Thursday, November 10, 2011

Exercise... Bad For The Immune System?!

Ever wonder why you felt cold-like symptoms the morning after a hard workout?  While exercise has always been known to provide a long list of health benefits, it may be surprising to hear that exercise can actually hurt the immune system.  Depending on the intensity, duration and the time spent on recovery, the immune response to exercise can be either good or bad.  Researchers who investigated the relationship between the immune system and exercise found that two changes typically occur.  Both cell production and their function changes after a bout of exercise (1,2,3,4,6).  Negative changes in the immune system tends to occur with continuous endurance exercises and interval training.  This news probably sounds ridiculous since that describes just about every type of aerobic workout, but don't worry, I'll explain how to prevent this problem from occurring.

If I were to say that neutrophil production increased as a result of exercise, this wouldn't mean much to anyone without knowing what a neutrophil is and what it can do.  To get around this, I compiled a list of immune cells which may be directly affected by exercise and describe each of their functions (5).
  • Leukocytes:  A fancy word for white blood cells.  The two major types of leukocytes are lymphocytes and phagocytes.
  • Neutrophil:  These cells are the kamikaze fighters of the body.  Naturally, there is a greater number of neutrophils than any other immune cell in the body.  This is a good thing because when a neutrophil engulfs an antigen to kill it, the neutrophil also dies.  Neutrophils can also kill bacteria through oxidative bursts which are highly oxidized chemicals toxic to bacteria.
  • Monocytes:  If a T cell happens to miss an antigen, a monocyte will find it and bring it back to the T cell.  Monocytes cannot destroy antigens- they can only find and deliver the antigens to the cells that can kill them.  Monocytes which leave the bloodstream turn into macrophages, a form which has the ability to destroy antigens through oxidative bursts or phagocytic activity.
  • Natural Killer:  Rather than targeting an antigen, NK cells engulf cells which look unnatural on the surface such as tumor cells.
  • B & T Cell:  The B lymphocyte and T lymphocyte cell is designed to identify specific antigens and once identified, they will multiply and destroy infected cells, respectively.
In high intensity continuous aerobic exercise lasting less than 45 minutes, neutrophil production and function is at its greatest.  This increase has been shown to remain higher than normal two to four hours after exercise (5).  During this time, the neutrophil also becomes more active, destroying antigens and removing waste that was caused by the exercise bout (4,6).

In contrast to improved neutrophil production and activity, monocytes do not see the same benefits.  Although the number of monocytes produced is higher after high intensity exercise, they become less functional as a macrophage, the form which is able to directly destroy antigens.  The specific function which appeared to become suppressed was its ability to use oxidative bursts to destroy waste products (3,4,6).  After a high intensity exercise bout, monocyte levels remain higher than normal for about two hours (5).

The same problem occurs with NK, T and B cells.  Although cell production increases after high intensity continuous aerobic exercise, the activity of all these cells drops below resting levels about two hours after the exercise bout.  NK cells lose function for two to four hours.  T and B cells lose function for one to two hours (5).  Losing function in these cells for such a great time leaves the body more susceptible to infection and sickness (1,4).

Longer duration high intensity exercise tends to exaggerate the effects found with shorter duration exercises lasting less than 45 minutes (5).  The only exception involves reduced function of the neutrophil (3,4,6).

Notice that the type of exercise associated with decreased immune function involved high intensity aerobic exercise which got worse as duration also increased.  In contrast to high intensity aerobic exercise, moderate intensity aerobic exercise was shown not to cause the cells of the immune system to lose function or slow cell production.  In fact, moderate intensity exercise either caused an improvement or no change to the resting levels of immune cell activity.

The key point to take away is to try to avoid doing too much high intensity exercise.  There has to be enough time dedicated towards moderate intensity workouts to allow the body to make the necessary repairs so that it can continue to meet the demands of training.  This idea might sound familiar because it's one of the key principles of exercise program design- recovery!

So next time the urge comes on to go all-out all the time, remember that it will come back to bite in the form of an injury related to residual waste which never had a chance to exit the body.  Try to remember that patiently moving through an exercise program will get the results better than rushing workouts through "quick fix" routines that focuses mainly on high intensity and less on moderate intensity.


    1. Nieman, D.C., L.M. Johanssen, J.W. Lee & K. Arabatzis: Infectious episodes in runners before and after the Los Ange-les Marathon. Journal of Sports Medicine and Physical Fitness. 30:316–328 (1990)
    2. Nieman, D.C., K.S. Buckley, D.A. Henson, et al.: Immune function in marathon runners versus sedentary controls. Med-icine and Science in Sport and Exercise. 27:5986–5992 (1995b)
    3. Nieman, D.C.: Immune response to heavy exertion. Journal of Applied Physiology.82(5):1385–1394(1997a).
    4. Woods, J.A., J.M. Davis, J.A. Smith, D.C. Nieman: Exercise and cellular innate immune function. Medicine and Science in Sport and Exercise.31(1):57–66(1999).
    5. Plowman, Sharon A., and Denise L. Smith. Exercise physiology for health, fitness, and performance. 3rd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2011. Print.
    6. Woods, J.A., J.M. Davis: Exercise, monocyte/macrophage function, and cancer. Medicine and Science in Sports and Exer-cise. 26(2):147–157(1994).