Meeting energy needs is a nutrition priority for athletes. Optimum athletic performance is promoted by adequate energy intake. This section will provide information necessary to determine energy balance for an individual. Energy balance occurs when energy intake (the sum of energy from foods, fluids, and supplement products) equals energy expenditure or the sum of energy expended as basal metabolic rate (BMR), the thermic effect of food, the thermic effect of activity (TEA), which is the energy expended in planned physical activity, and nonexercise activity thermogenesis. Spontaneous physical activity is also included in the TEA.
Athletes need to consume enough energy to maintain appropriate weight and body composition while training for a sport. Although usual energy intakes for many intensely training female athletes might match those of male athletes per kilogram body weight, some female athletes may consume less energy than they expend. Low energy intake (e.g., < 1800-2000 kcal·d−1) for female athletes is a major nutritional concern because a persistent state of negative energy balance can lead to weight loss and disruption of endocrine function.[7,8,9,10]
Inadequate energy intake relative to energy expenditure compromises performance and negates the benefits of training. With limited energy intake, fat and lean tissue will be used for fuel by the body. Loss of lean tissue mass results in the loss of strength and endurance, as well as compromised immune, endocrine, and musculoskeletal function. In addition, long-term low energy intake results in poor nutrient intake, particularly of the micronutrients, and may result in metabolic dysfunctions associated with nutrient deficiencies as well as lowered resting metabolic rate (RMR). The newer concept of energy availability, defined as dietary intake minus exercise energy expenditure normalized to fat-free mass (FFM), is the amount of energy available to the body to perform all other functions after exercise training expenditure is subtracted. Many researchers have suggested that 30 kcal·kg−1 FFM·d−1 might be the lower threshold of energy availability for females.[12,13,14,15]
Estimation of energy needs of athletes and active individuals can be done using a variety of methods. The Dietary Reference Intakes (DRI)[15,17] and the Dietary Guidelines 2005 (https://www.health.gov/dietaryguidelines/dga2005/report/HTML/D3_Disccalories.htm) provide energy recommendations for men and women who are slightly to very active, which are based on predictive equations developed using the doubly labeled water technique thatcan also be used to estimate energy needs of athletes (Fig. 2).
The Dietary Reference Intake (DRI) method for estimating energy requirement for adults.
Energy expenditure for different types of exercise is dependent on the duration, frequency, and intensity of the exercise, the sex of the athlete, and prior nutritional status. Heredity, age, body size, and FFM also influence energy expenditure. The more energy used in activity, the more calories needed to achieve energy balance.
Typical laboratory facilities are usually not equipped to determine total energy expenditure. Therefore, predictive equations are often used to estimate BMR or RMR. The two prediction equations considered to most closely estimate energy expenditure are the Cunningham equation (1980) and the Harris-Benedict equation. Because the Cunningham equation requires that lean body mass be known, sports dietitians typically use the Harris-Benedict equation. To estimate total energy expenditure, BMR or RMR is then multiplied by the appropriate activity factor of 1.8-2.3 (representing moderate to very heavy physical activity levels, respectively). Numeric guidelines such as these only provide an approximation of the average energy needs of an individual athlete. An alternative method for estimating exercise energy expenditure is to use metabolic equivalents (METs) recorded during a 24-h period. Any of these methods can be used to estimate energy expenditure for the determination of energy intake requirements and provide the sports dietitian with a basis to guide the athletes or active individuals in meeting their energy needs.
Cite this: Nutrition and Athletic Performance - Medscape - Mar 01, 2009.