Altitude exposure may lead to considerable weight loss. Most reports, showing weight losses of 3% in 8 days at 4300m and up to 15% after 3 months at 5300 to 8000m, appear to indicate that this weight loss is a function of both absolute altitude and the duration of exposure. Based on the available scientific evidence to date, it is concluded that altitude weight loss is because of an initial loss of water and subsequent loss of fat and muscle mass due to malnutrition. Up to 5500m, malabsorption of macronutrients does not occur. Up to altitudes around 5000m, weight loss from a reduction of fat and muscle appears to be avoidable by maintaining adequate dietary intake. Primary anorexia, lack of comfort and palatable food, detraining, and possibly direct effects of hypoxia on protein metabolism seem inevitably to lead to weight loss during longer exposures at higher altitudes. To minimise losses, it is advisable to acclimatise properly, reduce the length of stay at extreme altitude as much as possible and maintain a high and varied nutrient intake. With sojourns at intermediate altitude for training purposes, adequate energy intake should be maintained taking into account the decrease in aerobic training intensity and the increase in basal metabolic rate that ensue from the hypoxic environment.