THE effects of exercise on resistance to illness, from the common cold to cancer, have long been surrounded by controversy and conflicting evidence. While few would dispute the value of fitness in warding off heart disease and osteoporosis, when it comes to the immune system the facts are not nearly so clear. Nonetheless, millions of people believe that regular exercise keeps them healthier.
Many avid exercisers insist that their fitness routine helps them ward off colds and recover more quickly from the minor illnesses they do get. Unless they are too sick to move, they tend to keep running, cycling, swimming or whatever when others with similar symptoms would lie low.
Since 1985, the American Cancer Society has recommended regular exercise as part of its 10-step program to prevent cancer. Although the evidence supporting this advice is far less established than, say, that for stopping smoking, the society concluded that exercise cannot hurt and preliminary evidence suggests that it may help to ward off certain common cancers.
Exercise and Immunity
Last fall, the decision of Magic Johnson to return to professional basketball, which he later reversed, focused public attention on the effects of exercise on the immune system. The focus of the attention was not just on those infected with H.I.V., the virus that causes AIDS, but also on healthy people who might one day be invaded by disease-causing microorganisms or malignant cells. Experts who have studied the data say more research is needed before advising the public on exercise and infectious illness.
Meanwhile, these are some findings worth considering:
*Regular moderate exercise appears to help ward off minor infectious ailments, but exhausting exercising impairs the immune response. Some studies have found that runners who are committed to regular exercise have fewer infectious illnesses, but following heavy training or after running a marathon, runners tend to experience more illness than others. Likewise, a study of 61 conditioned athletes on a university crew team found more frequent and more severe upper respiratory infections among them than among 126 unconditioned cadets in the university's Reserve Officers Training Corps. In other words, for exercisers who overdo it, the damaging effects of stress can outweigh the benefits of physical activity.
*Moderate exercise, both in experimental animals and in people, often -- but not always -- results in a temporary increase in blood levels of various immune cells and substances that may improve resistance to infection. For example, men who worked out on an indoor bicycle had an immediate increase in the number of circulating white blood cells that destroy foreign microbes and that stimulate production of antibodies and other substances that attack invaders.
*One of the substances that rises after exercise is endogenous pyrogen, the protein that causes fevers. A temporary increase in body temperature is a common result of physical activity, and this "fever" may help to squelch an infection before it can take hold.
*Many of the beneficial immunological changes are more likely to occur in unconditioned people who exercise than in those who are already very fit. On the other hand, laboratory animals that are "conditioned" before being exposed to a disease-causing virus are less likely to get sick and die if they are then exercised to exhaustion.
So should you exercise when you feel sick? Based on available evidence, Dr. Edward R. Eichner, a professor of medicine at the University of Oklahoma in Oklahoma City, suggests this: if symptoms are restricted to the upper respiratory tract -- specifically, the nose and throat -- try a "test drive" at half speed. If the activity clears your head and you feel peppy enough and not in pain, it should be all right to finish the workout. But do not exercise if symptoms are below the neck or bodywide, like fever, muscle aches, loss of appetite or hacking cough. Your body will recover faster with rest.
Exercise and Cancer
With cancer the evidence is somewhat clearer, though hardly conclusive. Although exercise increases blood levels of natural killer cells that fight off cancer cells, researchers believe other mechanisms play a far more important role.
Several studies among men and women have indicted that those who are physically fit or who are physically active have lower death rates from cancer. A study of more than 10,000 men and 3,000 women examined at the Institute for Aerobics Research in Dallas found that those who were most fit on a treadmill test had much lower cancer death rates in the ensuing eight years. There was a fourfold difference in cancer deaths among the men and a sixteenfold difference among the women.
The strongest evidence for a protective effect of exercise involves colon cancer, a leading cause of cancer deaths among Americans.
In tracking deaths among more than 17,000 Harvard alumni, Dr. Ralph Paffenbarger Jr. found that those who said they exercised at moderate to high levels had 25 to 50 percent fewer cases of colon cancer than the least active men in the study. The main benefit of exercise to the colon is believed to be the increased rate at which body wastes and any cancer-causing substances they may contain pass through the colon in physically active people.
For women, exercise, particularly during teen-age and young adult years, seems to be associated with lower rates of breast cancer and various hormone-related cancers of the reproductive tract. Dr. Rose Frisch at the Harvard School of Public Health found that among nearly 5,400 female college alumnae, those who had been college athletes or who trained regularly had about half the risk of later developing breast cancer that nonathletes ran. Nonathletes also had higher rates of cancers of the uterus, ovary, cervix and vagina.
The main benefit of exercise in reducing cancer risk in women is believed to be a lower lifetime exposure to estrogen, which can stimulate growth of cells in the breasts and reproductive organs. Physical activity can change the hormone ratio and reduce body fat, which itself increases the amount of cancer-stimulating estrogens in the blood. As the cancer society says, exercise will not hurt and it may help, so get moving.
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