Altitude exposure leads to considerable weight loss. The different hypotheses that have been put forward to explain this phenomenon are discussed reviewing the literature: 1) a primary decrease of food intake due to loss of appetite caused, directly or indirectly, by hypoxia, changes of menus, comfort and habits, 2) a discrepancy between energy intake and energy expenditure due to an increased basal metabolic rate and/or high levels of activity which are not matched by an increased food intake, 3) a loss of body water due to increased insensible loss through increased ventilation in the mountain environment, decreased liquid intake, and/or changes in water metabolism, 4) an impaired absorption of nutrients from the gastrointestinal tract, and 5) a loss of muscle mass due to lack of physical exercise and/or direct effects of hypoxia on protein synthesis. It is concluded that altitude weight loss is due to an initial loss of water and subsequently to loss of fat mass and muscle wasting. Up to altitudes around 5000 m the weight loss from fat and muscle seems to be largely avoidable by maintaining adequate intake in a comfortable setting. Primary anorexia, lack of comfort and palatable food, detraining, and possible direct effects of hypoxia on protein metabolism seem to inevitably lead to weight loss during longer exposures at higher altitudes. In order to minimize losses it is advisable to acclimatize properly, to reduce the length of stay at extreme altitude as much as possible and to maintain a high and varied nutrient intake.