Why Do People Faint after Strenuous Physical Activity

Histamine is closely connected to lower blood flow and pressure which occur after exercising intensely and bring about fainting

By Alexandra Lupu on October 11th, 2006 15:01 GMT
A recent study run by researchers at the University of Oregon showed that strenuous exercise leads to fainting because of the overactivation of two histamine receptors in our bodies. Heightened levels of exercising make a large number of people pass out, and this does not refer only to people who are not usually active, but also highly to trained athletes.

Most of the individuals faint after high-levels of physical activity, when at exertion, because their blood pressure becomes lower and the blood flow to the brain also decreases. Passing out after exercising intensely is a condition known in medical terms as 'syncope' and may be one of the earliest signs of heart problems.

"There is reason to believe that histamine is the primary vasodilator contributing to post-exercise hypotension, but we cannot say for certain. Some people have problems regulating blood pressure during and after exercise. Trained athletes have had fainting bouts at the end of exercise. It may be that these result from a natural overactivation of these two receptors for histamine," explained lead researcher John R. Halliwill, Professor of Human Physiology at the University of Oregon.

Researchers found that the two histamine receptors which cause blood flow and blood pressure to become lower after training strenuously and bring about fainting are known as H1 and H2. But the passing out phenomenon occurring when people become exerted after intense exercising can be avoided by using two commonly known antihistamines prior to the physical activity. The two antihistamines are fexofenadine for the H1 histamine receptor and ranitidine, which works against the H2 receptor.

On the other hand, researchers also pinpointed the fact that maintaining the two histamine receptors normal throughout the training period might also be beneficial for the individual, as it may play an important role in the reduction of high blood pressure if exercising regularly.

"Activating these receptors might be an important part of the health benefits of daily exercise. The body tends to be very good at recycling mechanisms. The body may be using these same receptors for other things. A bout of exercise appears to turn on a program for remodeling blood vessels in the body, and these receptors may be an important part of that program," said Halliwill.
  
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