Running at altitude has many effects on the body.
Running at altitudes of 6,500 feet and higher causes a variety of different effects in the body. It is much more physically demanding on the body than running at sea level; while it can result in faster running times and an increased performance, it can also cause a loss of muscle mass, an increase in red blood cells and a faster heart rate.
Red Blood Cells
Training at a high altitude increases the red blood cell count in the body which enables the body to more efficiently and quickly carry oxygen around the body. This red cell count stays higher for a few days, even after returning to sea level, and can result in a significant speed advantage over those who haven't trained at altitude. This can be seen in the 1968 Mexico Olympics where athletes who had previously set records in running events over 800m were beaten by athletes native to high altitudes.
Oxygen Intake
When running at altitude you will notice it is harder to breathe. This is because the air is thinner at higher altitudes and contains less oxygen, so you aren't able to effectively replenish your blood with oxygen as you would at a lower elevation. This lack of oxygen results in you breathing harder and faster than normal. However, the increased breathing, or hyperventilation, does decrease the carbon monoxide in the blood which causes a higher blood pH and helps counteract lactic acid-induced fatigue which means your muscles won't tire as quickly.
Weight Loss
Running at a high altitude changes the body composition. A study has shown that runners living at altitudes higher than 4,500 feet lose around 100 to 200g of muscle mass a day. One reason for this is that your metabolism increases when you are at altitude which results in weight loss. To avoid this, a diet based on at least 60 percent carbohydrates is recommended. Some of this weight loss can also be attributed to dehydration, so runners need to make sure they stay adequately hydrated.
Heart rate
Running at a high altitude decreases atmospheric pressure on the human body which results in decreased oxygen diffusion through the lungs and into the bloodstream. The body compensates for this decreased oxygen with a faster heart rate and this increase in heart rate pushes the blood more quickly around the body. This in turn distributes oxygen to tissues in need much faster, however, as the body acclimatizes to the high altitude the heart rate will return to normal.
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