optimal athletic performance. However, foods of animal origin are excellent sources of protein, iron, zinc, calcium and vit.anill 3-12. These nutrients are not easily obtained on vegetarian intakes in amounts needed without intensive dietary planning. The diets of vegetarian athletes should be evaluated to ensure an intake of a wide variety of foods and food combinations that provide all required nutrients in adequate anmoout:s. Detraining Nutrition information and counseling should be available to those individuals who make a significant reduction in their level of physical activity: fromthe college athlete who stops training to the elite athlete who retires. Such former athletes may require 'retraining" to develop new eating behaviors, achieve a healthy nonathletic body weight and body fat composition, altd help t hem understand how to make food choices to promote goo(I health. SUMMARY The importance of diet and healthful food choices il optillizing health status, fitness levels, and athletic performance has been recognized by both participants and professionals. There continues to be a need for the interpretation of new research findings in this fast-growing discipline and for the dissemination of nutrition information and training techniques for a broad spectrum of individuals involved in various forms of physical activity. The registered dietitian who has specialized in exercise physiology and sports nutrition has the knowledge and counseling skills to act as the provider of this nutrition information . Additional i fornnation may be obtained in the Sports Nutritio. Ma.uall, 2nd edition, published by The American Dietetic Association and the Sports and Cardiovascular Nutrition dietetic practice group as well as in Sport NutritionfortheAthletes (f Co (rnada,published by the Sport NutritionAdvisory Committee of the Sports Nle(icine and Science Council of Canada. References 1. Healthy People 2000: National Health Pro,noltiou od Disease Prevention Objectives. Washington, DC: I!S Dept Health anl Hllmall Services; 1990. 2. Blair SN, Kohl HW, Paffenbarger RS, Clark D)(, Cooper KH, Gibbons LW. Physical fitness and all-cause mortality: a prospective study of healthy men and women. JAMA. 1989; 262: 2395-2401. 3. American College of Sports Medicine. Position statement: the recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness in healthy adults. Meed Sci Sports Ever. 1990; 22: 265-274. 4. American College of Sports Medicine. ACS.i F'itue.,s Book Champaign, Ill: Leisure Press; 1992. 5. American College of Sports Medicine. Gl idelt.es Jfor F rercisc O()tURNAL,OF THE AMERICAN I)IETETI( ASS()(IATON / 695
Testing and Prescription.Philadelphia, Pa: Lea &Febiger; 1991. 6. Consensus Panel addresses obesity question. JAMA. 1985; 254:1878. 7. National Research Council. Diet and Health. Implications for Reducing ChronicDiseaseRisk. Washington, DC: National Academy Press; 1989. 8. 1983 Metropolitan height and weight tables. Stat Bull MetropInsur Co. 1983: 64:3. 9. Weigley ES. Average? Ideal? Desirable? A brief overview of heightweight tables in the United States. JAm Diet Assoc. 1984; 84:417-423. 10. Doxey GE, Fairbanks B, Housh TH, Johnson GO, Datch F, Lohman TG. Body composition roundtable: Part 1. Scientific considerations. Natl Strength ConditionJ 1987; 9:12-15. 11. Position of The American Dietetic Association: optimal weight as a health promotion strategy. JAm Diet Assoc. 1989; 89:1814-1817. 12. Jensen MD. Research techniques for body composition assessment. JAm Diet Assoc. 1992; 92: 454-460. 13. Lohman TG, Roche AF, Martorell R, eds. Anthropometric Standardization Reference Manual. Champaign, Ill: Human Kinetics; 1988. 14. Mazess RB, Barden HS, Bisek JP, Hanson J. Dual-energy x-ray absorptiometryfor total-body and regional bone-mineral and soft-tissue composition. Am J Clin Nutr. 1990; 51:1106-1112. 15. Paffenbarger RS, Hyde RT, Wing AL. Physical activity, all-cause mortality, and longevity of college alumni. NEngl JMed. 1986; 314: 605. 16. Karch Fl, McArdle WD. Nutrition, Weight Controland Exercise. Philadelphia, Pa: Lea &Febiger; 1983. 17. Horton E. Metabolic fuels, utilization, and exercise. Am, J Clin Nutr. 1989; 49:931-932. 18. Durnin J. The energy cost of exercise. ProcNutrSoc. 1985; 44:273. 19. Wilmore JH. Body weight standards and athletic performance. In: Brownell KD, ed. Eating, Body Weight and PerformanceinAthletes: Disorders of Modern Society. Philadelphia, Pa: Lea &Febiger; 1991. 20. Sawka MN, Pandolf KB. Effects of body water loss on physiological function and exercise performance. In: Gisolfi CV, Lamb DR, eds. Perspectives in Exercise Science and Sports Medicine: Fluid Homeostasis During Exercise. Indianapolis, Ind: Benchmark Press; 1990:31-38. 21. Dietary GuidelinesforAmericans. 3rd ed. Washington, DC: US Depts of Agriculture and Health and Human Services; 1990. Home and Garden Bulletin No. 232. 22. Sherman WM, Lamb DR. Nutrition and Prolonged Exercise. In: Lamb DR, Murray R, eds. Perspectivesin Exercise Science and Sports Medicine: Prolonged Exercise. Indianapolis, Ind: Benchmark Press; 1988: 213-280. 23. Ivy J. Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. JAppl Physiol. 1988; 64:1480-1485 24. Zachwieja J. Influence of muscle glycogen depletion on the rate of resynthesis. Med Sci Sports Exer. 1991; 23:44-48. 25. Costill DL, Sherman WM, Fink WJ, Maresh C, Witten M, Miller JM. The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. Am J Clin Nutr 1981; 34: 1831-1836. 26. Sherman WM, Costill DL, Fink WJ, Miller JM. The effect of exercise and diet manipulation on muscle glycogen and its subsequent use during performance. Int J Sports Med. 1981; 2: 114-118. 27. Food and Nutrition Board. Recommended Dietary Allowances 10th ed. Washington, DC: National Academy of Sciences; 1989. 28. Lenon PWR. Effect of exercise on protein requirements. JSports Sci. 1991; 9:53-70. 29. Paul GL. Dietary protein requirements for physically active individuals. Sports Med. 1987; 8:154-176. 30. Butterfield G, Cady C, Moynihan S. Effect of increasing protein intake on nitrogen balance in recreational weight lifters. Med Sci Sports Exer. 1992; 24:S-71. Abstract. 31. Roe DA. Vitamin requirements for increased physical activity. In: Horton ED, Terjung RL, eds. Exercise, Nutrition, and Energy Metabolism. New York, NY: Macmillan Publishing Co; 1989: 172-179. 32. Singh A, Moses FM, Deuster PA. Chronic multivitamin-mineral supplementation does not enhance physical performance. Med Sci Sports Exer 1992; 24: 726-732. 33. Sherman AR, Kramer B. Iron nutrition and exercise. In: Hickson JE, Wolinsky I, eds. Nutrition in Exercise Sport. Boca Raton, Fla: CRC Press; 1989: 291-30(1. 34. Haymes, EM. Vitamin and mineral supplementation to athletes. Int J SportsNutr. 1991; 1: 146-169. 35. Clement DB, Sawchuk LL. Iron status and sports performance. 696 / JUNE 1993 VOLUME 93 NUMBER 6
Sports Med. 1984; 1:65. 36. Eichner ER. The anemias of athletes. Phys Sportsmed. 1986; 14(9):122-130. 37. Halioua L, Anderson JB. Lifetime calcium intake and physical activity habits: independent and combined effects on the radial bone of healthy premenopausal Caucasian women. Am .1 Cliv Notsr. 1989; 49:534-541. 38. Dalsky GP. The role of exercise in the prevention of osteoporosis. Comp Thr. 1989; 15:30-37. 39. DrinkwaterBL, Nilson K, et al. Bone mineral content of amenorrheic and eumenorrheic athletes. NEngl J Med. 1984; 311:277. 40. Myburgh KH, Hutchins J, Fataar AB, Hough SF, Hoakes TI). Low bone density is an etiologic factor for stress fracture in athletes. Ann Intern Med. 1990; 113:754-759. 41. Tipton CM. Consequences of rapid weight loss. In: Haskell W, Scala J, Whitman J, eds. Nutrition and Athletic Performance. Paleo Alto, Calif: Bull Publishing Co; 1982: 176-199. 42. Ahlman V, Karoven M. Weight reduction by sweating in wrestlers and its effect on physical fitness. J Sports Med Phys Fit. 1962; 7:58. 43. Costill DL, Saltin B. Factors limiting gastric emptying during rest and exercise. JAppI Physiol 1974; 37:679. 44. Williams MH. NutritioobrFitness aed Sport. 3rd ed. hlubuque, Iowa: Wm C Brown Publishers; 1992. 45. Murray R. The effects of consuming carbohydrate-electrolyte beverages on gastric emptying and fluid absorption during and following exercise. Sports Med. 1987; 4: 322-351. 46. Fruth JM, Gisolfi CV. Effects of carbohydrate consumption on endurance performance: fructose versus glucose. In: Fox EL, ed. Nutrient UtilizationDuringFxercise.C Columbus: Ross Laboratories; 1983. 47. Slavin JL, Joensen DJ. Caffeine and sports performance. Phys Sportsmed. 1985; 13:191. 48. Sherman WM. Carbohydrate feedings before and after exercise. In: Lamb DR, Williams MH, eds. Perspectives in Exercise Science and ,SportsMedicine. Vol 4. Ergonomics: Enhancement ofPerfor~7nnce il Exercise and Sport. Indianapolis, Ind: Brown-Benchmark; 1991. 50. Coyle EF, Hagberg JM, Hurley BF, Martin WH, Ehsani AA, Holloszy 10O.Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. JAppl Physiol. 1983; 55:230. 51. Ivy JL, Katz ST,, Cutler (L, Sherman WM, Coyle EF. Muscle glycogen synthesis after exercise: effects of time of carbohydrate ingestion. J Appl Physiol. 1988; 64:1480-1485. 52. Exercise During Pregolawcy and Postnatal Period: (ACOG) Home Exercise Prograims. Washington, DC: American College of Obstetricians and Gynecologists; 1985. 53. .Jarski RS, Trippet DL. The risks and benefits of exercise during pregnancy. J Fam Pract. 1990; 30:185-189. 54. Wolfe LA, Hall P, Webb KA, et al. Prescription for aerobic exercise during pregnancy. Sports Med. 1989; 8: 273-301. 55. Sady SP, Carpenter MW. Aerobic exercise during pregnancy. Spo rts Med. 1989; 7:357-375. 56. Fishbein EG, Phillips M. How safe is exercise during pregnancy? J Obstet Gynecol Neonatal Nuors. 1990; 19:45-49. 57. Huch R, Erkkola R. Pregnancy and exercise--a short review. LrJ Obstet Gynecol. 1990; 97:2()8-214. 58. Lovelady CA, Lonnerdal B, Dewey KG. Lactation performance of exercising women. Am J Clin N7ltr. 1990;52:103-109. 59. Saris WHM, Browns F. Nutritional concerns for the young athlete. In: Childrenand Exercise XII. Champaign, Ill: Human Kinetics; 1986. 60. American College of Sports Medicine. Opinion statement on physical fitness in children and youth. Med Sci Sports F:xer 1988; 20:422. 61. Calabrese LH. Nutritional and medical aspects of gymnastics. In: Weider, ed. Clin Sports Med 1985; 4(1): 23-30 * ADA Position adopted by the House of Delegates on October26, 1986, reaffirmed on October 24, 1991, and updated for simultaneous publication in the Jou7ral of The American Dietetic Association and the Joura.l of The CanadianAssociation. * Recognition is given to the following for their contributions: A lthors: Maureen Smith Plomden, MS, RD; Dan Benardot, PhD, RD Reviewers: The Canadian Dietetic Association (Lynn Gates, MSc, RPDt; Donna Nadolny, MSc, RPDt; Marilyn Booth, MSc:, RPDt; Pamela Lynch, PDt; Barbara Marriage, MSc, RD; Marielle Ledoux, PhD, dtp); Sports and Cardiovascular Nutritionists dietetic practice group; Kristine Larson Clark, PhD, RD; Kristin .. Reimers, MS, RD; Penny M. Kris Etherton, PhD, RD; Keith Wheeler, PhD