Give your body a break.

WebMD Feature

Feb. 5, 2001 -- The runny nose, stuffy head, and hacking cough: Those telltale signs of a cold or flu can be hard to ignore. Yet many exercise devotees find it tempting to do so (and they know who they are). But working out through an illness can backfire, depending upon what type of bug you've got and on how intensely you exercise.

When deciding whether it is better to hit the bed instead of the gym, exercise physiologists tend to subscribe to the "neck-up" rule. That is, moderate exercise gets the green light if symptoms are confined to the head, such as sneezing, headaches, and watery eyes.

One of the few human studies on the topic was published two years ago in Medicine & Science in Sports & Exercise, the official journal of the American College of Sports Medicine. Researchers at Ball State University in Indiana studied people infected with rhinovirus, the type of virus believed to be responsible for 30% to 50% of all colds, and who exhibited "head cold" symptoms. After 10 days, they found no difference in symptoms between the 34 people who exercised 40 minutes every other day and the 16 who did not exercise.

"If it's a sniffle and you're otherwise fine, it's okay to do some exercise," says Richard Cotton, chief exercise physiologist for the American Council on Exercise.

But caution should prevail when illnesses strike below the neck. The official recommendation from the American College of Sports Medicine is to lay low when symptoms include fevers, body aches, or extreme fatigue.

Evidence from animal studies and human case histories indicate exertion during the flu exacerbates the symptoms of the infection and prolongs the recovery period, Cotton says. "When [the illness] moves from the head to the chest, it's time to take time off," he tells WebMD.

Increased physical activity also speeds up the circulation of pathogens through the bloodstream, raising the risk of transmitting infections to the heart, says David Nieman, DrPH, a professor of health and exercise science at Appalachian State University in Boone, N.C., and a well-known researcher in exercise immunology. Overdoing it may even lead to death when a systemic infection is present.

The bottom line is the "concept of sweating it out with a fever is foolish," says Nieman. "You can make it more severe, and you risk a relapse."

Even if you are not ill, pushing too hard can leave you susceptible to colds and the flu. "Research has shown that individuals who engage in vigorous exercise are susceptible to upper respiratory infections," says LaGary Carter, DA, president of the American Society of Exercise Physiologists and professor of kinesiology and physical education at Valdosta (Ga.) State University. He says the process of warming the inhaled air you breathe can irritate the lining of the lungs and worsen coughs. The problem worsens when the outside temperature is cold.

Research also has shown that overexertion -- as may happen with long-distance running or heavy training for competitive sports -- can stress the immune system. Nieman conducted one of the largest studies to date on this topic in 1987. In that study, he and his colleagues followed 2,311 athletes who were training for the Los Angeles Marathon for two months before and for one week after the race. They compared runners who actually ran the marathon with those who had signed up but did not run for reasons other than illness. The researchers found that during the week after running, those who ran the race became sick at a rate six times greater than the athletes who did not run.

"These changes [in the immune system] last for half a day, and some can last up to three days," says Nieman, whose study was published in the Journal of Sports Medicine and Physical Fitness. Such work suggests that lack of sleep, poor nutrition, mental stress, or weight loss, open the window for infection to creep in and take hold.

Overdoing it also can come back to haunt you through the risk of post-viral fatigue syndrome, a condition Nieman -- a veteran of 58 marathons -- has witnessed firsthand. He describes how a fellow marathoner continued a race even though he had developed a fever the day before. "He then spent two years with post-viral [fatigue] syndrome," says Nieman. "He was so fatigued he could hardly do any exercise."

Nieman says he has worked with five other runners with the same condition, each having the common trigger of exercising too hard while sick.

So what's a fitness enthusiast to do? Besides eating well, getting plenty of sleep, and avoiding stress, taking vitamin C may be helpful. Nieman says more study is needed on its effectiveness, but says there is some evidence that vitamin C supplements may help if taken a few days before and after a session of high intensity exercise, such as an ultra-marathon. However, "for the average Joe Blow, it does no good at all," he says.

When the coast seems clear for a return to a workout, experts suggest easing back into a normal routine. If the cold was relatively mild, it may take less than a week to get back into full swing. However, those getting over a severe bout of the flu may want to take up to a month before attempting an intense workload.

The good news is, a healthy lifestyle that includes regular exercise may boost the immune system and help prevent illness. In three randomized trials with more than 150 women, Nieman compared sedentary participants with those who exercised moderately (for example, taking brisk, 45-minute walks five days a week). After 12 to 15 weeks, women who exercised regularly reported significantly fewer days with upper respiratory infection symptoms than those who were physically inactive. "The number of days of sickness in the walkers was just almost half of the [sedentary] control group," says Nieman. "This is huge statistically. There's no other supplement, no drug, that comes close to reducing the number of sickness days."

The message? Stick with a routine of regular, moderate exercise, but don't overdo it. Signs that you're pushing the envelope may include labored breathing, prolonged fatigue, or an elevated heart rate the morning following a hard workout.

"My grandmother said it best," says Harold W. Kohl III, PhD, director of physical activity and nutrition at the International Life Sciences Institute Center for Health Promotion in Atlanta. "Listen to your body. If it hurts, if there's discomfort where you usually don't have discomfort, it's worthwhile backing off until that passes."

Sarah Yang is a freelance writer in El Cerrito, Calif., who has written for CNN.com, the Los Angeles Times, and the San Francisco Examiner. She is a frequent contributor to WebMD.

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