Athletes and Altitude
VO2max is the body's maximal ability to extract oxygen from the air and deliver it to the tissues. Above 5000 feet the maximum work a person can do decreases by 3% for every 1,000 feet. This means your body's ability to utilize oxygen diminishes with increasing altitude. Even after acclimatization, this only improves a little bit and a person can never perform as well at altitude as they can at sea level. With increasing altitude, you need to take more air into your lungs, contributing to the breathless feeling that many athletes experience when first coming to altitude, and especially if trying to perform at the same intensity as at sea level. One of the processes in acclimatization important for athletes is the production of a hormone called EPO or erythropoietin. This hormone acts on the bone marrow to produce more red blood cells. These cells carry oxygen from the lungs to the tissues. Increasing these cells effectively increases the 'oxygen carrying capacity' in your blood. However, this process takes weeks. EPO is a big topic among competitive endurance athletes.
Competing at Altitude
For those athletes doing aerobic events over 5000 feet, 10-20 days of acclimatization at the performing altitude is ideal. Athletes participating in events over 12,000 feet must have acclimatization at an intermediate altitude prior to performance. Performing without acclimatization at this altitude could cause altitude sickness. Those participating in anaerobic sports (short intense events lasting less than 2 minutes, such as sprinting) at altitude do not require extended acclimatization, and may perform better because of lower air density.
Training at Altitude for Sea Level Events
Training at moderate altitude may enhance sea level performance in endurance athletes. This was noted after the 1968 Olympics in Mexico City (7600 feet) when some athletes found an improvement in their performance after return to sea level. However, other athletes did not notice any difference in performance. Studies have shown improved aerobic power in runners who trained at 6,000 feet for 10 days then performed at low altitude. Lower oxygen levels at altitude stimulate EPO leading to increased red blood cells or hematocrit. This effectively allows more oxygen to be carried to the tissues. Essentially, this is blood doping the natural way.
Recent studies do suggest benefits from 'training low' and 'sleeping high'. This includes hypoxic tents that many athletes are now using. These tents have a generator which extracts oxygen from the air, creating a moderate altitude environment. Athletes then sleep/lounge in them for several hours a day. This stimulates erythropoietin, thereby increasing the hematocrit (red blood cell count). Indeed the 'live high, train low' concept may allow the best combination. One study divided 39 competitors into three training groups; living and training at sea level, living at altitude and training at sea level, living at altitude and training at altitude. Both groups who lived at altitude had increased red blood cells and VO2max, but only the group who lived high and trained at sea level had improved race times. One explanation for this is that the live high, train low group is able to train harder and better utilize their blood's oxygen carrying capacity. Because of increased red cell production, all athletes training and/or sleeping at altitude should make sure they have appropriate iron stores and nutrition. This is especially true for women, as they tend to have lower iron stores due to periodic loss of blood through menstruation.
The response of erythropoietin to hypoxia is quite individual. While some have a significant response, others barely respond at all and may not have a benefit from sleeping and/or training at altitude.
While training at moderate altitude can be beneficial there are limitations. Athletic training above 8,000 feet is not generally recommended. At these higher altitudes, your exercise capacity decreases to the point that "deconditioning" can take place. Prolonged recovery time increases time required between training sessions. At even higher altitudes increasing catecholamines (epinephrine and norepinephrine), the body's stress hormones, contribute to weight loss and muscle wasting.
Other considerations of training at altitude
Nutrition becomes increasingly important when training even at moderate altitude. Rising stress hormone levels in response to lower oxygen levels place a higher demand for fuel on your body. As resting metabolism rates increase, athletes need to add to their caloric intake. Moderately increasing carbohydrate intake is key in replacing glycogen stores in your body after training and compensating for the increased caloric demand at altitude.
Hydration in the athlete is more important at altitude as well. Some athletes sweat 0.5 to 1.5 liters an hour. In addition, the lungs must humidify the air, which requires more water in the dry air at high altitude. To monitor hydration, a good rule of thumb for an athlete is to weigh themselves before and after training. Hydration to the pretraining weight will help athletes stay on top of their individual fluid needs.