Limits of energy turnover in relation to physical performance were addressed in terms of upper and lower limit, changes during a training programme and how to regulate energy balance at a changing energy turnover. At the lower limit we stress the risk of energy restriction to reach a desired body size or body composition for reduced performance, early osteoporosis and weight cycling. The upper limit of energy turnover can be increased by the consumption of energy dense carbohydrate-rich foods. Training induces an increase in energy turnover which can initially be more then twice the calculated energy costs from laboratory measurements of physical activity. In the long term, training can induce a reduction in resting energy expenditure even while the active cell mass (muscle mass) of the body has increased. The regulation of energy balance on a day to day basis seems to be better at high levels of energy turnover either as a consequence of the physiological need or as a consequence of the fact that athletes performing at a high level of energy expenditure have learned to ingest as much food as possible.