Altitude exposure decreases energy intake and thus induces an energy deficit resulting in the loss of body mass. The energy deficit is worsened when energy expenditure is increased by exercise because the increase is not matched by an increased energy intake. The indicated fuel for the optimal use of the rarefied oxygen at altitude is carbohydrate. For optimal taste and maximizing energy intake, fat is the best. Protein should rather be limited because of its high thermic effect. Intestinal malabsorption probably does not play a role in the energy deficit. Even though the sensation of exertion during climbing at high altitude is intense, the actual energy expenditure is not high.