Meeting energy needs is the first nutrition priority for athletes. Achieving energy balance is essential for the maintenance of lean tissue mass, immune and reproductive function, and optimum athletic performance. Energy balance is defined as a state when energy intake (the sum of energy from food, fluids, and supplement products) equals energy expenditure (the sum of energy expended as basal metabolism, the thermic effect of food, and any voluntary physical activity) (1). Inadequate energy intake relative to energy expenditure compromises performance and the benefits associated with training. With limited energy intake, fat and lean tissue mass will be used by the body for fuel. Loss of muscle results in the loss of strength and endurance. In addition, chronically low energy intake often results in poor nutrient intake, particularly of the micronutrients.
In the 1989 Recommended Dietary Allowances (RDAs) (2), mean energy requirements for women and men who are slightly to moderately active and between 19 to 50 years of age were established as 2,200 and 2,900 kcal per day, respectively. Expressed alternatively, normally active people are counseled to consume an energy intake of 1.5 to 1.7 times resting energy expenditure or at a rate of 37 to 41 kcal/kg body weight per day (2). Energy expenditure is influenced by heredity, age, sex, body size, fat-free mass, and the intensity, frequency, and duration of exercise.For athletes, the recommendation is made to evaluate the kind of exercise performed for its intensity, frequency, and duration, and then to add this increment to the energy needed for normal daily activity (3,4,5). For example, a 70 kg male runner who runs 10 miles per day at a 6-minute pace would require approximately 1,063 kcal per day to cover the energy expenditure of running (0.253 kcal/min/kg) (6), plus the energy cost of normal daily activities (70 kg x 37 to 41 kcal/kg body weight) for normal activity. Thus, this athlete would need approximately 3,653 to 3,933 kcal per day to cover the total cost of energy expenditure. Ultimately, however, numeric guidelines for energy intake, such as those cited above, can only provide a crude approximation of the average energy needs of an individual athlete. Any athlete needs to consume enough energy to maintain appropriate weight and body composition while training for a sport. Usual energy intakes for male endurance athletes range from 3,000 to 5,000 kcal per day (7). Although usual energy intakes for many intensely training female athletes may match those of male athletes per kg of body weight, some consume less energy than they expend. This low-energy intake can lead to weight loss and disruption of reproductive function, and is often seen with energy intakes of less than 1,800 to 2,000 kcal per day (6-11).
Although resistance exercise usually requires less energy than endurance exercise, the total energy needs of athletes participating in strength training and bodybuilding may be as high as those of endurance athletes because of their increased body size and high levels of fat-free mass. In circumstances in which an increase in lean body mass is the goal, energy intake must be sufficient to meet the needs for muscle growth. Thus, many strength athletes may need 44 to 50 kcal/kg body weight/day, and those in serious training may have even higher energy requirements (more than 50 kcal/kg body weight/day) (12,13).
