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Just Do It—But Don't Overdo It

Exercise is good for you. You're probably sick of hearing that message.

But did you know too much exercise can make you sick? In a study at the Los Angeles marathon, runners who finished fell victim to a cold, flu or other virus at a rate six times greater than a group that didn't race.

The proof of exercise's benefits is overwhelming. Still, there's such a thing as too much exercise. "Exercise, like any good thing, can be carried too far," says David Nieman, Ph.D., a health and exercise science specialist in Boone, N.C. He helped conduct the Los Angeles study.

Exercise creates stress on the body. In most cases that's good. Putting your body under frequent, mild doses of this stress improves it over time. That's why people who do regular moderate exercise get half as many colds as people who are sedentary, Dr. Nieman says.

"The immune system really likes activity," he says. "When you exercise ... immune cells circulate at a higher rate. It is like taking the Marines out of the barracks and having them travel through the bloodstream."

Too much stress

Over-exercising has the opposite effect. Dr. Nieman found the tipping point is 90 minutes or more of continuous activity at moderate to high intensity. At that point, stress weakens the immune system, making you more prone to illness. The effect can last up to 72 hours.

If you work out too much one day, you may get a cold. If you do so day after day, you can run into far more serious problems.

Pushing yourself beyond your physical limits is called overtraining. Sometimes it can be tough to tell the difference between a hard workout and overtraining. Signs that you're doing too much include a drop in performance, longer recovery time, sore muscles, loss of appetite, headaches, and trouble sleeping.

Continuing to overdo it can lead to joint pain, injuries to the soft tissues of the ligaments and tendons, and anemia. Women can suffer even more serious problems. Hormonal imbalances can shut down the menstrual cycle and weaken bones.

A few people, addicted to exercise, ignore the warning signs. Exercise addicts, also called compulsive exercisers, will keep working out while they suffer injuries or declining health, even after a doctor tells them to rest.

"The exercise addict is never satisfied," says Jack Raglin, Ph.D., a kinesiology specialist in Bloomington, Ind. "There's always this obsession with adding more." Exercise becomes the only priority rather than one of many. Dr. Raglin believes exercise addiction affects about 1 percent of the U.S. population.

Withdrawal symptoms

Stopping may cause withdrawal symptoms: depression, irritability, changes in sleep, or lack of appetite. Treatment requires professional help.

Although you need to know the dangers of too much exercise, you shouldn't use them as an excuse to avoid working out.

"The number of people who get themselves in trouble by over-exercising is quite small compared with the number of people who have health problems because they don't exercise enough," says Jeffrey Benson, M.D., a family practice physician in Freeport, Maine.

The government recommends 30 to 60 minutes of moderate to vigorous physical activity on most days. That's right on target, Dr. Nieman says, if your goal is to maintain weight and reap the health benefits linked to exercise. These include reduced risk of heart disease, certain cancers, and diabetes.

"If you do more than 60 minutes, the health benefits don't increase," he says. "If people are going to go beyond that, they should understand they are doing it for weight loss or peace of mind."