Exercise Can Damage Immunity

Hundreds of studies attest to effects of exercise on immunity, but most of them are useless as a general guide because they failed to control for exercise intensity in relation to the fitness status of the individual athlete.

Studies that have measured immune responses to light or moderate exercise report only mild and temporary changes. But studies on immunity after intense exercise show profound effects.

Monocyte concentrations in blood are increased threefold, indicating a big immune challenge. The lymphocyte proliferative response is suppressed suggesting that the immune system is being overwhelmed by the trauma of exercise. And the activity of natural killer cells is suppressed for hours afterwards. Because natural killer cells are your first line of defense, their suppression leaves you prey to opportunistic infections.

If the training is matched to the athlete, however, the immune system reacts to the trauma of exercise by growing stronger. Trained athletes in good health have a higher number of natural killer cells, and a higher level of killer cell activity than sedentary folk. They also have a higher base level of monocytes. Both animal and human studies show that training programs, carefully designed to provide sufficient stress to challenge the immune system but not enough to overwhelm it, result in stronger immunity.

But the average study simply recruits a bunch of subjects and arbitrarily decides exercise level and duration. If it is too light, nothing happens. If it is too heavy, immunity bombs. One recent study, for example, took young sedentary men from their habitual level of virtually zero exercise, to 40-50 minutes of aerobic exercise daily for five days a week. To you that might be a doddle, but to these guys it was boot camp. After 15 weeks, their natural killer cell activity was very depressed . Fifteen weeks of what was intense effort for them, damaged their health defenses and left them prey to infection.

When we first reviewed the evidence on sport and immunity in 1984, the Colgan Institute decided to track American, British, and Soviet athletes looking for evidence of immune suppression. We found plenty! Athletes from all three countries are more susceptible to infections than the general population. And as their training or competition intensity increases, so does their rate of illness.

Other researchers agree. Dr G. Asgiersson found that athletes are more subject to bacterial infections. Dr L. Fitzgerald of St. George's Hospital Medical School in London, reports that the immune systems of athletes at the top level of competition are often severely depressed, and they are especially subject to viral infections. Dr L. Salo found that elite swimmers become more susceptible to illness as the swimming season progresses and exercise intensity increases.

And there's the clue: exercise intensity. For example, marathon runners may be wonderfully healthy coming up to a marathon race. But after the intense effort of the race, many of them become ill in the following weeks. In one study, a third of all marathon finishers suffered an upper respiratory tract infection within two weeks after the race. In another just published study, Dr Gregory Heath and colleagues at the Centers for Disease Control in Atlanta and the University of South Carolina, counted upper respiratory tract infections in 530 male and female runners over 12 months. Frequency of infections was directly related to weekly mileage. The higher the mileage the more infections suffered by the athlete.

Soviet researchers report similar findings. After four months of intense competition, Soviet athletes (now Unified Team athletes), suffered a significant drop in the number and function of T-lymphocytes. Dr I. Surkina gives the example of one athlete from the Soviet ski team who showed the most severe depression of T-cell proliferation. During the five subsequent months, he suffered six different recurring infections. Think how that would devastate your training.

The next year the Soviets reduced the competitive season by decree. The athletes' immunity remained high and none became sick. But American and British athletes are not subject to government decrees. As emphasized by the rates of infection and injury before and during the Barcelona Games, far too many of us are chronically overtrained, and have chronically suppressed immunity.

Research studies agree. Elite American and British athletes have manv more days off for illness than club level athletes. In runners, infections can cause more days off training than injuries. In American marathon runners, the most elite and hardest training have the lowest lymphocyte counts. Both male and female members of the US cross-country ski team have poorer immunity than control subjects. Dr Rod Fry of the University of Western Australia, has just published an excellent review showing that elite athletes are often overtrained, immune suppressed, and prone to infections.

That seems to leave you caught between the proverbial rock and hard place. If you don't train intensely, you can't reach your potential. If you do train intensely, you devastate your immunity.