Nutritional Issues For Young Athletes

Introduction

Like adults, child athletes need adequate nutrition to maintain health and to optimize performance. In addition, their nutrition must provide for physical growth and maturation. This presentation is not meant to examine the adequacy of current nutritional intake among young athletes, or their eating pattern. For more information on these issues see a review by Nelson-Steen (7). "Nutrition" will be approached here from the vantage point of a physiologist, rather than that of a dietitian. We shall focus on several nutritional issues that are specific to the growing athlete: protein and calorie needs, children's use of energy sources during exercise and means of maintaining adequate fluid and electrolyte balance.

Protein and Calorie Requirements in the Growing Athlete

A major age-related difference in nutritional needs is in protein intake: while in adults the recommended intake is 0.8 grams per kg body mass per day, protein requirements are higher during the years of growth. Seven- to 10-year-old children need 1.2 grams per kg body mass per day and 11- to 14-year-old children need 1 gram per kg per day. Surveys among small groups of young athletes suggest that their protein intake is adequate, or even exceeds these requirements (2, 10).

Adult-based data have shown that differences in daily energy requirements among athletes depend on the volume of their training and the specific energy cost of their physical routines. While the same applies to athletes of all ages, there are no similar data for children. Nor is there much information on the energy a child athlete expends while performing a specific sports routine. Such scant information cannot serve as a clue for the daily energy demands in specific sports. It is likely that, as the proficiency of executing a specific routine increases, the energy cost decreases, but this author is not aware of any data that substantiates this notion for child athletes.

There is another reason to assume that energy requirements of child athletes are different from those of adults. As shown in several studies (5), the energy cost of walking and running at a given speed, when calculated per kg body mass, is considerably higher in children than in adults. The younger the child the higher the cost. It has been recently shown that the main reason for a relative "wastefulness" of energy in children is the lack of adequate co-ordination between their antagonist muscle groups during walking or running (3). It is likely, but not yet proven, that the same applies to other physical activities such as swimming, skiing and skating.

Use of Energy Sources during Exercise

Data based on respiratory (6), plasma substrate and muscle enzyme analyses suggest that, during exercise, children use relatively more fats and less carbohydrates than do adults. This may be one reason why children are less successful in activities that require anaerobic energy turnover. The cause for the above difference is not clear. Whether children's preference from fats as an energy source has any implications for nutritional recommendations has yet to be determined.

Fluid and Electrolyte Requirements

One implication of the increase in metabolic rate during exercise is the production of metabolic heat. Unless this extra heat is dissipated, body temperatures will increase and this, if extreme, may induce heat-related illness. Evaporation of sweat is the main avenue for heat dissipation for body cooling, it may result in excessive fluid and electrolyte losses, unless body fluids are replenished. Unfortunately, our thirst mechanism almost invariably underestimates the actual fluid requirements during prolonged exercise. This may result in "voluntary dehydration" (i.e., dehydration that occurs even when beverages are offered ad libidum). Dehydration induces several performance- and health-realted detrimental effects. While muscle strength and power are affected minimally by dehydration, one's ability to endure an activity over time is diminished. Of possibly greater relevance to figure skating is the decrease in mental acuity. For example, a dehydrated person may not notice certain visual cues (4).

Voluntary dehydration occurs in children (1, 9, 10) and adults, alike. One age-related difference, though, is that core body temperature during dehydration increases faster in children than in adults (1). It is therefore essential to prevent or ameliorate voluntary dehydration in child athletes.

Inappropriate fluid replenishment patterns may also result in electrolyte insufficiency, hyponatremia in particular. This will occur, for example, when the athlete replenishes sweat and urinary losses by drinking only water. One of the outcomes of hyponatremia is muscle cramps.

How can one prevent voluntary dehydration in child athletes? The main strategy is to enhance thirst and to educate the athlete (but also the coach, parents and team physician) to drink frequently, even when not thirsty. As recently shown (8, 9), children's thirst can be enhanced during exercise by flavoring their drink and by adding NaCl and carbohydrate in amounts found typically in sports drinks (i.e., 18 mmol per liter NaCl and 6% sucrose and glucose). Salt tablets contain excessive amounts of NaCl and therefore are counter-indicated. One should ascertain that the child arrives fully hydrated for a practice session or for competition, and enforce drink pauses (every 15-20 minutes) during prolonged activities.

Selected References

1. Bar-Or, O., R. Dotann, O. Inbar, A. Rothstein, H. Zonder. Voluntary hypohydration in 10- to 12-year-old boys. Journal of Applied Physiology: Respiratory Environmental Exercise Physiology 48:104-108, 1980

2. Delistraty, D.A., E.J. Reisman, M. Snipes. A physiological and nutritional profile of young female figure skaters. Journal of Sports Medicine and Physical Fitness 32:149-155, 1992.

3. Frost, G., J. Dowling, O. Bar-Or, K. Dyson. Cocontraction in three age groups of children during treadmill locomotion. Journal of Electromyography and Kinesiology 7:179-186, 1997.

4. Leibowitz, H.W., C.N. Abernathy, E.R. Buskirk, O. Bar-Or, R.T. Hennesy. The effect of heat stress on reaction time to centrally and peripherally presented stimuli. Human Factors 14:155-160, 1972.

5. MacDougall, J.D., P.D. Roche, O. Bar-Or, J.R. Moroz. Maximal aerobic capacity of Canadian school children: prediction based on age-related oxygen cost of running. International Journal of Sports Medicine 4:194-198, 1983.

6. Martinez, L.R., E.M. Haymes. Substrate utilization during treadmill running in prepubertal girls and women. Medicine and Science in Sports and Exercise 24:975-983, 1992.

7. Nelson-Steen, S. Nutrition for the school-aged child. Chapter 18 in Bar-Or (ed.) The Child & Adolescent Athlete. Oxford, Blackwell Science, 1996, pp. 260-273.

8. Rivera-Brown, A.M., R. Gutierrez, J.C. Gutierrez, W.R. Frontera, O. Bar-Or. Drink composition, voluntary drinking, and fluid balance in exercising, trained, heat-acclimatized boys. Journal of Applied Physiology 86:78-84, 1999

9. Wilk, B., O. Bar-Or. Effect on drink flavor and NaCl on voluntary drinking and rehydration in boys exercising in the heat. Journal of Applied Physiology 80:1112-1117, 1996

10. Ziegler, P.J., C.S. Khoo, P.M. Kris-Etherton, S.S. Jonnalagadda, B. Sherr, and J.A. Nelson. Nutritional status of nationally ranked junior US figure skaters. Journal of American Dietitican Association 98:809-811, 1998.