Reduced physical activity commonly occurs in patients with disease or chronic disabilities, in the elderly, and in certain patients with obesity. Surprisingly, information on the effect of inactivity on energy homeostasis is scarce and often difficult to interpret. In models of reduced physical activity, such as space flights, bed-rest and confinement, subjects frequently lose weight (<5 %), predominantly in the form of fat-free mass. In some cases this is compensated by an increase in fat

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... ss, which means that changes in weight are poor indicators of energy balance. The extent to which spontaneous reduction in energy intake (in most studies energy intake is fixed) compensates or overcompensates for the reduction in energy expenditure (mainly physical activity and to a small extent in BMR, typically <6 %) is largely underexamined. Preliminary observations suggesting that there is a preferential selection of low-energy-dense foods (low in fat) require confirmation under carefully controlled experimental conditions. It is concluded that a comprehensive and systematic evaluation is needed to address the effects and relevance of various degrees of physical inactivity to energy homeostasis, in relation to disease and space medicine.