Running at elevation presents a distinct physiological challenge due to the decrease in the partial pressure of inspired oxygen, making each breath less effective. This phenomenon begins to affect athletic performance noticeably at elevations generally above 5,000 feet (about 1,500 meters). Although the percentage of oxygen in the air remains the same, lower barometric pressure means fewer oxygen molecules reach the bloodstream. This reduced oxygen delivery directly impairs the body’s aerobic capacity—the maximum amount of oxygen the body can use during intense exercise. Acclimation is the necessary process of physiological adjustments the body must make to safely sustain running performance in a hypoxic environment.
The Immediate Physiological Impact of Altitude
The body initiates an immediate response to compensate for the lack of available oxygen. Within minutes of arrival, chemoreceptors detect the lowered oxygen content in the blood and trigger hyperventilation. This increased breathing rate attempts to raise oxygen pressure in the lungs, but it also causes an excessive loss of carbon dioxide, affecting the blood’s pH balance.
The cardiovascular system also reacts instantly by elevating the resting and submaximal exercise heart rate and increasing cardiac output. This mechanism speeds up the circulation of the existing, less-saturated blood, enhancing oxygen delivery to the working muscles. This elevated heart rate means the runner is working harder and less efficiently than at sea level, resulting in an instantaneous decline in endurance performance.
Performance reductions are immediate, with maximal oxygen uptake (\(\text{VO}_2\text{max}\)) decreasing by approximately 10% for every 1,000 meters ascended above 1,200 meters. Energy metabolism shifts slightly toward anaerobic pathways, which leads to increased lactate production and quicker fatigue during prolonged efforts. This initial shock also causes a temporary reduction in plasma volume as the body tries to concentrate red blood cells, contributing to fluid loss.
Establishing the Acclimation Timeline
The time required for a runner to acclimate is not a single fixed period but a series of distinct physiological adaptations occurring over weeks. Short-term functional adjustments begin almost immediately and are largely complete within the first week. This initial phase involves adjusting the body’s acid-base balance to accommodate hyperventilation and stabilizing fluid losses.
Within the first three to seven days, a runner’s ability to tolerate basic activity improves as enzymatic and fluid balance changes allow for better function. This short-term adaptation is sufficient for survival and light activity, but it does not restore significant aerobic performance. Runners should not expect to feel close to their sea-level fitness level during this time.
Mid-term performance restoration typically takes about two to three weeks, often considered the ideal minimum for training camps. During this period, cellular adaptations occur, including increased capillarization in muscle tissue and changes in mitochondrial function, improving the muscle’s ability to utilize limited oxygen. By two weeks, performance should return closer to sea-level norms, though still below maximum potential.
A long-term, maximizing adaptation involves significant hematological changes, specifically an increase in red blood cell count and total hemoglobin mass. This process, stimulated by the hormone erythropoietin (EPO), takes four to eight weeks to fully develop. The full benefit of increased oxygen-carrying capacity requires this longer duration, which is why professional athletes often spend a full month or more at moderate altitude.
Practical Strategies for Safe Training
Runners must implement several practical strategies to manage the initial stress of altitude safely and effectively. In the first week, a drastic reduction in training intensity is non-negotiable. Runners should reduce their pace by one to two minutes per mile compared to sea-level efforts, focusing instead on maintaining effort, not speed.
Instead of relying on pace or heart rate, which is artificially elevated, runners should use the Rate of Perceived Exertion (RPE) scale to guide their training. Training efforts should feel easy to moderate, avoiding hard workouts for the first several days. Avoiding over-exertion early on prevents compounding the body’s stress response and reduces the risk of illness and fatigue.
The hypoxic environment and compensatory breathing cause increased respiratory water loss, requiring a substantial increase in fluid intake. Runners should aim to drink an extra 1 to 1.5 liters of water daily, prioritizing hydration with electrolytes to offset fluid and mineral loss. Additionally, the body uses carbohydrates more readily at altitude, making it important to consume a diet slightly higher in quality carbohydrates to fuel training.
For runners traveling for a single competition, the timing of arrival is a significant strategic consideration. Arriving either just before the event (less than 24 hours prior) or waiting the full 10 to 14 days is generally recommended. The intermediate period (days three through five) is associated with the worst fatigue, dehydration, and poor sleep, making performance during this window detrimental.
Recognizing and Managing Altitude Sickness
The normal physiological strain of altitude must be distinguished from Acute Mountain Sickness (AMS), the most common form of altitude illness. AMS symptoms typically develop within 8 to 24 hours of arrival and resemble a severe hangover, including headache, nausea, loss of appetite, dizziness, and general fatigue. These symptoms are caused by the body’s struggle to adapt to lower oxygen levels.
If AMS symptoms are mild, primary management involves stopping exercise, resting, ensuring proper hydration, and taking common pain relievers for the headache. Symptoms that worsen or fail to resolve after a day of rest at the same elevation signal that immediate descent is required. Descent is the single most effective treatment for all forms of altitude sickness.
Runners must also be aware of the signs of the more severe, though rare, conditions: High-Altitude Cerebral Edema (HACE) and High-Altitude Pulmonary Edema (HAPE). HACE (swelling of the brain) is indicated by severe confusion, altered mental status, and loss of coordination. HAPE (fluid in the lungs) is characterized by a persistent cough, decreased exercise tolerance, and shortness of breath even at rest. Both HACE and HAPE are medical emergencies that require immediate descent of at least 1,000 meters and urgent medical attention.