Nutritional concerns for the child and adolescent competitor

REVIEW ARTICLE

Nutritional Concerns for the Child and Adolescent Competitor Heather J. Petrie, MS, Elizabeth A. Stover, MS, and Craig A. Horswill, PhD From the Gatorade Sports Science Institute, Barrington, Illinois, USA With exercise for sports competition in children and adolescents, acute nutrient needs will change. Fluid intake to ensure the replacement of water and minerals (electrolytes) lost in sweat is important. Energy needs also increase because of the elevated energy expenditure with physical activity. Arguably carbohydrate is the recommended source of training needs, although research has yet to be done to show performance benefits in young athletes on a high-carbohydrate diet. In the majority of sports, an increased intake of food naturally occurs to accommodate the day-to-day nutrient needs of young athletes, and unlike non-athlete, young competitors typically come closer to meeting their requirements for micronutrients. Nonetheless, certain athletic groups may be at risk for shortfalls in their diet. Compared to athletes in team sports, participants in weight-control sports may be at greater risk of failing to meet requirements for energy, protein, and some micronutrients. Endurance athletes, particularly female distance runners, may have intake deficits for the minerals iron and calcium. Acute issues such as heat illness and chronic concerns that include impaired growth and development, and the risk of injuries that include stress fractures may be an outcome of inadequate nutrition during physical training. Nutrition 2004;20: 620 – 631. ©Elsevier Inc. 2004 KEY WORDS: creatine monohydrate, strength training, nutritional supplementation, ergogenic aid, phosphocreatine

INTRODUCTION Sport opportunities for youths continue to grow over time and throughout the world. In the United States between 1992 and 2002, participation in organized sports by children increased by 8.4% and by adolescents increased by 15.4% (National Sporting Goods Association: http://www.nsga.org/public/pages/index.cfm/ pageid⫽158). In Europe, depending on the country, 53% to 98% of children ages 6 to 11 y are involved, at least occasionally, in sports (COMPASS: http://w3.uniroam1.it/compas/). The percentages are nearly identical for adolescents ages 12 to 16 y. This is somewhat perplexing in light of the increase in childhood and adolescent obesity that covers the globe,1,2 but perhaps it is a promising sign. It has been shown that adolescent athletes maintain healthier nutritional habits than non-athletic peers, and the investigators concluded that intervention programs are wise to include athletic and nutritional components.3 Most children and adolescents who are strongly committed to sports are not concerned about nutrition as it relates to energy balance and obesity. This may be due to the natural selection process of the lean, naturally coordinated child achieving success and thereby continuing with the sport. Nonetheless, nutrition is a major component of their training because of the interactions among nutrition, growth and development, achieving optimal performance, and avoiding injuries and problems that may arise due to nutritional deficiencies. This review focuses on the nutritional concerns of young athletes by examining what is known about the acute and chronic needs (general nutrition) of young athletes. Specific nutritional issues are summarized according to three categories of sports: endurance sports (swimming, running, cycling, duathlon, and triathlons), strength and weight-class sports that emphasize individ-

Correspondence to: Craig A. Horswill, PhD, Gatorade Sports Science Institute, 617 W. Main St., Barrington, IL 60010, USA. E-mail: [email protected] Nutrition 20:620 – 631, 2004 ©Elsevier Inc., 2004. Printed in the United States. All rights reserved.

ual performance (gymnastics, figure skating, track sprints and field events, weight lifting, wrestling, martial arts, boxing), and team sports (American football, soccer, field hockey, volleyball, basketball, lacrosse, and baseball). Where possible, the review focuses on information available for children ages 7 to 12 y and adolescents ages 13 to 18 y. In many cases, because of the limited number of studies in children and teens, the papers reviewed include older adolescents (18 to 20 y).

DIETARY REFERENCE INTAKES In summarizing macro- and micronutrients, we focus on the dietary reference intakes (DRIs) for the needs of children and adolescents.4 Definitions of the terminology related to the DRIs can be found elsewhere in this issue of Nutrition. For comparison purposes in Tables II through IV, we list the estimated average requirement as the point of reference. Use of other standards such as the recommended daily allowance would not be appropriate because the recommended daily allowance is designed to address the needs of individuals. The estimated average requirement refers to an average daily nutrient intake that is estimated to meet the requirements of 50% of the healthy individuals in a particular age or gender group.

GENERAL NUTRITIONAL CONSIDERATIONS FOR YOUNG ATHLETES Fluids and Hydration Fluid turnover during a 24-h period in adults has been estimated at 2 to 3 L/d.5 To our knowledge, no research has been done to determine daily turnover rates in children or adolescents by using tracer methodology. Ballauff et al.6 estimated water balance by measuring intake versus output in children ages 6 to 11 y and reported turnover to be approximately 1.6 L/d. Based on a urine 0899-9007/04/$30.00 doi:10.1016/j.nut.2004.04.002

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TABLE I. SWEAT RATES IN CHILD AND ADOLESCENT ATHLETES* Subjects Triathletes59† Triathletes59† Triathletes59† Triathletes59† Team and individual sports118§ Team and individual sports118§ Tennis, martial arts, wrestling, boxing, track and field, and swimming11 Tennis case report49

Exercise conditions

Mean age (y)

Laboratory-based, duathlon simulation (run-cycle-run) Laboratory-based, duathlon simulation (run-cycle-run) Laboratory-based, duathlon simulation (run-cycle-run) Laboratory-based, duathlon simulation (run-cycle-run) Laboratory-based, run, cycling, elliptical trainer Laboratory-based, run, cycling, elliptical trainer Cycling on an ergometer outdoors in a tropical climate Field-tennis play

12.5–14.8

Sex (n)

Sweat rate (mL/h)

Environmental conditions

Male (7)

640 ⫾ 370‡

NA

12.5–14.8

Female (8)

510 ⫾ 226‡

NA

15.0–17.1

Male (9)

1260 ⫾ 170‡

NA

15.0–17.1

Female (8)

710 ⫾ 379‡

NA

13 ⫾ 1

Male (9)

937 ⫾ 342

DB: 26.5°C; RH: 27.3%

17 ⫾ 1

Female (6)

567 ⫾ 165

DB: 26.5°C; RH: 27.3%

11–14

Male (12)

566 ⫾ 110

17

Male (1)

2500

DB: 33.0°C; RH: 58.5%; WBGT: 30.4°C; Black globe: 44°C DB: 31.6°C; RH: 62%; 50% cloud cover

* Mean ⫾ standard deviation. † Significant age effect on sweat rate. ‡ Converted from kilogram per hour assuming 1:1 for kilogram:1000 mL. § Sweat rates were significantly lower than those of adults who exercised under the same conditions and intensity. DB, dry-bulb temperature; NA, not available; RH, relative humidity; WBGT, wet-bulb globe temperature

volume of 22 g · kg⫺1 · d⫺1 and the expected volume needed per kilocalorie of energy expended, the investigators suggested that fluid intake should be greater than what they observed in the young subjects. The children studied were homebound and not participating in sports at the time of measurements. The effect of sports on fluid needs of young athletes would increase because of sweat losses during training sessions and competitive events. Table I summarizes the few studies on sweat rates in young athletes and indicates that fluid needs may increase above baseline needs by 0.5 to greater than 1.0 L/d. The importance of maintaining adequate fluid balance is in part to prevent dehydration and sustain normal cardiovascular and thermoregulatory functions required for exercise performance. Compared with adults, children may experience greater heat stress when exercising in hot environments because of greater heat accumulation from the environment. Children have a greater ratio of surface area to body mass and absorb environmental heat more readily than do adults.7 In addition, non-athletic children generate a faster metabolic rate and greater heat production during physical activities as compared with adults working at the same relative workload.8 It is unclear whether child and adolescent athletes would also be less work efficient. Within the same age group, athletes might differ from non-athletic children by having a greater efficiency of movement and ability to relax antagonistic muscle groups, thereby conserving energy and producing a less impeded force production. Further, the sweating capacity is typically reported to be lower in children.9 With maturation, there appears to be an increase in sweat rate when adjusting the data for body surface area.10 In addition, young athletes who are acclimated to hot environments may have higher sweat rates,11 although this has yet to be confirmed in well-controlled studies. Aside from increasing the risk of heat illness, dehydration is strongly associated with fatigue during exercise. Dehydration of a minimum of 2% of body weight in adults is often reported to decrease endurance and work capacity.12 A 1% decrease in body weight from exercise-induced sweating decreases endurance in children.13 Thus, dehydration decreases performance by affect-

ing the cardiovascular system, thermoregulation, and central fatigue (perceived exertion). It is not clear what level of dehydration affects endurance performance of adolescents, but one might expect adolescents to follow a pattern similar to that of the adult.

Energy and Macronutrients ENERGY. Children and adolescents need adequate energy intake to ensure proper growth, development, and maturation (DRI in press: http://www.nap.edu/books/0309085373/html/). The athletic or very active child or adolescent generally will have needs in excess of this level due to the greater energy expenditure from the physical activity. It is difficult to establish a DRI for energy for this group because of large interindividual variability, especially because the onset of the growth spurt, which is a major impetus for increased energy requirements, is unpredictable in adolescents. The estimated energy requirement set by the Food and Nutrition Board has recommended an energy intake based on equations that consider the individual’s age, reference height and body weight, and a physical activity classification of sedentary, moderately active, active, and very active. Among 9- to 13-y-old children, energy requirements range from 1415 kcal/d for the sedentary 9-y-old girl to 3038 kcal/d for the very active 13-y-old boy. Among adolescents ages 14 to 18 y, energy requirements range from 1718 kcal/d for the sedentary 14-y-old girl to 3804 kcal/d for the very active 18-y-old boy. These estimates for energy requirements are subject to variability within and between individuals (http://www.nap.edu/books/0309085373/html/). Physical training requires additional calories exceeding those needed for growth and basal energy of children and teens. Routine exercise training may elevate total daily energy expenditure through mechanisms independent of the training demands. In obese children, a daily exercise program increased energy expenditure by 12% more than estimated for the training session.14 This effect has not been investigated in young non-obese athletes. There

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is some evidence showing that, like the obese children, non-obese, non-athletic children undergoing a 6-wk training period may increase total daily energy expenditure more than expected.15 With very heavy training in teen and child athletes on elite squads, behaviors may change to compensate and conserve energy, i.e., the athletes may be less active during non-training periods of the day, but this remains to be studied. PROTEIN. Recommended protein intakes for sedentary children and adolescents are presented in Table II. Although sufficient protein intake is important to provide essential amino acids to support growth, especially for the maintenance and development of lean body mass, adequate energy is also critical. Inadequate energy will cause protein to be used as a substrate for energy rather than for synthesizing lean tissues. It is generally recommended that adults obtain at least 12% to 15% of their dietary energy from protein.16 This recommendation also appears reasonable for the child and adolescent athlete. For example, a sedentary 9-y-old girl (reference weight of 29 kg in accordance with the DRI in press) has an energy requirement of 1390 kcal/d. By consuming 12% to 15% of her daily energy as protein, she will ingest 1.44 to 1.78 g · kg⫺1 · d⫺1. A very active 18-y-old boy (reference weight of 67.2 kg; DRI in press) has an energy requirement of 3804 kcal/d. With 12% to 15% of his energy as protein, he will ingest 1.70 to 2.12 g · kg⫺1 · d⫺1. Future research is needed to confirm this recommendation as being adequate, particularly when a growth spurt occurs during intense training or the competitive season. In addition, subsequent research is needed to determine what level of protein is adequate if energy intake is inadequate due to excess expenditure or restricted energy intake to reduce body fatness and simultaneously maintain lean body mass. Heavy and regular exercise training increases the daily protein requirements in adult athletes.17 No data are available on the requirements for young athletes. In one of the few studies on children (non-athletes), protein turnover was measured during the baseline period and immediately after a 6-wk period of mild training.15 The children engaged in a mild exercise training program that consisted of walking 45 to 60 min for 5 d/wk for 6 wk. The intensity was not accounted for, but the young subjects walked 3.2 to 6.4 km/d. Rather than resulting in an increased need, exercise training decreased rates of protein synthesis and breakdown to conserve protein, perhaps because the children did not compensate with an increased energy intake to match the increased need due to the training. Young athletes train at higher intensities and attain greater volumes of work than walking for up to 60 min; hence, protein needs may increase for sports. The combination of the heavy training and increased protein intake also may influence protein turnover and perpetuate the need for greater intake. In most cases, athletes will spontaneously increase their food intake for energy and naturally meet their protein needs. Assessments of protein intake in young athletes have shown that on average and per kilogram of body weight, most athlete groups obtain enough protein and easily meet the estimated higher needs (Table II). CARBOHYDRATE. The DRIs for carbohydrate (Table II) are based on the amount needed to provide glucose to the brain and do not consider needs of the muscle for glycogen replacement. In adults performing prolonged or repeated high-intensity exercise, carbohydrate is considered the limiting fuel for maintaining work rate.18 –20 Children are thought to lack the full development of glycolytic capacity,21–23 so fat may play as important a role as carbohydrate in supporting performance. The difference between childhood and adulthood in muscle enzymatic capacity for glycogenolysis may disappear during the adolescent period. Compared with values in adults, little or no difference has been seen in muscle glycolytic enzymes in adolescents ages 13 to 15 y.24

Nutrition Volume 20, Numbers 7/8, 2004 Whether young athletes benefit from a high-carbohydrate diet remains to be determined. This does not mean that foods containing high amounts of carbohydrates are not important in the diet of children and teen competitors. Grain-based foods, vegetables, and fruit contribute significant amounts of carbohydrate fuel for exercise, fiber, minerals, and vitamins and contribute to restoring muscle glycogen that is needed for training and competition. Because of the importance of carbohydrate as substrate for highintensity training, we recommend that young athletes consume at least 50% of their total daily energy intake as carbohydrate. FAT. Although there is no adequate intake (AI) or recommended daily allowance set for total fat, there are AI levels set for the essential fatty acids, linoleic acid, and linolenic acid. The AI levels for boys ages 9 to 13 y are 12 g/d and 1.2 g/d for linoleic acid and linolenic acid, respectively. For adolescent boys (14 to 18 y), the levels are slightly higher at 16 g/d and 1.6 g/d, respectively. For young girls, AI levels are 10 g/d and 1.0 g/d for linoleic acid and linolenic acid, respectively. Adolescent girls have AI levels of 11 g/d and 1.1 g/d, respectively, for these two essential fatty acids. It has been recommended that 25% to 30% of total daily calories come from fats.25 Unsaturated fats should contribute most of the fat-derived energy from the diet, with saturated fats providing no more than 10% of the total daily calories.26 Dietary fat also aids in the absorption of essential fat-soluble vitamins and carotenoids, all of which are needed for health. Restricting fat intake in the healthy, non-obese child could impair growth and development; however, it is not clear whether this is a direct effect of fat restriction or energy restriction.27 Athletes, young or adult, may reduce dietary fat intake to create an energy deficit for the purpose of weight control. Decreasing the energy intake by reducing the fat contribution may be a sound strategy provided the adolescent athlete has an additional amount of body fat exceeding the minimum amount needed for health. The level recommended as the minimum to sustain health and fitness is 7% for males and 14% for females.28 Completely abstaining from certain foods that can contain high amounts of fat, i.e., dairy products and red meat, is not recommended. Elimination of such foods from the diet also could create deficits in intake of highquality protein, calcium, magnesium, iron, zinc, chromium, vitamin B12, and fat-soluble vitamins, which are critical to optimal growth. Although manipulation of macronutrient content of the diet to influence performance continues to be studied, increasing the intake of dietary fat at the expense of carbohydrate has seldom, if ever, provided an advantage in physical performance over that of a high-carbohydrate diet in adults.29 –33 Similar to the experimental research on carbohydrate diets in children and adolescents, no one has investigated the effect of adjusting fat intake on performance in youth. Nonetheless, dietary fat provides energy for growth needs of children and adolescents and contributes essential fatty acids to their diet. In addition, the young athlete might be able to afford a slightly higher fat intake than the sedentary counterpart because of the increased energy expenditure during training.

Micronutrients Micronutrients perform the same actions in athletes that occur in the non-athlete. For example, vitamins such as niacin or B12 serve as cofactors for facilitating metabolic reactions regulated by enzymes and the synthesis of specific tissues. Minerals play key roles in formation of body tissues (calcium in bone), maintenance of fluid balance within specific compartments (sodium for extracellular fluid space and potassium for intracellular fluid space), and excitation of tissues (action potentials and signal transmission in nerve and muscle tissue). Readers are referred elsewhere for comprehensive reading on basic nutrition.25,26,34 –36

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TABLE II. ESTIMATED AVERAGE REQUIREMENTS AND MACRONUTRIENT INTAKES PER DAY IN YOUNG ATHLETES*

Sport

Sex

EAR EAR EAR EAR Endurance sports Walking (n ⫽ 7)119 Swimming (n ⫽ 22)69 Mixed endurance† (n ⫽ 33; iron deficient)47 Mixed endurance† (n ⫽ 93; normal iron stores)47 Swimming (n ⫽ 5)71 Swimming (n ⫽ 5)60 Swimming (n ⫽ 15)61 Cross-country running (n ⫽ 9)71 Strength and weight class sports Gymnastics (n ⫽ 29)121 Gymnastics (n ⫽ 22)121 Figure skating (n ⫽ 12)122 Gymnastics (n ⫽ 7)120 Figure skating (n ⫽ 48)123 Gymnastics (n ⫽ 88)82 Figure skating (n ⫽ 16)122 Figure skating‡ (n ⫽ 18)83 Figure skating (n ⫽ 46)123 Wrestling (n ⫽ 18)86

M F M F

Judo (n ⫽ 16)125 Gymnastics (n ⫽ 26)85 Karate (n ⫽ 16)124 Karate (n ⫽ 29)124 Judo and Wrestling (n ⫽ 10)84 Team sports Ice hockey (n ⫽ 49)37 US football (n ⫽ 46)126 US football (n ⫽ 88)126 Volleyball (n ⫽ 65)127 Soccer (n ⫽ 8)128 Basketball (n ⫽ 13)129

Age (y)

Energy (kcal)

Energy (kcal/kg)

9–13 9–13 14–18 14–18

CHO (g)

PRO (g)

100 100 100 100

PRO (g/kg)

Fat (g)

0.77 0.73 0.75 0.73

M,F M F

8–10 11.3 ⫾ 2.3 17.2 ⫾ 1.3

1844 ⫾ 243 NA 2757 ⫾ 776

NA 58.5 44.2 ⫾ 12.4

NA NA 368 ⫾ 101

70 ⫾ 0.3 NA 85.1 ⫾ 26.9

NA 2.32 1.4 ⫾ 0.4

NA NA 104.9 ⫾ 39.6

F

17.4 ⫾ 1.4

2611 ⫾ 852

42.7 ⫾ 13.9

352 ⫾ 127

77.5 ⫾ 24.6

1.3 ⫾ 0.4

99.2 ⫾ 36.6

F F F F

18.8 ⫾ 1.0 19 ⫾ 2.4 19.6 ⫾ 1.2 20.4 ⫾ 2.2

2405 ⫾ 1022 3136 ⫾ 508 2275 ⫾ 665 2312 ⫾ 945

36.9 ⫾ 15.7 NA 34 ⫾ 11 42.4 ⫾ 17.3

NA (68% of E) 362 ⫾ 109 NA

93.3 ⫾ 27.9 (11% of E) 80 ⫾ 25 84.5 ⫾ 31.6

1.4 ⫾ 0.4 NA 1.2 ⫾ 0.4 1.6 ⫾ 0.6

NA (21% of E) 60 ⫾ 24 NA

F F F

7–10 11–14 14 ⫾ 1.6

1,651 ⫾ 363 1706 ⫾ 421 1674 ⫾ 684

NA NA NA

219 ⫾ 57 227 ⫾ 64 241 ⫾ 88

68 ⫾ 17 67 ⫾ 20 66 ⫾ 27

NA NA NA

60 ⫾ 16 62 ⫾ 18 53 ⫾ 34

F F F M

14.5 ⫾ 0.3 15 ⫾ 2.4 15.8 ⫾ 0.6 16 ⫾ 1.5

1553 ⫾ 315 1632 ⫾ 734 1267 ⫾ 136 2325 ⫾ 907

NA NA NA NA

207.1 ⫾ 101 243 ⫾ 111 156 ⫾ 22 308 ⫾ 126

50.3 ⫾ 6.4 70 ⫾ 35 80.7 ⫾ 14.7 94 ⫾ 35

NA NA 1.89 ⫾ 0.35

58.1 ⫾ 18 45 ⫾ 28 39.2 ⫾ 12.2 82 ⫾ 43

F

16 ⫾ 1

NA

212 (170, 253)

64 (64, 65)

NA

62 (50, 74)

NA

374 ⫾ 122

104 ⫾ 41

NA

86 ⫾ 34

41.1 ⫾ 11.8 26.8 ⫾ 17.4 NA

367 ⫾ 123 209 ⫾ 136 333 ⫾ 367 180 ⫾ 61 266 ⫾ 60.8 37.5 ⫾ 109 302 ⫾ 152 239 ⫾ 177 182 ⫾ 174

94 ⫾ 28 61 ⫾ 41 103 ⫾ 104 53.5 ⫾ 20 64.7 ⫾ 16.1 89.8 ⫾ 24.5 107 ⫾ 54 71 ⫾ 51 56 ⫾ 54

1.43 ⫾ 0.4 0.96 ⫾ 0.6 NA

104 ⫾ 36 66 ⫾ 45 100.5 ⫾ 126 47.4 ⫾ 29 65.1 ⫾ 18.8 83.1 ⫾ 28.9 98.0 37.5 40.6

56.8 ⫾ 13.6 43 ⫾ 16 48 ⫾ 21 25.7 62 ⫾ 12 30 ⫾ 8

NA 302 ⫾ 125 366 ⫾ 170 195 ⫾ 88 526 ⫾ 62 NA

NA 91 ⫾ 34 133 ⫾ 77 62 ⫾ 26 142 ⫾ 23 (165% of RDA)

2.2 ⫾ 0.5 1.5 ⫾ 0.6 1.9 ⫾ 1.0 1.0 ⫾ 0.4 NA NA

1630 (1621, 1639)

M

17.2 ⫾ 3

2649 ⫾ 861

M

16.0 ⫾ 1.9

M F F M M

18.4 ⫾ 6.4 19.7 ⫾ 0.1 19.7 ⫾ 1 20.1 ⫾ 1.3 21.6

Pre 2713 ⫾ 785 WL 1707 ⫾ 1107 3804 ⫾ 764 1381 ⫾ 556 1947 ⫾ 398 2763 ⫾ 741 Pre 2517.6 GWL 1577.8 RWL 1317.6

M M M F M F

12.5 ⫾ 0.5 12–14 15–18 14–19 17 ⫾ 2 19 ⫾ 0.3

2437 ⫾ 502 2523 ⫾ 936 3365 ⫾ 1592 1648 ⫾ 780 3952 ⫾ 1071 1995 ⫾ 151

34.9 ⫾ 8.1 42.3 ⫾ 12.7 NA

1.17 ⫾ 0.38 1.38 ⫾ 0.46 NA

NA 109 ⫾ 59 154 ⫾ 90 73 ⫾ 49 142 ⫾ 17 NA

* Mean ⫾ standard deviation for sports listed. † Mixed endurance: athletes participating in running, rowing, swimming, cross-country skiing, orienting, and pentathlon. ‡ Numbers in parentheses are 95% confidence intervals. CHO, carbohydrate; E, energy; EAR, estimated average requirement; F, female; GWL, gradual weight loss; M, male; NA, not available; Pre, preseason diet before weight loss; PRO, protein; RWL, rapid weight loss; WL, in-season diet during a decrease of approximately 3.5% of body weight; US, United States

VITAMINS. The estimated average requirements for vitamin intake in children and adolescents are presented in Table III. According to research on dietary intake of young athletes, most consume an amount of vitamins that achieves or comes close to

achieving the daily requirements and more so than their nonathletic peers.37,38 Current research does not support increased vitamin requirements because of the physical activity or training that young athletes undertake. Because energy intake is typically

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TABLE III. ESTIMATED AVERAGE REQUIREMENTS AND SELECTED VITAMIN INTAKES PER DAY IN YOUNG ATHLETES* B1 (mg)

B2 (mg)

Niacin (mg)

B6 (mg)

B12 (␮g)

Folacin (␮g)

C (mg)

E (mg)

D (␮g)‡

445 420 630 485

0.7 0.7 1.0 0.9

0.8 0.8 1.1 0.9

9 9 12 11

0.8 0.8 1.1 1.0

1.5 1.5 2.0 2.0

250 250 330 330

39 39 63 56

9 9 12 12

5 5 5 5

NA NA 1.9 ⫾ 0.9

NA NA 2.2 ⫾ 0.7

NA NA NA 20.0 ⫾ 8.4

NA 2.2 ⫾ 1.0 2.0 ⫾ 0.9 NA

NA 5.0 ⫾ 3.7 4.7 ⫾ 4.9 NA

NA 346 ⫾ 115 301 ⫾ 128 309 ⫾ 179

78.2 74.7 ⫾ 42.5 75.0 ⫾ 58.0 178.9 ⫾ 131.6

NA NA NA NA

NA NA NA NA

1,031 ⫾ 688 1127 ⫾ 750 5518 ⫾ 4494 6196 ⫾ 6749 1913 ⫾ 1137 6617 (3118–10116) 2106 ⫾ 1877

1.4 ⫾ 0.3 1.5 ⫾ 0.5 1.5 ⫾ 0.7 2.1 ⫾ 0.8 0.8 ⫾ 0.1 2 (1–2) 1.1 ⫾ 0.3

1.8 ⫾ 0.5 1.8 ⫾ 0.6 1.9 ⫾ 0.8 2.5 ⫾ 1.1 1.2 ⫾ 0.3 2 (2–3) 1.1 ⫾ 0.4

17.4 ⫾ 3.9 18.2 ⫾ 6.5 18 ⫾ 8 26 ⫾ 12 NA 22 (17–27) NA

NA NA 1.4 ⫾ 0.8 1.6 ⫾ 1 NA 2 (1–2) NA

NA NA 3⫾2 5⫾4 NA NA NA

NA NA 250 ⫾ 146 233 ⫾ 138 NA 200 (147–253) NA

129 ⫾ 95 145 ⫾ 85 126 ⫾ 96 112 ⫾ 103 86 ⫾ 66 NA 71 ⫾ 38

NA NA 2.7 ⫾ 2.4 3.2 ⫾ 4 NA NA NA

NA NA 3.4 ⫾ 3.0 3.8 ⫾ 3.8 NA 3.2 (1.3–5.0) NA

1680 ⫾ 1820 6063 ⫾ 6130¶ 8025 ⫾ 8658¶ 544 ⫾ 495 934 ⫾ 1021

2.0 ⫾ 0.6 1.5 ⫾ 1.0 2.2 ⫾ 1.6 0.9 ⫾ 0.5 3.91 ⫾ 0.88

2.8 ⫾ 0.8 NA NA 1.2 ⫾ 0.6 2.48 ⫾ 0.56

36 ⫾ 8 NA NA 20 ⫾ 19 35.1 ⫾ 6.7

NA NA NA 0.8 ⫾ 0.7 3.30 ⫾ 0.74

NA NA NA 2.3 ⫾ 1.7 10.7 ⫾ 9.2

NA NA NA 163 ⫾ 98 905 ⫾ 286

161 ⫾ 85 103 ⫾ 93 180 ⫾ 239 93 ⫾ 91 520 ⫾ 173

11.6 ⫾ 4.2 NA NA NA 46 ⫾ 22.0

4.0 ⫾ 2.0 NA NA NA NA

1069 ⫾ 437

* Mean ⫾ standard deviation for sports listed. †Retinol activity equivalents or retinol equivalents. ‡ Adequate intake. § Endurance: athletes participating in running, rowing, swimming, cross-country skiing, orienting, and pentathlon. 㛳 Numbers in parentheses are 95% confidence intervals. ¶Values listed in reference as international units. EAR, estimated average requirement; NA, not available; US, United States.

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EAR males, 9–13 y EAR females, 9–13 y EAR males, 14–18 y EAR females, 14–18 y Endurance sports Swimmers (n ⫽ 22, 11.3 y, male)69 Endurance§ (n ⫽ 33, 17.2 y, iron-deficient female)47 Endurance§ (n ⫽ 93, 17.4 y, normal iron stores, female)47 Swimmers (n ⫽ 15, 19.6 y; female)61 Strength and weight-class sports Gymnastics (n ⫽ 29, 7–10 y)121 Gymnastics (n ⫽ 22, 11–14 y)121 Figure skating (n ⫽ 16, 14 y; female)122 Figure skating (n ⫽ 16, 16 y; male)122 Karate (n ⫽ 16, 19.7 y, female)124 Figure skating㛳 (n ⫽ 18, 16 y, female)83 Karate (n ⫽ 29, 20.1 y, male)124 Team sports Ice hockey (n ⫽ 49, males, 12.5 y)37 US football (n ⫽ 46, males, 12–14 y)126 US football (n ⫽ 88, males, 15–18 y)126 Volleyball (n ⫽ 65, female, 14–19 y)127 Soccer (n ⫽ 8, male, 17 y)128

A (␮g)†

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TABLE IV. ESTIMATED AVERAGE REQUIREMENTS AND SELECTED MINERAL INTAKES PER DAY IN YOUNG ATHLETES*

EAR males, 9–13 y EAR females, 9–13 y EAR males, 14–18 y EAR females, 14–18 y Endurance sports Swimming (n ⫽ 22, 11.3 y, male)69 Endurance† (n ⫽ 33, 17.2 y, irondeficient female)47 Endurance† (n ⫽ 93, 17.4 y, normal iron stores, female)47 Swimming (n ⫽ 5, 18.8 y, female)71 Swimming (n ⫽ 15, 19.6 y, female)61 Cross-country running (n ⫽ 9, 20.4 y, female)71 Strength and weight-class sports Gymnastics (n ⫽ 29, 7–10 y, female)121 Gymnastics (n ⫽ 22, 11–14 y, female)121 Gymnastics (n ⫽ 26, 19.7 y)85 Figure skating (n ⫽ 19, 14 y, female)122 Figure skating (n ⫽ 48, 15 y, female)123 Figure skating (n ⫽ 15, 16 y, male)122 Figure skating‡ (n ⫽ 18, 16 y, female)83 Figure skating (n ⫽ 46, 17.2 y, male)123 Karate (n ⫽ 16, 19.7 y, female)124 Karate (n ⫽ 29, 20.1 y, male)124 Team sports Ice hockey (n ⫽ 49, males, 12.5 y)37 US football (n ⫽ 46, males, 12–14 y)126 US football (n ⫽ 88, males, 15–18 y)126 Volleyball (n ⫽ 65, female, 14–19 y)127 Soccer (n ⫽ 8, male, 17 y)128

Ca (mg)

Cu (mg)

Fe (mg)

Mg (mg)

P (mg)

Se (␮g)

Zn (mg)

1300 1300 1300 1300

540 540 685 685

5.9 5.7 7.7 7.9

200 200 340 300

1055 1055 1055 1055

35 35 45 45

7 7 8.5 7.3

NA 1066 ⫾ 606㛳

NA 1.5 ⫾ 0.5

12.8 15.0 ⫾ 4.6

NA NA

NA NA

NA NA

NA NA

785 ⫾ 402

1.4 ⫾ 0.5

14.5 ⫾ 4.8

NA

NA

NA

NA

NA 1246.6 ⫾ 387

1.4 ⫾ 1.1 NA

31.4 ⫾ 21.7§ 18.5 ⫾ 9.3

NA NA

NA NA

NA NA

NA NA

NA

1.5 ⫾ 0.9

89.5 ⫾ 94.5§

NA

NA

NA

NA

NA

1135 ⫾ 237

NA

NA

1116 ⫾ 388

NA

NA 197 ⫾ 94

961 ⫾ 331 990 ⫾ 509

NA NA

NA 8⫾4

840 ⫾ 297

NA

11 ⫾ 3

867 ⫾ 403

NA

11 ⫾ 4

683 ⫾ 296 904 ⫾ 499

NA NA

11.8 ⫾ 5 13 ⫾ 6

873 ⫾ 457

NA

13 ⫾ 7

NA

NA

NA

NA

1032 ⫾ 497

NA

19 ⫾ 10

211 ⫾ 101

1164 ⫾ 591

NA

9⫾5

828 (558–1096)

NA

16 (10–21)

224 (164–284)

983 (759–1208)

NA

7 (5–9)

1237 ⫾ 638

NA

21 ⫾ 14

NA

NA

NA

NA

461 ⫾ 126 421 ⫾ 198

NA NA

8.2 ⫾ 2.0 10.8 ⫾ 2.9

113 ⫾ 24 154 ⫾ 50

940 ⫾ 186 1259 ⫾ 331

NA NA

NA NA

1510 ⫾ 390 1261 ⫾ 655

1.6 ⫾ 0.6 NA

16 ⫾ 4 15 ⫾ 11

375 ⫾ 80 482 ⫾ 287

NA NA

NA 3211 ⫾ 1404

15 ⫾ 3 13 ⫾ 8

1737 ⫾ 1359

NA

20 ⫾ 12

634 ⫾ 437

NA

4102 ⫾ 2204

17 ⫾ 12

935 ⫾ 497

NA

7.9 ⫾ 4.0

207 ⫾ 130

NA

NA

7⫾5

1072 ⫾ 246

2.41 ⫾ 0.66

22.0 ⫾ 5.5

500 ⫾ 115

2113 ⫾ 460

2721 ⫾ 588

19.7 ⫾ 4.6

* Mean ⫾ standard deviation for sports listed. † Endurance: athletes participating in running, rowing, swimming, cross-country skiing, orienting, and pentathlon. ‡Numbers in parentheses are 95% confidence intervals. § Includes supplement use. 㛳 Significantly higher calcium intakes in iron-deficient group. Ca, calcium; Cu, copper; EAR, estimated average requirement; Fe, iron; Mg, magnesium; NA, not available; P, phosphorus; Se, selenium; US, United States; Zn, zinc.

elevated to meet the demands for training, requirements for several of the B vitamins may be similarly increased. Table III summarizes the available literature on vitamin intake of young athletes. The vitamins K, pantothenic acid, and biotin are not reviewed because of the absence of survey data in the literature. MINERALS. General mineral requirements are listed in Table IV as are descriptive data from survey research on mineral intake of young athletes. The minerals fluoride, chromium, iodine, man-

ganese, and molybdenum are not summarized due to the lack of survey data on intake among young athletes. Research on adults has shown that, with the exception of those minerals lost in high amounts in sweat, elevated metabolism through exercise does not increase mineral requirements.39,40 Similarly, in children, the need for minerals is not thought to change because of initiating training for sports participation. As described below, intake of electrolytes lost in high volume due to sweat concentration and/or large losses of sweat may need to increase to prevent deficits from occurring.

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In the diet of children and teens, iron and calcium are two minerals frequently identified as being deficient and could affect health and physical performance, particularly in female athletes.41 It is now thought that peak bone mass, achieved in females by their late 20s, is critical to preventing or minimizing the risk of osteoporosis as women age.42 Calcium intake during the period of childhood to adulthood will be key to achieving an optimal peak bone mass. Inadequate intake during training and competition as a youth has been associated with stress fractures, although it is difficult to arrive at a cause-and-effect relation. Inadequate intake of energy, protein, and vitamin D that parallel low-calcium intake may influence other factors (estrogen hormone levels) that also negatively affect bone health. Low iron intake, particularly during the adolescent years of females when the menstrual cycle has started, may be a cause of poor performance. Inadequate iron intake that does not necessarily produce overt anemia can still decrease physical or mental performance in youth.43 Chronic inadequate intake can lead to low stores of iron that impair muscle metabolism44 and affect cognitive function.45 The recommendation is to obtain an adequate intake and optimize conditions and forms of iron that are readily absorbable. Increasing the consumption of iron in the diet would be assumed to alleviate nutritional deficiencies, but the interactions with other nutrients should not be ignored. For example, Telford et al.46 used the food-frequency questionnaire to assess typical eating patterns of 78 adolescent and young adult athletes. The investigators found that the variation of plasma ferritin among athletes was not influenced by total iron or meat intake. The best predictor of iron status was the proportion (not absolute intake) of total protein in the diet. Possible explanations for this finding were that carbohydrate and fat ingestion were negatively correlated with plasma ferritin, so that these substrates may inhibit iron absorption if consumed in high proportions, whereas protein works to facilitate the absorption of iron. Malczewska et al.47 found among adolescent female endurance athletes that those with iron deficiency had significantly greater calcium intake than did non-athletic peers with normal iron stores. Calcium is well known to interfere with the absorption of heme and non-heme forms of iron.48 These findings point to the need of further research on potential interactions of nutrients that optimize or impede bone health and development. In addition, electrolytes lost in sweat may pose another mineral balance problem for those athletes undergoing prolonged training in which sweat losses are great. Whereas children and teens have been reported to have lower sodium concentrations in the sweat lost during exercise, the losses may be quite variable across individual athletes and depend on the sweat rate in determining the absolute loss during an exercise session. A case report of an elite junior tennis player (age 17 y) indicated a loss of approximately 90 mM of sodium during slightly less than 2 h of play.49 This was approximately the same as the total daily intake of sodium (87 to 174 mM depending on the day of evaluation). Although the player’s sweat sodium concentration was relatively low at 36 mmol/L, the high sweat losses (close to 5 L) contributed to an acute sodium deficit. The recommendation for this player, who had a history of exercise-induced muscle cramps when playing in the heat, was to increase sodium and fluid intake to match losses during play or compensate for the losses at meals between matches. MULTIVITAMIN AND MULTIMINERAL SUPPLEMENTS. Teenage athletes are very likely to use supplements in hopes of enhancing recovery and avoiding illness. A study of 1355 Korean adolescent athletes found that approximately 36% of the male and female athletes use vitamins and mineral supplements.50 In the United States, the average prevalence of such supplement use by athletes has been estimated to be 46%.51 The analysis spanned several studies that included elite, collegiate, and high school

Nutrition Volume 20, Numbers 7/8, 2004 athletes, and the prevalence among the younger athletes alone appears to be lower than that among elite athletes. Despite the interest in and use of supplements by adolescents, research has not shown that vitamin supplements enhance growth, lean body mass, or physical performance in healthy, wellnourished adult or adolescent athletes.52–55 The work of Telford et al.56 demonstrated this in late-adolescent athletes with average ages of 18.6 y (supplement treated) to 19.3 y (placebo). In that study, athletes, involved in a mixture of sports, were randomly assigned to receive a placebo or a multivitamin, multimineral supplement that provided 100% of the daily requirements. Treatment was provided for an approximately 8-mo period and was found to increase blood indices of vitamin B1, B6, B12 and folic acid status, and ensured the subjects of achieving their daily needs. Nonetheless, measurements of sprint speeds, flexibility, maximum oxygen uptake, shoulder strength, and swim time, for the swimmers who participated, were no different before and after treatment. The lone performance improvement occurred for the vertical jump in the female basketball players. These players also exhibited an increase in skinfold thickness and body weight (presumably body fatness) compared with their placebo control peers. It is more likely that an overall improvement in their nutritional status, as shown by the apparent gain in body fatness coincidental with the supplement treatment, explained the improved leg power. Our recommendation is that, because young athletes generally consume more food for energy, they obtain adequate amounts of vitamins and minerals from dietary sources. Supplements are not needed. In the next sections, however, we address some of the minerals that may fall short in adolescent athletes involved in certain sports.

NUTRITION SPECIFIC TO YOUNG ENDURANCE SPORT ATHLETES Endurance sports include running, cycling, swimming, and crosscountry skiing and are distinguished from other sports by the duration of the training and competition. In addition, the intensity of effort is sustained at a fairly constant submaximal intensity. The prolonged nature of these sports requires attention to intakes of adequate fuel and fluids. With regard to micronutrients, research has shown that iron and calcium status or intakes may require special consideration by this group. Fluids The duration and intensity of endurance training, especially when it occurs in a hot, humid environment, can predispose the endurance athlete to significant fluid and electrolyte loss.57 Similar to adults, when children or adolescents are provided only water to drink they do not replace their losses as well as they do with a flavored drink or a flavored carbohydrate-electrolyte solution.11,58 The presence of sodium in a beverage not only enhances the drive to drink but also helps to replace some of the sodium lost in sweat. Older adolescents tend to sweat more than younger adolescents.10,59 A group in Australia studied the sweat losses of male and female elite du- and triathletes ages 15 and older versus those younger than 15 y.59 The older group tended to have greater sweat rates then the younger group, with the older males having a mean sweat rate of nearly 1.3 L/h, which was higher (P ⬍ 0.05) than that of the other groups. Interestingly, both age groups were permitted ad libitum consumption of water during a simulated duathlon race, and neither group was able to maintain euhydration despite unlimited access to water. Energy Few studies have explored energy expenditure and, consequently, the needs of endurance training in adolescents. Trappe et al.60 used

Nutrition Volume 20, Numbers 7/8, 2004 the doubly-labeled water technique to assess the energy cost of high-volume swim training in a group of older adolescents (mean age 19 y). These female athletes swam nearly 20 km/d to expend approximately 5600 kcal and had an energy intake of approximately 3100 kcal. It is likely that athletes under-reported intakes, and with only 5 d of observation there was insufficient time to determine whether body mass was declining as a result. Regardless, this study provided direct evidence for the large energy demands of endurance sports. Ousley-Pahnke et al.61 found that, even during the tapering phase of swim training, energy expenditure was very high at 2342 ⫾ 158 kcal. There is also research to suggest that children require more energy per kilogram of body weight during physical activity than do adults.62 Many studies that have used dietary records have found that young athletes, in particular female athletes, have low energy intakes.63 Inadequate energy intake in combination with high energy expenditures can create a negative energy balance that can produce a delay in the onset of puberty, short stature, low bone mineral density, increased risk of injury and slower recovery, menstrual irregularities, dehydration, and nutrient deficiencies. For further information, readers are encouraged to consult other references that go beyond the scope of this paper.48,64 – 66 Macronutrients Little research has been conducted on the protein or carbohydrate needs of the child and adolescent athlete. Consistent with studies on adult athletes, the needs of both macronutrients are likely greater due to exercise.16,67 It is recommended that the adult endurance athlete generally consume carbohydrate in an amount of 5 to 10 g · kg⫺1 · d⫺1 to ensure adequate replenishment of glycogen levels and protein in an amount of 1.2 to 1.7 g · kg⫺1 · d⫺1 to maintain lean body mass. As the intensity of the physical activity increases, so does the body’s reliance on carbohydrate as fuel. Stored carbohydrate is a special consideration for the endurance athlete because it is available in such limited supply. During sustained exercise at fairly high intensity, over time the endogenous carbohydrate supplies are depleted and will force the athlete to reduce the level of intensity, if the exercise is to be sustained (http://www. nap.edu/books/0309085373/html/). When glycogen stores are depleted, the athlete can obtain as much as 5% to 10% of energy needs from oxidation of body protein during exercise. Because this is an undesirable response in growing young athletes, the recommendation of adequate carbohydrate intake to maintain adequate muscle and liver glycogen concentrations is further justified.68 The DRI recommends a range of 0.73 to 0.85 g of protein per kilogram of body weight for sedentary boys and girls ages 9 to 18 y (http://www.nap.edu/books/0309085373/html/). It is not yet known how these requirements increase in children due to endurance training. Based on research conducted on endurance and strength in adult athletes, many sports nutritionists and exercise physiologists recommend protein intakes for these groups that exceed the DRI (1.2 to 1.6 g · kg⫺1 · d⫺1 and 1.2 to 1.7 g · kg⫺1 · d⫺1, respectively).16 They also recommend 1.5 g · kg⫺1 · d⫺1 of protein for the adolescent in the midst of a growth spurt. Provided the athlete is consuming adequate energy from a variety of foods, consumption of adequate protein generally is not an issue.

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include increased fecal and urinary losses, increased erythrocyte hemolysis and turnover, increased gastrointestinal blood losses, and increased blood volume due to training. If present, these factors would increase the iron needs of a young athlete.70 Elevenyear-old boys participating in 6 mo of intense endurance swim training combined with cross-training experienced a progressive and significant decline in serum ferritin when compared with age-matched non-athletic controls. Such a decline occurred despite dietary iron intakes that met the recommendation of 12 mg/d.69 Rowland et al.70 found similar results among six cross-country running teams during their competitive season, with approximately 40% of female and 17% of male runners classified as iron deficient, not anemic. The incidence of iron deficiencies increased as the season progressed. Marginal iron status can inhibit optimal performance. Dietary surveys of athletes often report iron intakes to be inadequate.71 It should be noted that dietary records are inherently inaccurate, with underreporting being a common problem, especially among females.72 However, it has been observed that many groups of endurance athletes eat significantly more food and subsequently may obtain more micronutrients than their sedentary counterparts.47 Although exercise itself does not increase the body’s need for dietary calcium per se, the body of an exercise- or diet-induced amenorrheic female athlete may require more. This type of athlete may be at greater risk of decreased bone mineral density and increased stress fractures. Therefore, greater calcium intakes, such as 1500 mg/d, may be necessary.68 These athletes may undergo not only significant energy expenditures but also diminished energy intakes. This may be due to a number of factors, including participation in a sport such as long-distance running, which attracts individuals who are more likely to be obsessed with control of body weight. The intense training of some endurance athletes has been found to decrease concentrations of estrogen and testosterone.73 Of particular concern is the decline in estrogen in some female athletes, which can lead to decreases in bone mineral density. When decreased estrogen is coupled with low intakes of total energy, protein, and calcium, the athlete may be placed at serious risk for stress fractures and possibly premature osteoporosis.73

NUTRITION SPECIFIC TO STRENGTH AND WEIGHT CLASS SPORTS Strength and weight-class sports are defined by repeated bouts of high intensity or maximal exercise typically lasting from a few seconds to 3 min. These sports are anaerobic in nature, meaning that the majority of the energy required to perform is derived from the adenosine triphosphate/creatine phosphate system and from the breakdown of glycogen stores used for anaerobic glycolysis. Because these sports require maximum effort in a minimal amount of time, great caloric expenditure can occur in a very short period. Hence, it is important that athletes in these sports consume adequate calories to fuel the activity but not deplete the calories needed to sustain growth and maturation. As with all athletes, it is important that athletes involved in anaerobic sports maintain adequate hydration and consume the proper amounts of macro- and micronutrients to support optimal performance and optimal growth.

Selected Micronutrients Puberty increases the requirement for iron due to increases in hemoglobin mass, tissue deposition, growth spurt, and onset of menstruation in females.48 Iron depletion and deficiency have been observed in endurance athletes.47 Iron status depends on a balance of intake and absorption versus loss. Regular, intense, endurance exercise may predispose the athlete to greater iron losses. The proposed mechanisms through which iron losses may be increased

Hydration Dehydration can result in decreased athletic performance and greater strain on the cardiovascular system. Typically, during athletic activity, dehydration occurs due to fluid intake not matching fluid losses (i.e., sweating). However, for athletes engaged in weight-class (wrestling and martial arts) and esthetic sports (gymnastics and figure skating), dehydration is often a voluntary prac-

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tice used to reduce body mass. This practice can greatly increase the heat stress placed on the athlete, especially if working out in a heated room.74 –76 In addition, in vitro research has suggested that dehydration of cells can lead to the initiation of protein and glycogen breakdown as a means to adjust cellular osmolarity in response to decreased intracellular fluid volume.77 Conversely, an increase in cellular fluid volume can initiate protein synthesis. Berneis et al.78 found that the latter effect is strong in humans. Thus, it is important for young athletes to match their sweat losses with fluid intake and to avoid dehydration as a means to decrease body weight.

Nutrition Volume 20, Numbers 7/8, 2004 that are associated with triggering an eating disorder include dieting for prolonged periods, frequent changes in body weight, heavy training volume, and traumatic events.94 Third, Deutz et al.95 showed that, in a group of 42 gymnasts and 20 runners, those who engaged in caloric restriction and/or skipping meals had lower resting energy expenditures and higher body fat percentages than did athletes who did not engage in those behaviors. Steen et al.96 reported a similar trend of depressed resting metabolic rate in a cross-sectional study of “weight cutting” adolescent wrestlers, although Melby et al.97 reported this to be a transient effect. Clearly, it is more beneficial for the lean athlete to match caloric expenditure with caloric intake to maintain body mass and promote lean mass development.

Energy and Substrate for Work and Growth For athletes participating in anaerobic sports, much of the energy needed to fuel the activity is derived from glycogen stores in the muscle. If carbohydrate intake is low, glycogen stores will become depleted and performance will decline more rapidly. Therefore, it is important for these athletes to consume adequate amounts of carbohydrate in the diet. It has been advised that athletes engaging in high-intensity exercise consume a diet that consists of 65% to 70% carbohydrate.16 In young adult and older adolescent athletes who manipulated body weight for their sports, hypocaloric, highcarbohydrate diets were associated with enhanced physical performance compared with an isocaloric, low-carbohydrate diet to achieve a similar amount of weight loss.79 – 81 In weight classification sports, a major concern is the effect of caloric restriction on growth and development and on the performance of young athletes. Athletes who do not meet their caloric needs are often deficient in several nutrients.82– 86 The implications this has on growth and performance are not fully understood. Many factors, such as genetics, nutrition, and training intensity and volume contribute to the growth potential of an individual.87 Due to these many factors, the effect of caloric restriction of young athletes is hard to determine. Bass et al.88 compared the growth rate of elite prepubertal female gymnasts with that of moderately active prepubertal female controls. The growth rates of the gymnasts were lower than those of the controls for seated height, femur length, and tibial length. Theintz89 reported that gymnasts who had inferior growth were able to “catch up” once they reduced or discontinued training. A cross-sectional study on adolescent male wrestlers found no difference in linear growth and maturation when compared with recreationally active adolescent males, perhaps because of such catch-up growth.90 The effects of caloric restriction on performance during highintensity exercise are inconsistent. Roemmich and Sinning90 found reductions in knee and elbow strength from preseason to late season in a group of adolescent male wrestlers who engaged in dietary restriction throughout their season. However, another study on adolescent male wrestlers reported no significant change in anaerobic power during the season despite significant weight loss that averaged about 5% of preseason body weight and an estimated 3% reduction in fat-free weight.91 Interestingly, neither study showed an increase in muscle function, which might be expected during a time of growth and heavy training in adolescents. In addition to the impact caloric restriction may have on growth and development, there are other concerns that make the practice inadvisable for young athletes. First, many young athletes have deficient intakes of calcium and iron. This deficiency also has been found in athletes in strength and weight class sports, especially when caloric restriction is involved.85,92 This could have implications for amenorrhea, decreased bone mineral density, growth rate, and impaired performance.64 A second concern is the development of an eating disorder. Eating disorders are more common in athletes than in non-athletes.93 Among athletes, those who engage in esthetic and weight class sports are at greater risk for developing an eating disorder than are athletes involved in sports in which weight and lean body mass are not emphasized.94 Common factors

Protein Adequate protein intake is another important consideration for athletes in strength and power sports. Protein is necessary for the development of muscle that is needed to excel in these sports. In addition, when glycogen stores are low, the body will increase the usage of protein stores for energy.64 It is accepted that athletes require more than the recommended intake for sedentary individuals, but to date, few experimental studies have been conducted specifically on the needs of young athletes. Laskowski and Antosiewicz98 reported that a group of 12 adolescent, elite, judo athletes receiving a protein supplement increased peak work capacity, peak anaerobic power, and work output significantly compared with the control group (i.e., no protein supplementation). The supplement group received soy protein that contributed to an intake of approximately 2.0 g · kg⫺1 · d⫺1 of protein for 4 wk of training compared with the control intake of 1.5 g · kg⫺1 · d⫺1. These findings warrant further investigation with a more rigorous research design. The treatments were not matched for energy intake, and the conclusions of the study are in contrast to the review of adult studies showing no improvement in performance when additional protein is consumed.99

NUTRITION SPECIFIC TO TEAM SPORTS Team sports are characterized by high-intensity, intermittent efforts that are repeated over the duration of the competitive event. The duration of competitive events runs from 30 to 90 min, depending on the sport and level of the youth sport. The combination of the high intensity and repeated nature of the efforts require power, strength, and endurance to maintain high level performance throughout the game. As a consequence, nutritional needs include replacement of fluids and adequate macronutrient intake to prevent fatigue due to dehydration and energy depletion and nutrition to promote recovery and maintenance of the lean body mass that is critical for force production for power, speed, and strength. Fluids In adults, fatigue in team sports may be dictated by dehydration and/or substrate depletion. The effect of dehydration, per se, on performance in child or adolescent athletes has not been investigated, but it would be expected to have a similar negative outcome as in adults.12,100 –104 Rico-Sanz et al.105 demonstrated that, with a 1-wk strategy of drinking to increase body water, late-adolescent soccer players (mean age 17 y) could decrease the effect of environmental heat stress on core temperature while playing a match. Performances of isokinetic strength and a soccer-specific task were not influenced by the increased hydration status. Regardless of the pregame hydration status, players lost approximately 2.9% of body weight as sweat during the match and showed

Nutrition Volume 20, Numbers 7/8, 2004 decreased performance (fatigue) as compared with the prematch assessment for a soccer-specific skill test. Energy and Substrate for Work The task of estimating energy expenditure needs for young athletes in team sports is complicated because assessment requires a steady-state condition and body position. In team sports, the efforts are largely spontaneous and require abrupt changes in body position (jumping and dodging) and speed (acceleration and deceleration), making it difficult, if not impossible, to use traditional methods such as indirect calorimetry. In studies on adults in which the exercise protocol is controlled for work (laboratory-based study) or the amount of exercise time has been accounted for, depletion of muscle glycogen content or a drop in blood glucose with intermittent repeat efforts of high intensity appears to contribute to fatigue. Several investigators have measured reduced glycogen content when comparing muscle samples obtained from a biopsy before and after soccer, ice hockey, and shuttle-run protocols that simulate the efforts of team sports.106 –108 Using dietary intervention in the form of a sports drink, carbohydrate ingestion during the high-intensity efforts helped to sustain sprint capacity109 –111 and maintain leg power (sprint speed and jumping ability)111–113 and motor skill performance in the later period of the performance test.111,113 Two initial reports have supported this benefit in adolescent athletes,114,115 but additional research is needed to confirm the effect. The muscle biopsy technique is not feasible in children or adolescents; hence, researchers have employed non-invasive methods such as nuclear magnetic resonance to assess muscle and explore the capability for glycolytic metabolism in children and late-adolescent athletes. Using nuclear magnetic resonance, lateadolescent athletes have been shown to reduce muscle glycogen by approximately 35% during a simulated soccer game that lasted an average of 42 min.116 The investigators reported associations between net glycogen used and time to exhaustion. They concluded that glycogen is an important factor in soccer-specific efforts and that dietary carbohydrate is an important consideration in the planning of training meals. Subsequently, the researchers found that, although an intake of 4.8 g of carbohydrate per kilogram of body mass nearly restored muscle glycogen in pregame levels, a reduction of about 10% from initial levels remained.117 The investigators recommended additional carbohydrate in the diet to prevent daily deficits.

SUMMARY AND FUTURE RESEARCH Nutrition plays an important role in the health and performance of young athletes. Fluid and energy balance are acute concerns of the junior athlete. A fluid deficit created by sweat loss has implications for acute problems such as heat illness and fatigue during prolonged athletic events. Acute energy and fluid imbalances will decrease the physical work capacity of children and adolescents during training and competition, particularly during sustained endurance efforts. Future research pursuits are required to confirm the performance benefits of energy (carbohydrate) and fluid intake during exercise in youths, especially during intermittent, highintensity efforts in stop-and-go sports. Chronic inadequate intake of protein, fat, or carbohydrate, trace minerals, or vitamins has implications for longer term consequences such as linear growth and bone health. The requirement for protein intake relative to energy intake needs clarification for athletes in whom energy balance may be compromised due to growth spurts, excess energy expenditure due to a high volume of training, or energy-intake restriction for a weight class sport. Heavy, intense exercise training could have a detrimental effect on bone health when intakes of calcium and protein are inadequate in maturing children. Research on optimizing dietary strategies will

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help resolve this issue. Additional research is needed to determine whether the recommendations applied to adult athletes for nutrient intake, such as consuming a high-carbohydrate diet, are beneficial to the performance and the athletic development of child and adolescent athletes. Moreover, the great research attention paid to the effects of nutrition and training on immune and inflammatory responses in the adult athlete only recently has found its way to the young athlete. The effects of nutritional supplementation (i.e., vitamins that act as antioxidants) in children and teens have yet to be explored for aspects of recovery after exercise and effect on immunologic and inflammatory stresses.

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