Exercise: benefit or risk?

Exercise: Benefit or Risk?

Suppose someone were to offer you a magical substance guaranteed to lengthen and improve the quality of your life? This stuff would cut in half your risk of heart disease, make a dramatic improvement in the delivery of oxygen to your body muscles, decrease your mental anxiety and depression, and lower your blood pressure and cholesterol levels. Would you believe that such a potion really existed? If medical evidence supporting these claims could be provided, would you be willing to sacrifice to obtain this health-saving nostrum?

Such a potion does exist, and although time and effort are involved, it can be obtained quite inexpensively. This "wonder drug" is exercise, particularly aerobic (exercise that increases oxygen intake), such as brisk walking, jogging, swimming, bicycling, and aerobic dance. And the spectacular benefits are available to all who use it three to five times a week.

However, the use of this "drug" has been associated with many potential side effects, especially when too much is taken. Pain in muscles, joints, and tendons is a real possibility. Use during hot and humid weather is not advised. Large doses during pregnancy may harm the growing fetus. And for those with a high risk for heart disease, use of this substance can imperil life itself.

You've probably read newspaper reports about exercise author Jim Fixx and the famous basketball player Pete Maravich, both of whom died of heart attacks while they were exercising. Countless other media reports have emphasized the injuries of top athletes, the potential for heat exhaustion and collapse by runners during races, and other ill effects associated with exercise. Many Americans, especially the millions who have joined the exercise movement, have become confused by such reports and are searching for answers.

Medical science has carefully weighed both the benefits and risks of exercise to provided answers to most of the questions being asked by the public. Before summarizing this evidence, an important underlying principle should be understood: For adults who regularly engage in moderate exercise programs, the benefits far outweigh the risks. However, with increasing amounts and intensity of exercise, the risks expand manyfold (see figure). The bottom line? When you engage in daily moderate aerobic exercise, such as brisk walking, for 30 to 45 minutes, you will realize many more benefits than risks. However, if you undertake an aerobic exercise program that is excessive in amount (usually more than 45 minutes) and intensity (i.e., very hard running or aerobic dance), the potential for experiencing the various risks of exercise escalate sharply.

Notice that the chart accompanying this article balances the benefits and risks for 12 areas of health and disease. A "surety rating" is given with each benefit or risk; the rating is an estimate of the strength of the medical evidence supporting the various statements. A brief summary of each of the benefits and risks for the 12 areas follows.

Cardiorespiratory system

The cardiorespiratory system comprises the heart, lungs, and blood vessels. The chief duty of these organs is to deliver oxygen to the working muscles during exercise. The evidence from numerous research papers is very firm in support of what is called the "training effect," which refers to the fact that with regular aerobic exercise, the cardiorespiratory system is able to deliver more and more oxygen to the active muscles.

HEART DISEASE AND RISK FACTORS

Regular aerobic exercise has been found to be very important in helping people to control their blood pressure and blood lipid profiles. There is even some evidence to suggest that smokers who start exercising may have an easier time in quitting their habit. The Centers for Disease Control in Atlanta, Georgia, recently surveyed all of the medical literature that examined the relationship between exercise and heart disease. They concluded that the risk for heart disease is doubled for inactive people as compared with active people. In fact, regular aerobic exercise was found to be just as important as smoking cessation, high blood pressure control, and blood lipid management in preventing heart disease. However, for people who are at high risk for heart disease, vigorous aerobic exercise has been found to be a promoter of sudden death from heart attack during the exercise bout.

So in light of this information, what is the practical advice about exercise? People who smoke, have high blood pressure, high blood-cholesterol levels, and/or who have parents or siblings who have died before age 50 from heart disease should have a thorough checkup before starting an exercise program. If the physician gives approval for exercise, a moderate exercise program such as brisk walking is safer than vigorous forms of exercise such as running.

OBESITY

Notice that the chart lists surety ratings of "1+" for each of the associated exercise benefits for obesity. Counter to what many people think, exercise has actually been shown to be less important than diet in both preventing and treating obesity. Most studies have shown that while aerobic exercise improves the overall health of the individual who is trying to lose weight, change in dietary habits - especially reduction in dietary fat intake (oils, margarines, high-fat dairy and meat products, etc) - provides the real power in the battle of the bulge. Exercise affects the basal metabolic rate very little after the session is over. In fact, after a two-mile jog, only 10 to 15 extra calories are expended during recovery (which takes about 30 minutes). And despite what you may have heard, regular exercise training does not decrease the appetite. Although you may feel like eating less immediately after the exercise bout, as the weeks pass by, active people tend to eat slightly more, not less, than inactive people. There is a small risk that obese people may injure themselves during exercise because of the excessive strain on their muscles and joints. For this reason, non-weight-bearing exercises such as bicycling or swimming are recommended for overweight people.

PSYCHOLOGICAL

Evidence is mounting that people who engage in regular aerobic exercise have increased feelings of vigor and improved self-concept, with lowered feelings of anxiety and depression. There is some research to suggest that one's ability to think and reason is improved during and immediately following exercise, but this is probably short-term. There is little if any evidence to support the contention that sleep is improved when people start up an exercise program. Neither is there evidence to suggest that people with substance-abuse problems can more easily conquer their habits of abuse with exercise. Some people can develop what has been called "exercise addiction" by exercising excessively. Despite feelings of chronic fatigue and associated problems in performance at work or in social relationships with friends and family, such individuals continue to persist in their excessive program of exercise.

MUSCULOSKELETAL 

One of the chief risks associated with excessive exercise is injury of the joints, ligaments, tendons, and muscles of the feet and legs. Most studies show that activities that require jumping and running, such as aerobic dance and jogging, promote the most injury. Moderate exercise can improve the health of the bones, however, increasing their amount of mineral content. The amount of minerals in the spinal bones usually decreases as one gets older, but regular exercise has been shown to help slow down this loss. A few studies suggest that elderly women who have osteoporosis (very low mineral content in the bones, leading to fractures) can partially reverse this problem by adopting a regular exercise program. Some younger women who exercise excessively, however, tend to lose their menstrual periods, which then leads to a decrease of bone mineral content. Dr. Barbara Drinkwater, of Seattle, has reported that the bone mineral content of 25-year-old women runners who had lost their periods was equal to that of women twice their age. Many sports medicine doctors recommend that women ease back on their exercise until their normal menstrual periods return, to protect their bone mineral content.

NUTRITION

Do people who start exercise programs also tend to make improvements in their diet? There is little or no evidence to support this common belief. Does regular exercise increase the need for vitamins and minerals above what can be obtained from the diet? Once again, there is little support for this sentiment. Iron, however, is an exception to this rule. In some athletes who exercise vigorously for long periods of time, a mild iron deficiency can develop. Upon the advice of a physician, these people may use a mild iron supplement.

DIABETES

Diabetics who exercise have been found to have a better ability to control their blood glucose levels. Some diabetics who require insulin injections, however, can develop very low blood glucose levels during exercise, leading to coma, if they do not carefully balance their life-style habits. This is why diabetics should consult with their physicians before starting an exercise program.

CANCER

A few recent studies are suggesting that bowel and breast cancers, two of the most common cancer killers, are less likely to develop in active people. One reason may be that food passes more quickly through the intestinal tract of active people. More research is needed to explore this exciting benefit of regular exercise.

IMMUNE SYSTEM 

Preliminary research suggests that moderate exercise such as brisk walking may promote a better functioning of the immune system, while excessive exercise such as marathon running may depress immune function. In a study conducted on 2,300 Los Angeles marathon runners, flu and colds were found to be increased 600 percent in the runners who ran the marathon as opposed to runners who trained but decided not to run it.

REPRODUCTION

Does regular exercise increase the sex drive? There is no evidence to support this common claim. To the contrary, some evidence suggests that excessive exercise may actually decrease the sex drive. As stated earlier, excessive exercise by women can lead to loss of menstrual periods. However, if the woman reduces the amount of exercise, menstrual periods usually return. Although researchers have found that the health of the pregnant woman improves with moderate exercise during pregnancy, there appears to be no support for the belief that the baby will be healthier. However, pregnant women who exercise excessively may actually interfere with the normal development of their growing fetus.

ENVIRONMENT

Exercising under hot and humid conditions can be very dangerous, leading to heat exhaustion and even death from heat stroke. Although studies show that we can adapt somewhat to exercising in the heat, it is best to avoid exercising heavily when the air temperature and humidity are high. Air pollution, especially ozone, is another environmental problem that has been associated with a temporary decrease in ability to breathe air properly.

AGING

The life span is defined as the number of years one is genetically endowed to live. Life expectancy is the number of years one is expected to live at birth and is related to both genetic and lifestyle factors. Although there is no evidence that exercise will lengthen the life span, many studies show that active people can expect to live two or more years longer than their peers because of a lower risk of dying from heart disease and other causes. Even more important, perhaps, is the fact that an active elderly individual can possess the exercise capacity of a person 20 to 40 years younger than themselves, improving the quality of life in their remaining years.

So what is the conclusion of this matter of exercise? Many broad claims have been made regarding the health benefits of regular exercise. But these must be balanced against the risks posed by too high a dosage. In other words, we must be careful not to dilute the elixir with unsubstantiated claims or to make it too potent by taking too much.

David C. Nieman, D.H.Sc., M.P.H., F.A.C.S.M., is chairman of the Department of Health Science at Loma Linda University's School of Public Health.