Nutrition and physical activity - Jornal de Pediatria

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Jornal de Pediatria - Vol. 76, Supl.3, 2000 S349

0021-7557/00/76-Supl.3/S349

Jornal de Pediatria Copyright

© 2000 by Sociedade Brasileira de Pediatria

REVIEW ARTICLE

Nutrition and physical activity Claudia Ridel Juzwiak1, Valéria C.P. Paschoal2, Fábio Ancona Lopez3

Abstract Objective: to review data about the nutrition of physically active children and adolescents. Method: review of literature using MEDLINE and LILACS data base and the keywords sport or physical activity and nutrition and children or adolescents. Results and Conclusion: involvement of children and adolescents in sports activities is important for their growth and development process, which should be regularly assessed. Na adequate diet should provide enough energy and nutrients so that young athletes can meet their requirements. The diet should suit the different stages of training, before, during and after competition. Hydration should be planned carefully as children present a less efficient thermoregulation than adults and can dehydrate more quickly, specially during sports practice. Female athletes can present menstrual alterations and inadequate bone growth due to excessive training associated with inadequate energy intake. Eating disorders are found among some groups of athletes. Physical activity should be encouraged as part of the treatment of overweight children. J. pediatr. (Rio J.). 2000; 76 (Supl.3): S349-S358: exercise, adolescent nutrition.

Introduction Participation of children in sports is an essential part of the growth and development process. Physical exercises helps prevent several diseases such as obesity, diabetes, and hypertension, and offers children an opportunity for leisure, social integration and development of their aptitude, leading to enhanced self-esteem and confidence.1 It is important that physically active children and adolescents consume enough energy and nutrients to meet their needs in terms of growth, tissue maintenance, and performance of their intellectual and physical activities. 1-6 Nowadays, early participation of young people in competitions and their involvement in intensive training programs calls for health professionals’ attention to the

adoption of eating behaviors that could be deleterious to health such as dehydration, inadequate weight control practices, nutritional disorders and uncontrolled use of substances seen as ergogenic.

Growth and development Height, weight and body composition One of the major concerns during childhood and adolescence is to ensure the achievement of expected growth and development. Regular physical training, or even the involvement in every-day relatively moderate physical activities, together with other environmental variables, influence the establishment of a genetically-determined growth pattern.7-8 Their action on muscles and bones is an important factor for the increase in bone mass peak during adolescence and, consequently, for the prevention of

1. Associate professor of Neurology, FAMED, PUCRS. 2. Student, Graduation Program in Neurosciences, PUCRS. 3. Municipal Health Secretariat, Porto Alegre, RS.

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osteoporosis in adulthood. However, their effects do not seem to increase or reduce final height values.9-11 To verify whether growth is adequate, weight and height should be measured regularly and assessed in terms of weight/height ratio, according to reference standards and body mass index (BMI).3-4,12 Regular physical activity is important for controlling ponderal index and has been associated with the reduction of fat mass and increase in lean mass. However, it is difficult to distinguish between the effects of training and the expected effects of an increase in lean mass caused by growth and maturation.11 Several aspects such as bone density, body water content and composition of tissues that form lean mass are different in adults and children. Although methods for body composition assessment have been developed in consideration to these differences,13-14 body fat content and weight should not be used as a criterion for the participation of young people in sports or for determining weight requirements, as they could cause impairment to normal growth and development.3-5,13 Among the existing body composition assessment methods, the most practical ones are skinfold and circumference measurements.15 Slaughter and col. 16 developed an equation by using the measurements obtained from a sampling of young people, which may be used for adolescents at large. The measurements of several other skinfolds, without conveerting them into fat content, can also be used to check changes in body fat composition of these adolescents.13-14 Slaughter et al.16 propose an equation for children and adolescents that is formulated as follows: a) For children whose addition of triceps and subscapular skinfolds is < 35 mm Boys: Fat content (%) = 1.21 x (addition of skinfolds) – 0.008 x (addition of skinfolds)2 - constant Girls: Fat content (%) = 1.33 x (addition of skinfolds) – 0.013 x (addition of skinfolds)2 - constant

Prepubertal Pubertal

Constants for boys White Non-white Constants for girls White Non-white

1.7 3.2

3.4 5.2

2.0 3.0

Postpubertal

5.5 6.8

b) For children whose addition of triceps and subscapular skinfolds is > 35 mm Boys: Fat content (%) = 0.783 x (addition of skinfolds) + 1.6 Girls: Fat content (%) = 0.546 x (addition of skinfolds) + 9.7 When estimating body fat content, the errors associated with the method being used should be considered. We should not forget that skinfold increases with normal growth in non-athletes while the opposite may occur in athletes.14 Changes in body composition should be monitored by qualified personnel in order to assess its effects on health and performance.3-4,13 During the pubertal period, primary and secondary sexual characteristics are developed, at the same time that physical growth process continues, culminating in the second growth enhancement stage. At this stage, the assessment of nutritional status is more complex, as criteria are not so clearly-defined as in childhood.17 In general, participation in sports activities is defined according to chronological age. In the case of adolescents, an individual with late sexual maturation usually competes with a peer with early sexual maturation, that is, someone who is taller and stronger.11 Boys and girls increase their body fat content during the pubertal period.13 Individuals with early sexual maturation are usually fatter, especially girls , staying fatter, taller and presenting reduced aerobic capacity when they reach adulthood.18,19 Kemper and col.,19 in a longitudinal study with 200 young people (13 to 22 years old, during 9 years), concluded that individuals with late sexual maturation had greater energy intake and an activity pattern slightly higher than those adolescents with early sexual maturation, resulting in a lower body fat content than the first group. 19 A negative energy balance, caused by inadequate energy intake or energy restrictions associated with certain sports, may inhibit the production of growth factors typical of normal growth and development. It was observed that 6 days of energy restricition (35 kcal/kg/day) in children between eight and 11 years old, resulted in a negative nitrogen balance and reduction in IGF-I and IFGBP3 circulating levels. The authors suggest that these hormonal changes may indicate a resistant growth hormone status, which could impair normal growth and development on the long run. Components of physical aptitude The components of physical aptitude are related to growth and sexual maturation. Physical exercise ability increases throughout the pubertal period due to an increase in strength, ability and resistance. These changes involve linear growth and alterations in body and physiological composition, for example, a greater control over body temperature, which improves peformance.

Nutrition and physical activity - Juzwiak CR et alii

Boys increase their aerobic capacity (VO2max) during adolescence, reaching their peak between the ages of 18 and 20. In postpubertal adolescent girls, VO2max per unit of body weight decreases, while body fat increases, hemoglobin tends to decrease and so does voluntary activity. Aerobic capacity (in ml of O2/kg/min), measured through an endurance test in male and female adolescents, from different countries, is approximately 45 to 55 in boys and 36 to 46 in girls. Age and aerobic capacity have an inverse relation. The chances for improving aerobic capacity through training are greater in adolescents than in children, and its peak occurs at the growth enhancement stage.18 There is higher efficiency of movements after the pubertal period. Even with declining aerobic capacity, performance in sports activities may reach its peak a few years after the pubertal period due to better motor coordination, movement saving and better strategies. Aerobic threshold is greater after the pubertal period and is a linear function of lean body mass.18

Nutritional aspects General considerations There are a variety of factors that influence the quality of a diet. In the case of schoolars and mainly adolescents, we should take into consideration that there is a quest for greater independence at this age, and food selection is one of the areas in which adolescents can show their determination and express their preferences. Peer influence, training time, and other daily activities are factors that may cause young athletes to choose foods, which despite being their favorite one, are not very nutritious, resulting in a suboptimal energy and nutrient intake.5,6,20 Snack intake is very common in this age group and corresponds to approximately 20% of daily total energy intake.18,21-22 Soares et al.23 reported a snack intake of 25 - 28% between major meals, when they assessed the food intake of 67 competitive swimmers from Rio de Janeiro and São Paulo, aged between 15 and 25 years. It is important to evaluate the frequency and quality of these snacks in order to improve their nutritional quality and their energetic contribution to the diet.23 Energy Maintenance of energy balance should a constant worry. Young athletes are commonly affected by energy imbalance, which, if maintained for a long period, may cause serious effects on health such as low height, delayed pubertal period, nutrient deficiency, dehydration, irregular menstrual periods, bone changes, higher incidence of lesions, and higher risk for the development of nutritional disorders.6 Although accurate energy requirements of young athletes is not well-established yet, their energy requirements may be estimated by current food intake analysis, growth rate, gender, age and intensity of physical activity.1,6

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Several studies have attempted to document the energy and nutrient intake of young athletes and demonstrated that these individuals’ food intake analysis indicates a lower energy intake in relation to their age, according to RDA (Recommended Dietary Allowances).24 Thompson,6 in a review on energy balance of young athletes, reports that such results were based on these athletes’ own diet records, which means that the accuracy of reported values depend on the athletes’ capacity of registering their food intake correctly, their tendency to report lower energy intakes and on the use of restrictive diets among athletes in certain modalities (gym, dancing, figure skating, fighters).6 RDA 24 may be used to estimate calorie requirements for normal growth and development (Table 1 and 2), although recent findings, through the double labeled water (DLW) method used in young people, suggest that RDA might be overestimating requirements for this age group.6

Table 1 -

Prediction equations for energy expenditure at rest, according to weight

Sex/Age

Equations to derive kcal/day

Men 0-3 years 3-10 years 10-18 years

(60.9 x W*) - 54 (22.7 x W*) + 495 (17.5 x W*) + 651

Women 0-3 years 3-10 years 10-18 years

(61.0 x W*) - 51 (22.5 x W*) + 499 (12.2 x W*) + 746

* W = weight in kg Source: Nrc, Food & Nutrition Board, Nas, 1989

Table 2 -

Calorie and protein needs for normal growth and development

Age 1-3 4-6 7 - 10 11 - 14 (women) 11 - 14 (men) 15 - 18 (women) 15 - 18 (men)

Calories (kcal/kg)

Protein (g/kg)

102 90 70 55 47 40 45

1.2 1.1 1.0 1.0 1.0 0.9 0.9

Recomended Dietary Allowances, NRC, 1989

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Henry et al.25 proposed equations to calculate basal energy requirements by means of variables such as skinfolds, circumferences, fat mass, fat-free mass, in addition to Tanner stages, and observed that weight was the variable which had the best correlation with basal metabolism rate. For boys, the most accurate equation was that which used skinfold data (suprailliac, triceps and subscapular), and for girls, the most accurate equation was the one that used weight, height, fat-free mass and age (Tables 3 and 4).25

Macronutrients It is recommended that a diet for young athletes should provide between 55 and 60% of total energy in the form of carbohydrates, 12-15% of proteins and 25-30 % of lipids.2-4

To calculate energy spending during exercise, we should consider the kind of sport practiced, frequency, duration, participation or not in competitions. Equivalence tables on energy spending in exercise may be used to calculate extra calories necessary for the day, as the one proposed by Baror26 (Table 5).

Protein requirements are greater in young people than in sedentary adults.6 RDA (24) serve as a guide for a protein intake that ensures normal growth. There are no studies that specify protein allowances for young athletes; however, Ortega2 suggests that protein intake in adolescents who practice sports should be 2 g/kg/day, which is double the recommendation for sedentary adolescents.2 In general, protein allowances are easily achieved since there is a validation of protein intake, not only through food ingestion, but also via nutritional supplements, which results from the existing association between the intake of this nutrient, muscle mass gain and strength.18

Inadequate energy intake is also associated with marginal intake of macronutrients and micronutrients, especially carbohydrates, pyrodoxin, calcium, folate, zinc and magnesium. Such association has deleterious consequences on growth such as increased risk for the development of diseases, reduction of metabolic rate, exacerbating the need for more restrictive diets as to obtain the desired weight loss, which is totally contraindicated.5,6 Food restrictions are common among athletes who compete in sports where body composition and esthetics are determining factors for success.3-4,6 As an example, young greco-roman wrestlers, who repeatedly restrict their diet drastically and regain weight later, present a 14% reduction in basal metabolic rate per unit of lean mass.18

These carbohydrates should preferably be complex (4045% of calories) and in a smaller proportion, simple (1015%).2 Inadequate intake of carbohydrates may result in insufficient muscle glycogen stores and early fatigue, in addition to the use of protein stores for energy production.6,27

Young people have higher levels of glycerol in their blood, increased use of free fatty acids and lower breathing exchange ratio during exercises, indicating an increased use of fats. Despite this increased use of fat during exercises, it is not recommended that the dietary ingestion of this nutrient be higher than 30% of total calorie value, and saturated fatty acids should contribute with less than 10% of this value. 6,28 A low energy intake due to reduced fat ingestion is very common among athletes who follow restrictive diets.5

Table 3 - Regression equations to estimate TMB (kj/day) in children aged between 10 and 15 years old Gender

Equations according to gender

R2

PR

W x 66.9 + 2876 FFM (kg) x 105.4 + 2230 W x 54.6 + H x 18.8 + 576 FFM (kg) x 91.1 + FM (kg) x 29.4 + 2422 W x 78.5 + SI (mm) x 45.3 - TR (mm) x 54.99 - SB (mm) x 38.3 + 294

0.61 0.62 0.62 0.63 0.67

575 567 563 558 527

W x 47,9 + 3230 W x 21 - H x 11 + FFM (kg) x 8007 - A x 154.6 + 5319 FFM (kg) x 96.7 - gender x 383.9 + FM (kg) x 21.4 - A x 136 + 3949

0.52 0.57 0.60

519 480 522

Boys

Girls

R2 = concordance level between TMB measured and estimated values; RSD = residual standard deviation (kg); W = weight (kg); H= height (cm); A = age (years) ; FFM = fat-free mass; SI = suprailliac skinfold; TRI= triceps skinfold; SB = subscapular skinfold; gender = female (equivalent to 0). Source: Henry et al., 1999

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Table 4 - Regression equations to estimate TMB (kj/day) in children according to their pubertal stage Pubertal stage

Equations according to gender

R2

PR

Boys PU DG3

W x 60 - A x 194 + WC (mm) x 50.7 + 2892 AMC (cm) x 270 + log soma 5 folds (mm) x 1450 - 1803

0.61 0.69

471 519

Girls MD1 10-15 years Premenarche Premenarche

W x 69.9 - 5230 W x 50.6 - ME x 170.9 + 3161 W x 53.6 + 3031 W x 97.07 - FM x 74.6 - A x 121.2 + 3452

0.52 0.52 0.57 0.61

416 416 485 462

R2 = concordance level between TMB measured and estimated values; RSD = residual standard deviation (kj); W = weight (kg); A = age (years) ; PU = pubic hair; GD3= genital development stage 3 (Tanner|); MD1 = mammary development stage 1 (Tanner); WC = wrist circumference; AMC = arm muscle circumference; ME = Menarche (premenarche = 0 and postmenarche = 1); FM = fat mass. Source: Henry et al., 1999

Micronutrients There are no specific nutrient recommendations for young athletes. RDA 24 and DRI29,31 are used as standards to check adequation, although they have little specificity. In addition to low energy intake, calcium intake should be controlled. An adequate intake of this nutrient is extremely important for these athletes who are at growth stage so that they reduce stress fractures and later risk for developing osteoporosis. This factor is especially important among female athletes who present primary amenorrhea, which is associated to a lower bone density. Among athletes, gymnasts are the ones who have a higher frequency of bone damage.6,18 A calcium intake less than 500 mg results in lower retention of this nutrient in adolescents, which is a common intake for this group. For better bone development and reduction in stress fracture, Bernadot32 suggests that levels higher than those preconized by RDA 24 should be ingested, especially by elite gymnasts. The new calcium DRI29 suggests higher intake rates for this nutrient by adolescents, considering the dramatic increment of bone mineral content during this period, however it does not take into account requirements in terms of sports practice. Calcium intake may be insufficient, especially among young people who reduce the intake of dairy products and present a high intake of proteins and foods that provide a large amount of phosphorus (ex. carbonated drinks).2 Special attention should be paid to iron intake during adolescence. Quick increase in lean mass, blood and red

cell counts results in higher iron requirement for myoglobulin and hemoglobin, with higher probability for the occurrence of anemia at the growth enhancement stage. Higher iron reserves due to greater lean mass and menstrual losses in girls justify the need for higher iron intake at this age. 18 Insufficient iron intake may hinder oxygen transport capacity, thus reducing performance and interfering with training if iron deficiency progresses into anemia. 6 Female adolescent athletes are at greater risk for iron deficiency due to their greater physiological needs, low energy intake (in relation to sports that require weight control), inadequate iron intake, and iron loss due to the practice of sports (hemolysis by impact). Even when the analysis of daily intake shows adequate ingestion and the blood test indicates a normal hemoglobin count, it is necessary to monitor body iron stores (ferritin) due to low iron bioavailability.6,18 Intake of vitamin A, vitamin C and magnesium may be below recommended levels in children who are not in the habit of eating fruits and vegeatbles. Diets that present severe caloric restriction may be deficient in pyrodoxin, calcium, folate and zinc.1,18

Hydration Hydration is essential to maintain health and physical performance. Due to increased loss of water and electrolytes

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Table 5 - Calorie equivalent in children, representing kcal per 10 minutes of activity Weight (kg) Activity

20

25

30

35

40

45

50

55

60

65

Basketball (game) Gym Cross-country skiing Cycling 10 km/h 15 km/h Field hockey Figure skating Horseback-riding Walking Trotting Galloping Ice hockey Judo Jogging 8 km/h 10 km/h 12 km/h 14 km/h Sitting, playing calmly Soccer (game) Squash Swimming (30 m/min) Breaststroke Crawl Backstroke Table tennis Tennis Volleyball (game) Walking 4 km/h 6 km/h

34 13 24

43 17 30

51 20 36

60 23 42

68 26 48

77 30 54

85 33 60

94 36 66

102 40 72

110 43 78

15 22 27 40

17 27 34 50

20 32 40 60

23 36 47 70

26 41 54 80

29 46 60 90

33 50 67 100

36 55 74 110

39 60 80 120

42 65 87 130

8 22 28 52 39

11 28 35 65 49

13 33 41 78 59

15 39 48 91 69

17 44 50 104 78

19 50 62 117 88

21 55 69 130 98

23 61 76 143 108

25 66 83 156 118

27 72 90 168 127

37 48 11 36 -

45 55 12 45 -

52 64 76 14 54 64

60 73 88 15 63 74

66 79 91 15 72 85

72 85 99 113 16 81 95

78 92 107 121 17 90 106

84 100 115 130 18 99 117

90 107 123 140 19 108 127

95 113 130 148 20 117 138

19 25 17 14 22 20

24 31 21 17 28 25

29 37 25 20 33 30

34 48 30 24 39 35

38 49 34 28 44 40

43 56 38 31 50 45

48 62 42 34 55 50

53 68 47 37 64 55

58 74 51 41 66 60

62 80 55 44 72 65

17 24

19 26

21 28

23 30

26 32

28 34

30 37

32 40

34 43

36 48

Source: BAR-OR, 1983.

through sudoresis, sportspersons should ingest fluids before, during and after training and competition periods.3-5 Water and electrolyte requirements for adults are wellstudied in the literature, but there is little information about these needs in children.33 During dehydration, young athletes have a faster increase in their internal temperature if compared to adults. This indicates that children have a less efficient thermoregulation than adults, which is possibly due to their reduced rate of sudoresis, higher surface area/ body mass ratio (which leads to enhanced heat exchange with the environment) and higher production of metabolic heat. These findings emphasize the necessity to guarantee

fluid intake by children and adolescents during physical exercise.6,32 Children usually lose less than 350 - 400 ml/hr/m2 of body surface area. This low rate of sudoresis is the result of reduced sweat production by the gland rather than by the number of heat-activated sweat glands. The transition of sweat patterns from childhood to adulthood occurs at the beginning and middle of the pubertal period. Some studies show that fluid intake by children and also by adults during long-lasting exercises was not enough to repor the losses, even when drinks were offered ad libitum. Although, for ethical reasons, dehydration levels have not exceeded 2 -

Nutrition and physical activity - Juzwiak CR et alii

3% of body weight, voluntary hydration rate among adolescents and adults was similar.33 Children may avoid dehydration during long-lasting and intermittent exercises by ingesting fluids every 15 - 20 minutes.28 For long-lasting sports activities, especially those over 90 minutes, hydroelectrolytic drinks with adequate carbohydrate concentration (6 - 8%) and osmolarity, may be used, having as advantage pleasant flavor and the enhanced fluid intake by young people.2,27 In a study that assessed children’s fluid flavor preference during physical exercise, Meyer et al.33 found that male and female Canadian children aged nine through 13 preferred grape beverages to plain water and orange or apple drinks. This preference was consistent at rest, after an oxygen uptake test and a progressively provoked mild dehydration process. Grape was the flavor that led to highest rehydration levels. Dehydration can also the result of insufficient energy intake in association with low fluid intake. It can also be intentionally caused with the aim of quickly reducing ponderal weight (sports requiring weight control), through increased sudoresis, fluid restriction and use of diuretics, with possible harmful effects on kidneys when performed on a regular basis. Body water loss through sweating higher than 5% of body weight is associated with fatigue and puts athletes at risk for heatstroke and even death. 6,18,35 Water homeostasis is more efficient after the pubertal period, but the importance of water for thermoregulation should not be underestimated. Children should be instructed on the consequences of dehydration and encouraged to get into the habit of drinking water regularly during the practice of sports. The use of an unshareable bottle should be encouraged.

Specific nutrition pautas for competition sports It is important to guarantee adequate energy reserves for children and adolescents involved in competitions. It is not recommended that children use the classic technique of carbohydrate overcompensation, which comprises a glycogen reserve depletion phase and a carbohydrate-free diet (replaced with a diet high in proteins and fat) followed by a high intake phase, due to the side effects caused in the first phase such as fatigue and irritability. It is suggested that 3 - 4 days before competition, special attention should be paid to the intake of carbohydrate-rich food.28 On the day of competition, athletes should be told to eat carbohydrate-rich and low-fat foods. Easily digestible and low-fiber foods should be chosen. Unknown foods (e.g.: regional) should not be selected at this time. The last meal should occur 3 - 4 hours before the event to guarantee gastric emptying. Selected foods should be easily digestible, preferably high in carbohydrates, with either high or low glycemic index.

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Up to two hours before competition, a snack high in low glycemic index carbohydrates may be ingested.1,35 Hendelman et al. 37 did not observe any effect on physiological response to submaximal exercise (75 minutes of bicycle) when carbohydrate-rich foods were offered 10 minutes before competition to untrained adolescents aged on average 14. 5 years. Respiratory quotient, heartbeat rate, glycemia and lactate concentration. Water may and should be ingested before the event. A fluid intake pattern should be established before competition so that the adolescents start the exercises with an optimal hydration level. Children’s preferences should be respected provided that the selected food has adequate characteristics. Nervousness and anxiety may affect children’s acceptance of the food.2 It is of fundamental importance that hydration be maintained during the event. Carbohydrate-rich foods or beverages may be ingested during long-lasting activities. Temperature and beverage flavor contribute to higher intake.33 Fluids and foods, especially those high in carbohydrates, should be offered in order to replenish water and restore muscle glycogen reserves, especially during the first hours after the event, taking advantage of glycogen synthetase activation. High glycemic index foods allow more efficient muscle glycogen restoration.37

Specific nutritional aspects of female athletes Menstrual alterations such as delayed menarche, primary amenorrhea, inadequate bone growth and fracture proneness are very common among female athletes, and are a consequence of insufficient nutrition and excessive training, mainly in the case of sports in which esthetics, strength and weight are determining factors. 5-6 The existing interrelationship between nutritional, menstrual and bone disorders is known as the female athlete triad.39 Menstrual dysfunctions include subclinical ovulatory disorders such as luteal phase deficiency and anovulation, in addition to clinical disorders such as oligomenorrhea and amenorrhea.6 The etiology of these menstrual anomalies is multifactorial. Some factors associated with menstrual irregularities include stress (related to lifestyle and athletic performance), inadequate energy intake, low body fat content and strict training program.1,39 Physically active and amenorrheic adolescents may not be worried about the long-term risk for osteoporosis, but may be persuaded to change their food intake and load of exercise training when warned against the risks of stress fracture and problems with hypothermia during exercise in cold climates.13

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Although some athletes and coaches believe that menstrual cycle suspension indicates optimal body fat content or optimal level of training, this situation is actually deleterious to bone health and may lead to infertility and other reproductive problems, immunofucntional changes and a greater risk for cardiovascular diseases. 6

Nutritional disorders Anorexia and bulimia The high incidence of nutritional disorders, menstrual dysfunctions and use of inadequate weight control practices reported by young athletes suggests that many of them do not meet their daily energy and diet requirements. The practice of sports such as gymnastics, ballet, figure skating, body-building and wrestling were identified as a stimulus for borderline energy intake, aimed at keeping body weight low, and compatible with the criterion for body image related to these activities.6 The presence of a modified eating behavior, frequently associated with other symptoms such as vomiting, use of laxatives, extreme weight loss, amenorrhea and other physiological and psychological symptoms, may characterize a real nutritional disorder.6,18 The prevalence of these disorders is significantly higher among athletes than in the population at large, and especially among female athletes, corresponding to 32 - 63%.3,4,6,38,39 Bulimia is characterized by binge-eating behavior followed by lack of control and excessively quick food intake within a short period of time, alternating with purge behavior or use of laxatives and diuretics. This kind of behavior is difficult to diagnose as patients present secretive behavior and normal weight, especially at the beginning. Anorexia nervosa is a more severe syndrome and has more harmful effects on health. The classic clinical status involves changes in body image, and constant and compulsive food restriction or even fasting, resulting, in many cases, in death.18 These nutritional disorders may manifest themselves in their hardly diagnosable clinical or subclinical form and should be a concern to professionals who deal with these athletes.38,39

Obesity There are multiple interactions between physical activity and obesity. Physical activity reduces the risk for obesity by acting on energy balance regulation; influences body weight distribution, conserving or maintaining lean mass, in addition to its effects on weight loss.41 Participation in sports and increase in physical activity are often recommended for the treatment of childhood

Nutrition and physical activity - Juzwiak CR et alii

obesity, followed by nutritional guidance and education. Psychosocial factors and low physical aptitude may discourage obese children from physical activity. Physical hypoactivity creates a vicious cycle: inactivity - positive calorie balance - obesity - reduced physical activity greater inactivity.9,41 Children should be encouraged to have a more active life when they are still very young. The practice of early exercise during growth process prevents the formation of new adipose cells.41,42 Children following an antiobesity treatment should associate calorie intake and low intensity and long-lasting physical activity, also considering the recreational aspect. 44 Weight loss and positive cardiovascular and respiratory effects resulting from physical conditioning in obese children cause a reduction in physiological effort during exercise. As to the psychosocial aspect, it improves self-image, selfconfidence, sociability, and also provides reduced effort perception.

Final considerations Several adolescents use food supplements and, sometimes, drugs, in order to achieve better performance, build up their muscles and reduce body fat content, thinking that if these substances are “natural”, they cannot cause any harm and that “if a small amount works well, a larger amount will work even better”. Health professionals who deal with these young athletes are supposed to warn them against the risks associated with the use of such substances and show them the importance of a balanced diet.45,46 Adolescents, family members and coaches should be well-informed so that adequate eating habits can be adopted, thus maintaining health and providing positive effects on intellectual and sports performance.

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