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How Exercise Affects Immunity


Exercise as it relates to immunity is a mixed message. Regular, moderate-intensity physical activity has been shown to help protect people against some diseases, particularly those that involve the upper respiratory track (like colds). However, too much exercise can have the opposite effect and reduce immunity. The keys are (1) knowing how much exercise is enough, (2) when exercise is appropriate and when it's not, and (3) which types of exercise are appropriate for your particular situation. Here are the details.

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Exercise and Immunity by the Numbers

The number of upper respiratory infections the average adult gets per year.

The number of weeks a person should wait before engaging in intensive physical training after serious “below the neck” symptoms (coughing, vomiting, diarrhea) caused by a respiratory infection.

25 – 50
The percentage of decrease in sick time for active people who complete at least 45 minutes of moderately-intensive exercise most days of the week

How Exercise Works to Boost Immunity

Most of the research on exercise and immunity has been done with colds and there's a considerable amount of research showing that regular, moderate exercise enhances the immune system. Several studies reported that recreational exercisers and athletes had a lower incidence of colds when they were engaged in a running program.

Exercise has been shown to increase the production of macrophages, which are cells that attack the kinds of bacteria that can trigger upper respiratory diseases. More recent studies show that there are actually physiological changes in the immune system that happen when a person exercises. Cells that promote immunity circulate through the system more rapidly, and they're capable of killing both viruses and bacteria. After exercising, the body returns to normal within a few hours, but a regular exercise routine appears to extend periods of immunity.

Some people believe that the temporary rise in body temperature that occurs during exercise may inhibit the growth of bacteria. This process allows the body to fight infection more effectively. Exercise also slows the release of stress-related hormones, and stress is shown to increase the likelihood of illnesses.

Dr. David Nieman, an exercise immunologist at Appalachian State University, is one of the country’s most respected authorities in this area. One of his studies showed that people who walked at 70 to 75 percent of their VO2max for 40 minutes per day reported half as many sick days because of colds or sore throats compared to people who didn't exercise.

However, too much exercise appears to negatively affect immunity. One study found that 90 minutes or more of high-intensity exercise (marathons, endurance races) makes a person more susceptible to illness for up to 72 hours after working out. During exercise, the body produces two hormones, cortisol and adrenaline, that raise blood pressure, elevate cholesterol levels, and temporarily weaken the immune system. 

Heavy, long-term exercise could increase the amount of white blood cells and increase the presence of stress-related hormones. Marathon and triathlon athletes are particularly vulnerable to increased susceptibility to infection, although susceptibility doesn't automatically lead to infection.

Exercising While Sick

It's typically safe to exercise at a low intensity if you have "above-the-neck" symptoms (runny nose, sneezing, sore throat). If those symptoms diminish during the first few minutes of exercise, the intensity may be increased. Exercise isn't recommended for people with "below-the-neck" symptoms (fever, sore muscles or joints, vomiting, diarrhea, or a cough that produces mucous). If you have those symptoms, let the cold run its course before you resume physical activity.

Wait two weeks after symptoms subside before engaging in intensive training. When you do return to training, allow two days of below-normal exercise intensity for each day you were sick before jumping back into your normal training routine.

Dr. Nieman recommendations include:

  • Exercise, but don't overtrain, if you just have a head cold.
  • Don’t exercise if your cold or other illness is “systemic”—that is, involves other parts, systems, or organs of the body.
  • Engage in moderate exercise before getting a flu shot. The physical activity could improve your body’s response to the vaccine and enhance your immunity to the flu.

Other Factors Impacting Immunity

The risk of infection is amplified when other factors related to immune function are present, including exposure to disease agents during travel, lack of sleep, severe mental stress, malnutrition, and weight loss. To counter the increased risk of infection, athletes should consider the guidelines below, each of which has a separate connection to the immune system and protection of the host against disease-causing agents.

  • Keep other life stresses to a minimum.
  • Keep vitamin and mineral stores in the body at optimal levels. 
  • Avoid overtraining and fatigue.
  • Get adequate sleep on a regular schedule.
  • Avoid rapid weight loss.
  • Avoid putting your hands to your eyes and nose.
  • Avoid sick people and large crowds when possible.
  • Get a flu shot during winter months and when flu epidemics (like H1N1) are present.

Endurance Athletes and Immunity

For endurance athletes, especially ultra-endurance athletes, the key to avoiding an impaired immune system is allowing your body and your immune system time to recover (also referred to as active recovery and passive recovery). Athletes with symptoms of overtraining, such as increased resting heart rate, slower recovery heart rate, irritability, or continuing fatigue, should reduce the intensity and frequency of endurance training to create a balance between exercise and

Take-Home Message

Exercising at a moderate intensity at least 40 minutes per day helps the body resist a variety of diseases and conditions. High-intensity endurance activities that last 90 minutes or longer may increase your susceptibility to infection for up to three days. As with diet and other health-related behaviors, a program of exercise that consistently and progressively challenges the systems of the body without severely compromising them is the most sensible training strategy.

Jim Brown, Ph.D. has written 14 books on health, medicine, and sports. His articles have appeared in the Washington Post, New York Post, Sports Illustrated for Women and Better Homes & Gardens. He also writes for the Duke School of Medicine, UCLA School of Medicine, Cleveland Clinic and Steadman-Hawkins Research Foundation.


Tags: Health