Energy balance is, at the core, a simple concept—if energy availability from food and beverage intake exceeds expenditure, the excess energy will be stored in the body, primarily as fat, unless concomitant exercise or physical maturation favors greater storage as lean body mass; if energy expenditure exceeds intake, body energy stores will be lost.
Energy balance is best understood as a dynamic equation rather than a static one, wherein changes in energy expenditure often evoke compensatory voluntary or involuntary changes in energy intake. Conversely, changes in energy intake evoke compensatory changes in energy expenditure.
Total daily energy expenditure is the sum of the RMR, the TEF, and PAEE, including exercise and nonexercise activity thermogenesis.
Energy intake is influenced by neuroendocrine factors that regulate hunger and satiety. However, personal, social, and environmental factors, including the availability of high-energy–dense foods and low levels of physical activity, can override the homeostatic regulation of body weight and contribute to increased body fat.
Weight loss is usually associated with an increase in hunger and a reduction in energy expenditure— called the energy gap—which is a major contributor to the common occurrence of weight regain. Regular exercise may help attenuate the energy gap by contributing to increased energy expenditure and enabling higher energy intake without accompanying weight gain.
Understanding the behavioral and metabolic features of the elite or recreational athlete is important in developing the most effective individualized approaches to facilitate his or her achievement of a healthy and competitive body weight and composition.