At altitudes above 4,000 meters, appetite loss is not mental fatigue or lack of discipline — it is a predictable physiological response driven by hypoxia and high-altitude metabolic stress. As oxygen levels decline, the body reorganizes its priorities to maintain essential functions: oxygen delivery, cardiovascular stability, and neurological protection. Digestion becomes secondary

In low-oxygen environments, gastrointestinal blood flow decreases sharply, slowing gastric motility and suppressing hunger signals. Hypoxia also reduces ghrelin — the hormone responsible for appetite — which means climbers stop feeling hunger even when their energy levels are critically low. On top of this, rapid breathing at altitude alters blood pH, triggering high-altitude nausea in up to 70% of climbers and making food even harder to tolerate.

Energy expenditure, however, increases dramatically. Metabolic rate can rise by 20–30%, with daily caloric needs often reaching 3,500–6,000 kcal/day, creating a severe caloric deficit that affects strength, decision-making, and overall climbing performance. Dehydration, common due to cold dry air and accelerated respiration, further reduces digestive efficiency and amplifies appetite suppression.

This appetite collapse is part of the acclimatization process: the body shifts energy toward oxygen transport, red blood cell production, and ventilatory adaptation. Eating becomes a low-priority task from a survival standpoint, even though fuel is essential for performance.

To minimize the deficit, climbers should rely on small, frequent carbohydrate doses (easier to metabolize under hypoxia), maintain aggressive hydration with electrolytes, choose warm and easily digestible foods, increase caloric intake at lower camps when appetite is still normal, and use simple sugars during ascent for rapid energy. Appetite loss combined with headache or nausea should always be treated as an early warning sign of acute mountain sickness.