How Exercise Affects Your Immune System

  • By Paul Moore
  • Published August 20, 2010

As August once again seems intent on giving us a good dose of almost Autumnal weather, sniffle season begins. That might particularly be the case if you’ve just finished a race of some sort. So what happens to your immune system following (or during) a bout of heavy exercise? Jeffrey Sankoff explains.

Written By Jeffrey Sankoff
The health benefits of a regular exercise programme are hard to overstate. It has long been known that maintaining a regular exercise programme strengthens the cardiovascular, respiratory and skeletal systems. Then why is it that so many athletes succumb to colds after participating in endurance events? Can a triathlon actually make you sick? This question is one that I have been asked frequently and the answer isn’t straightforward.

The immune system is very complex. It consists of several types of cells that circulate in our blood and are collectively referred to as white blood cells. The immune system serves two purposes: to destroy anything foreign in the body and to kill cells that have mutated in some way. The first of these functions relates principally to fighting off infection while the second prevents the development and spread of many cancerous cells.

To combat an infection, the immune system has to be able to first recognise anything in the body that does not belong there. Then other immune system cells are alerted to the presence of the foreign material so that they can produce the necessary components to kill it.

Exercise has long been known to affect the immune system. Scientific experiments on animals and humans have demonstrated that when they exercise regularly at moderate intensity for periods of up to an hour a day, the number of immune cells circulating in the blood increases. Furthermore, the effectiveness of those cells is heightened, leading to greater virus killing ability. Researchers have reported that these effects can result in a nearly 50 per cent reduction in the number of colds experienced by those who exercise regularly versus those who do not.

However, prolonged efforts at high intensity cause countervailing effects. When the body is stressed in this manner, the adrenal glands secrete two hormones into the bloodstream, cortisol and epinephrine (adrenaline). These hormones are crucial to maintaining high-level physical effort over a prolonged period of time. Together they liberate energy stores, improve cellular metabolism and elevate the heart rate and blood pressure, and thereby enhance performance. Unfortunately, these hormones also cause a significant decrease in the effectiveness of the circulating immune cells, and this effect lasts as long as several days.

As a result, it is not at all uncommon for triathletes to become ill in the week after a long-course race. While these hormonal effects are seen even in Olympic-distance racing, the effects are more pronounced in longer races.

Knowing that we are more susceptible to illness after such a race, is there anything that can be done to improve our chances of not getting sick? Here are a few things you can try:

AVOID EXPOSURE: You can’t get sick if you aren’t exposed to someone who is already. If a family member is ill, try to keep some distance.

WASH YOUR HANDS FREQUENTLY AND THOROUGHLY: Research has shown that the routine use of hand sanitisers can reduce infection rates.

TAKE VITAMIN C: Although the magnitude of the effect of once-daily 500 or 1,000 mg of vitamin C supplements is open to question, taking this vitamin may help prevent infection or reduce the severity and duration of symptoms if you become infected.

Train hard, train healthy.

Jeffrey Sankoff, MD, is a two-time Ironman triathlete an ER rdoctor at the University of Colorado Health Sciences Centre in Denver, Colo. For more information, visit his website at

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Paul Moore

Paul Moore

Paul Moore is the Online Editor for Triathlete Europe. When not glued to a computer he can be found writing books - most recently Ultra Performance: The Psychology of Endurance Sports and The World's Toughest Endurance Challenges. Both are available on Amazon. Paul has also written Ultimate Triathlon: A complete training guide for long-distance triathletes which is also available on Amazon.