Altitude Acclimatisation: How Your Body Adapts to High-Elevation Hiking

Altitude hiking is among the most physiologically demanding forms of recreational exercise. As elevation increases, barometric pressure falls and the partial pressure of oxygen decreases — meaning each breath delivers less oxygen to the lungs. At 3,000 metres, oxygen availability is approximately 30% lower than at sea level. At 5,000 metres (the altitude of Everest Base Camp), it is roughly 50% lower. The body responds to this hypoxic stress with a cascade of physiological adaptations — some occurring within hours, others over days or weeks — collectively known as acclimatisation.

Acute mountain sickness (AMS). Acute mountain sickness is the most common altitude-related illness, affecting 25–40% of people who ascend rapidly to 2,500–3,500 metres. Symptoms include headache (the cardinal symptom), fatigue, loss of appetite, nausea, and dizziness. AMS is caused by mild cerebral oedema (swelling of brain tissue) in response to hypoxia. It is generally self-limiting and resolves with rest and acclimatisation. The Lake Louise Score is the standard clinical tool for assessing AMS severity. The primary risk factor is the rate of ascent — ascending too quickly does not allow time for physiological adaptation.

Serious altitude illness: HACE and HAPE. High-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE) are rare but life-threatening conditions that can develop from untreated AMS or rapid ascent to very high altitude. HACE involves severe brain swelling and presents with ataxia (loss of coordination), altered consciousness, and eventually coma. HAPE involves fluid accumulation in the lungs and presents with breathlessness at rest, cough, and pink frothy sputum. Both conditions require immediate descent and emergency medical treatment. Recognising the warning signs — worsening headache despite rest, ataxia, confusion, breathlessness at rest — is potentially life-saving.

The acclimatisation process. Over 3–7 days at altitude, the body undergoes deeper adaptations. Erythropoietin (EPO) production increases, stimulating red blood cell production and increasing oxygen-carrying capacity. Breathing patterns shift to maintain better oxygenation during sleep (periodic breathing is common at altitude and is normal). Muscle cells increase mitochondrial density and improve oxygen utilisation efficiency. These adaptations explain why athletes train at altitude to improve sea-level performance — the increased red blood cell mass and improved oxygen efficiency persist for weeks after returning to lower elevations.