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Should You Train When You’re Sick?

When in doubt, it's better to be safe than sorry.

When in doubt, it’s better to be safe than sorry. 

When most people get sick they grab some blankets and warm soup, turn on the TV and curl up on the couch. But, runners aren’t most people. Plenty of athletes are determined not to miss training days even if they have a cold or the flu.

That can be OK, say doctors, as long as you’re smart about it.

“For your personal safety, ‘head and up’ is a good guideline,” says Dr. Alex Koch, an associate professor in exercise science at the Lenoir-Rhyne University. He’s referring to the often-cited rule that if your illness is just in your head (sniffles or watery eyes), then you should be fine to do light or moderate exercise. But, if it’s below your head (coughing or body aches), then you definitely need to rest. If you have a high fever, your rest period should extend a few days beyond when the fever goes down or you can risk doing serious damage to your body.

It’s not that clear-cut, though, says Koch. There are some strains of upper respiratory illnesses that are not exacerbated by illness and some that are. “But, you don’t know what exact strain of an illness you have,” he says, so better to be safe than sorry.

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The question, once you’re not feeling well, really becomes: What’s the benefit of working out if you’re sick?

“An elite athlete might not want to take time off training if they can avoid doing so, whereas a recreational exerciser might not want to run the risk of feeling worse after a workout when they are unwell,” says Mike Gleeson, PhD., a professor of exercise biochemistry at Loughborough University, who has done studies on the effects of exercise on immune system function. Whether or not you opt to rest an entire week, just one day or not at all depends on where you are in your training, how close you are to your target races and what your goals are.

“It’s a very tough decision to make,” admits Koch.

Trying to train while sick definitely puts you at risk of prolonging the illness and doing more harm than good for your fitness. But, if you’re able to simply tone down the workouts and take it a little easier, you can come out of it without having lost much training time.

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Of course, there are things to think about besides your personal health and fitness. With the flu epidemic having already claimed over 100 lives in California (one of the few states that tracks adult flu deaths), it’s important to be careful about spreading germs. If you’re sick and go to the gym or the pool, you could easily get others sick and spread your illness.

Exercising can also diminish your immune function and temporarily hurt your immune system, depending on how hard you work out.

“The effects of a bout of prolonged strenuous exercise on immune function lasts between 3 and 24 hours,” says Gleeson.

Moderate exercise of 30-45 minutes can actually help boost some aspects of immunity, explains Gleeson. But, prolonged strenuous exercise of 90 minutes or more at or above 70 percent of your maximum heart rate can inhibit immune cell functions.

Studies measuring the effects after at least an hour of activity at 75 percent of VO2 max have shown a decrease in the activity of natural killer cells, drops in the antibody levels in saliva, and T-cells and B-cells being unable to divide as well as they should, says Koch. A study also found that after completing a marathon, runners were six times more likely to get a respiratory infection. That helps explain why it always seems like you get sick the week after finishing a race or during a rest period.

The effects can be greater if blood sugar levels fall, says Gleeson, so drinking a carbohydrate beverage during your workout can help. But, mostly, you should be very careful after a hard workout or race.

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It also means that if you do get sick, you should be careful about not compromising your immune system any further. Having the flu for weeks isn’t going to help you run faster.

“The more you run, the more likely you are to get sick,” Koch says.

To avoid getting sick — and practice good habits in general — get a flu shot, wash your hands regularly, sleep plenty and eat healthy foods with enough calories. Low-calorie diets can correlate to increased risks of infection, says Koch.


About The Author: 

Kelly Dunleavy O’Mara is a journalist/reporter and former professional triathlete. She lives in the San Francisco Bay Area and writes for a number of magazines, newspapers, and websites. You can read more about her at