Elderly subjects have a lower energy requirement compared to young adults as a result of lower physical activity and a lower basal metabolic rate. A lower energy intake in the elderly could lead to undesirable low intakes of essential nutrients. The reduction of the energy turnover is caused by, or a consequence of, a decrease in active cell mass. Thus, the effect of exercise training on habitual activity across age ranges, and age-related changes in body composition in relation to habitual activity level, are investigated here. The focus is on data on physical activity and body composition obtained with doubly labelled water. The results suggest that exercise training does not affect energy requirement in the elderly and the age-related decrease in fat-free mass is not delayed in subjects with a relatively high habitual activity level. Beneficial effects of exercise training in the elderly are endurance, flexibility, range of motion and balance control, all contributing to a delay in the age-induced impairment of personal mobility. Energy intake will inevitably go down as a result of a reduction of the energy requirement. Thus, the nutrition of elderly subjects needs more attention with regard to the essential nutrients than the nutrition of younger adults.