Thousands of endurance athletes travel each year to compete in events held at moderate altitude — from trail races in the Swiss Alps (1,500–2,500m) to mountain marathons in the Colorado Rockies (2,000–3,000m) to Andean ultramarathons above 3,500m. The majority arrive 1–3 days before the event — a timing that places them precisely in the highest-risk window for acute mountain sickness (AMS) and the greatest physiological impairment from altitude exposure without meaningful acclimatisation.
What Happens to Performance at Altitude
Above approximately 1,500m, atmospheric oxygen partial pressure begins to meaningfully decline. The physiological consequences for endurance athletes:
- VO₂max decline: VO₂max decreases approximately 1–2% per 100m above 1,500m. At 2,500m (1,000m above the threshold), VO₂max is reduced by 10–20% compared to sea-level values — a performance-limiting decrement that cannot be overcome by fitness alone.
- Lactate accumulation: At reduced PaO₂, the lactate threshold occurs at a lower fraction of VO₂max — meaning the pace that feels like threshold at sea level generates near-maximal lactate response at altitude.
- Increased ventilation: The hypoxic ventilatory response increases breathing rate at rest and exercise, which itself has a metabolic cost (respiratory muscles consume oxygen) and contributes to the fatigue of altitude.
- Dehydration acceleration: Altitude increases insensible fluid loss through respiratory passages — the drier, colder air at altitude and elevated ventilation rate can triple respiratory water loss compared to sea level. Athletes at 3,000m lose approximately 500ml per hour more fluid than at sea level through breathing alone.
Acute Mountain Sickness: Recognising the Signs
AMS is the most common altitude illness and affects 25–30% of athletes ascending rapidly to 2,500m and 50–60% at 3,500m without acclimatisation. Symptoms typically begin 6–12 hours after arrival at altitude and peak at 24–48 hours:
- Headache (the cardinal symptom — present in all AMS cases)
- Nausea, loss of appetite, or vomiting
- Fatigue and weakness disproportionate to activity level
- Dizziness and difficulty with balance
- Sleep disturbance (periodic breathing at night is universal above 3,000m)
Mild AMS resolves with rest, hydration, and acclimatisation over 24–48 hours. Severe AMS with ataxia (unsteady gait) or altered consciousness (HACE — High-Altitude Cerebral Edema) requires immediate descent and medical attention. Never attempt to race through symptoms of severe AMS.
The Timing Dilemma: Arrive Early or Arrive Late
The classic altitude racing strategy debate: arrive early to acclimatise, or arrive very late to minimise exposure before symptoms peak. The evidence supports both approaches, depending on lead time:
- Arrive 2–3 weeks early: Full altitude acclimatisation requires 2–3 weeks minimum at the race altitude. Plasma volume, erythropoietin response, and ventilatory acclimatisation are substantially complete. This is the optimal approach — but rarely logistically feasible for recreational athletes.
- Arrive 24–36 hours before the race: If early arrival (2+ weeks) is not possible, arriving less than 24 hours before the race start may be the second-best option. The initial physiological response to altitude exposure is most severe from hours 6 to 48 — arriving the day before places your race in the early part of the peak symptom window, but you may not yet have accumulated full AMS impact. Many athletes perform adequately on "early arrival" altitude exposure (within 24 hours).
- Avoid the 2–7 day window: Arriving 2–7 days before a race at altitude places your race day precisely in the period of maximal AMS symptom burden and maximal physiological impairment before acclimatisation benefits have developed. This is the worst-case timing.
Prevention: Medications and Supplements
- Acetazolamide (Diamox): 125–250mg twice daily, beginning 24 hours before ascent and continuing for 48–72 hours at altitude. Acetazolamide is the most evidence-supported AMS prophylaxis and reduces AMS incidence by approximately 50%. Requires a prescription. Side effects include increased urination, tingling extremities, and altered taste of carbonated beverages.
- Ibuprofen: 600mg three times daily beginning at ascent — reduces headache severity in AMS but does not address the underlying mechanisms. Appropriate for symptom management, not as a substitute for acclimatisation.
- Ginkgo biloba: Some evidence for modest AMS reduction at 120mg twice daily, though less consistent than acetazolamide.
- Hydration: Maintaining adequate hydration at altitude reduces AMS severity and supports plasma volume during the acclimatisation process. Increase fluid intake by 500–750ml per day above your sea-level baseline when at altitude above 2,500m.
Altitude Race Pacing and Fueling Adjustments
When racing at altitude without full acclimatisation, pace adjustments are non-optional. A practical pacing reduction framework: reduce target pace by 4–6% per 1,000m above 1,500m. A runner targeting 5:00/km at sea level should target approximately 5:15–5:20/km at 2,500m and 5:30–5:40/km at 3,000m. Fueling needs change modestly at altitude — the reduced caloric burn at slower race paces offsets the slightly elevated resting metabolic rate, and in-race fueling protocols remain similar. The larger dietary concern at altitude is appetite suppression (common, especially in the first 24–48 hours), which can lead to inadequate carbohydrate intake in the days before the race. Use the Race Day Nutrition Planner to set your altitude-adjusted race pace and calculate the corresponding gel timing and carbohydrate targets for your modified finish time.