DEPARTMENTS: Wouldn't You Like To Know
My daughter is a successful competitive high school swimmer who trains at least a couple hours each day, including swimming and some resistance training. She was at an apparently healthy weight before the start of the season, but she has unintentionally lost 15 lbs during the past month as the training levels have increased (currently 118 lbs and 5′9″). She swims in the morning and the afternoon for a total of about 2.5 hours each day plus the normal schedule of school classes. I’m worried about the drop in her body weight, which appears to include loss of muscle. She complains of feeling tired all the time, plus her periods, which were quite regular, have been absent the last couple months. Some of the other parents said this is just part of being at a higher level of competition, but I’m worried. Is this situation normal?
Competitive athletes are very focused on their training programs, but being a successful athlete also requires special attention to nutrition. Meeting energy requirements is vital for optimal performance (5), as well as for growth, development, and maturation in youth (1,2). The importance of sufficiently fueling athletes is addressed in a joint position stand, “Nutrition and Athletic Performance,” by the American College of Sports Medicine, American Dietetic Association, and Dietitians of Canada. A low energy intake for active females is a particular concern because of the resulting weight loss and disruption of normal endocrine function (5). You have observed this firsthand with your daughter’s unintentional drop in body weight and the interruption of her menstrual cycle. Addressing both issues — weight loss and menstrual status — is important.
When adequate energy intake is not maintained, the body will use fat and lean tissue for fuel. For athletes, the loss of lean mass negatively impacts strength and endurance (5). Energy availability is a consideration and reflects dietary intake minus exercise energy expenditure normalized to fat-free mass (FFM). More simply, energy availability is the amount of energy available to the body for all body functions AFTER subtracting the energy cost of exercise training (5). For females, the target seems to be an energy availability at least 30 kcal per kg FFM per day (5). Inadequate dietary intake has been identified as the primary nutritional concern for female athletes (10). Good nutrition is necessary for normal growth. Body mass index (BMI) can give some insight; BMI-for-age charts are used for youth between ages 2 and 20 years. Currently, your daughter’s BMI is 17.4, which is about the 15th percentile for her age (7). Thus, she is on the lower end of the healthy range. When training levels increase, caloric intake should increase to meet the energy costs of exercise in addition to the continued energy the body needs for basic functions.
The first step is to determine needed caloric intake. Various methods are commonly used by sports dieticians (5). One method uses the Harris-Benedict equation to estimate resting metabolic rate (RMR) that is then multiplied by an activity factor to estimate the total energy expenditure (see Box 1 to see how this could be applied). Another method involves the use of metabolic equivalents (METS) (see Box 2). Realize that both of these methods provide estimates because energy expenditure is influenced by heredity, age, body size, and FFM (5). In addition, selection of either a given activity factor or MET values for activities throughout the day requires judgments and approximations. Using these two methods, the approximate total energy expenditure for your daughter, a 118-lb (54 kg) female, is estimated between 3,135 and 3,241 kcal. For purposes of discussion, a value of 3,200 kcal will be used.
The next step involves consuming sufficient foods and beverages to match this requirement. This can be a challenge for a young student athlete who is juggling training along with a busy schedule of classes and studying. Heavy training with multiple workouts each day may require more than three meals and three snacks per day (5). Increasing portion size at meals (e.g., extra slice of toast at breakfast, bigger serving of pasta at dinner) as well as nutritious snacks between meals and after workouts can help provide needed calories. Some snack examples include fruits (oranges, bananas, apples); smoothies; whole-grain bagel with peanut butter; trail mix with granola, nuts, and dried fruit; slice of pizza with green peppers; and instant oatmeal with low-fat milk and slivered almonds (9).
Athletes must focus on selecting a variety of healthy foods to help fuel the body for training as well as health. When the body doesn’t have enough calories from the diet and once carbohydrate stores start to drop, the body targets fat stores and even protein (muscle). Menstrual function becomes disrupted as the body goes into an “emergency” mode, trying to conserve energy (4). Consultation with a physician experienced in treating female athletes can rule out other causes for menstrual cycle disruption (e.g., pregnancy, polycystic ovary syndrome, pituitary tumor) (2,4).
Matching caloric requirements is only one part of getting ready for training and competition. Athletes also must consider the composition of meals and snacks. In general, athletes can follow recommendations found within the Dietary Guidelines for Americans (5). For teenagers, the Dietary Guidelines recommend the following (14):
* 45% to 65% carbohydrate
* 10% to 30% protein (note that for adults, this is 10% to 35%)
* 25% to 35% fat (note that for adults, this is 20% to 35%)
Athletes need between 6 and 10 g of carbohydrate per kilogram body weight per day (5). For your 118-lb (54 kg) daughter with a high level of activity as you described, this is a carbohydrate intake between 324 to 540 g or 1,296 to 2,160 kcal. Consuming 500 g of carbohydrate (toward the upper end of the range) will provide 2,000 kcal, which is 62.5% of total kcal (see Box 3 for how to calculate this). Carbohydrates are an important source of energy during higher intensity physical activity. If body stores become depleted, the body can then turn to protein for up to 5% to 10% of energy needs, an undesirable shift if one strives to maintain lean body mass or wishes to ensure adequate protein availability for growth among youth (12).
Adult endurance athletes typically need about 1.2 to 1.4 g of protein per kilogram of body weight per day (5). Resistance training also can increase the body’s protein requirement with recommended intakes of 1.2 to 1.7 g of protein per kilogram of body weight each day (5). These ranges also appear to be appropriate for youth (12). With the high level of endurance training sustained in swim training plus some resistance training, it would be reasonable for your daughter to target approximately 1.5 g of protein per kilogram of body weight. This amounts to 81 g of protein each day, or 324 kcal, which accounts for 10% of total calories (see Box 3 for how to calculate this).
The rest of caloric intake will be from fat. Total fat intake for teens should be between 25% and 35% (14). Thus far, carbohydrate has accounted for 62.5% of total calories, protein for 10%, and thus 27.5% remains for fat. This amounts to 880 kcal or 98 g of fat. Fat intake for youth should be divided equally between saturated, polyunsaturated, and monounsaturated fats (5).
In addition to the calorie-producing nutrients of carbohydrates, proteins, and fats, the other nutrients also are of importance — vitamins, minerals, and water. Vitamins and minerals that have been found to be lacking in athletes’ diets include vitamins D, C, and E; the B vitamins; β-carotene; and iron, zinc, magnesium, and selenium (5). In addition, for female athletes experiencing the absence of menstrual cycles, concerns regarding bone density highlight the value of sufficient calcium intake (1,500 mg/day may be needed) (12). Problems associated with the inadequate intake of vitamins and minerals are mostly seen in situations in which calories are restricted or when diets are unbalanced or are of low nutrient density. In such cases, a multivitamin-and-mineral supplement might be considered. Realize that supplements will not improve performance for athletes already consuming a nutritious diet (5).
Avoiding dehydration is another consideration for athletes. Not only is adequate hydration important to avoid heat illness (e.g., heat exhaustion, heat stroke), but dehydration can compromise performance (5). The steps for hydrating before exercise depend on the amount of fluid deficit brought on by previous exercise (3). In general, consuming 5 to 7 mL of fluid per kilogram body weight of water or sport beverage at least 4 hours before exercise will allow for fluids to be absorbed by the body and excess to be excreted in the urine (5). During exercise, sweat rates can vary widely, with ranges reported from 0.4 to 1.8 L/hour (3) or even up to 2.4 L/hour (5). This may be hard to notice in swimming when sweating is not evident given the water environment where exercise occurs. One tool is to check body weight before and after exercise to determine weight loss caused by sweating. Given the wide range of sweat rates, no single recommendation is possible. The goal is to prevent dehydration — as determined by no more than a 2% reduction in body weight from before to after exercise. Beverages with both electrolytes and carbohydrates can be helpful for both performance and to maintain an appropriate fluid-electrolyte balance (12). After exercise, normal meals and beverages will typically help restore fluid levels, but in cases of excessive dehydration, 16 to 24 oz (450–675 mL) of fluid can be consumed for each pound (0.5 kg) of body weight lost (5).
Attention to nutrition is an important key to optimal performance. The calculations and general recommendations are provided with the intent to show the importance of consuming sufficient nutrients and embracing a balanced diet. Carbohydrates are vital as fuel for high-intensity training and competition. Protein is important for many functions in the body, including structure of muscles and even an energy source when caloric intake is too low. Fat is another fuel source for low-intensity activities and is needed for normal body functions and is a component of cell walls throughout the body. Adequate vitamins, minerals, and water are required for the body to function and are necessary for top-level performance. A sports dietician can be helpful in creating an individualized nutritional plan. You also can use the MyPlate Web site where individualized food plans can be constructed (13).
Proper nutrition is a concern for all young people. Because of the high levels of obesity — approximately 17% of children and adolescents are now obese (8) — the typical focus is on calorie reduction and weight loss. However, for many young athletes, consuming sufficient calories is of major importance for health and for performance. By addressing the potentially related issues of energy imbalance (as evidenced by unintentional weight loss) and menstrual cycle disruption, you can help ensure that your daughter’s health and her swim times are optimal.
The author acknowledges the review of an early draft of this column by Stella Volpe, Ph.D., R.D., LD/N, FACSM.
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© 2012 American College of Sports Medicine
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