Energy balance refers to the balance between energy expenditure and energy intake. It can be measured on a day-to-day basis, but it probably makes more sense to measure it over a period of several days or weeks. When energy intake exceeds energy expenditure, the energy balance is positive and weight gain will occur. When energy intake is below energy expenditure, the energy balance is negative and weight loss will result. Generally, athletes are good at maintaining body weight and thus are in energy balance most of the time. Over the longterm, energy balance is maintained in weight-stable individuals even though this balance may be either positive or negative on a day-to-day basis. People who want to lose weight should increase energy expenditure relative to energy intake. In many activities in which body composition or body weight is believed to be important (gymnastics, dancing, bodybuilding, and weight category sports such as judo and boxing), participants often try to maintain a negative energy balance to lose weight. Thus, the energy intakes in these activities can be very low. At the other extreme are endurance sports such as triathlon, cycling, cross-country skiing, and ultra-endurance running, which require extremely high energy expenditures and energy intakes. In these sports, maintaining energy balance on a day-to-day basis is crucial for performance.