According to most experts, working out with the flu does more harm than good.
No Such Thing as Sweating it Out
“The best information we have says complete bed rest is best,” David Nieman, professor of health and exercise science and director of the human performance lab at Appalachian State University told the Los Angeles Times. And regarding the old saying that you can sweat out the flu through intense exercise: “That’s a myth," he says. “A big myth.”
Increasing your body temperature may be dangerous. “If your core temperature is already up and you dare to add to it, you have to keep in mind that viruses like to multiply with higher body temperatures.
“We know that heavy exertion causes a transient downturn in immune function that can last from a few hours to a day,” adds Neiman. Intense workouts can cause stress hormone levels like cortisol to increase, which causes immune cells to function less efficiently. “During this downturn, if you have a virus, it will multiply at a higher rate and make you sick.”
Fever is the limiting factor, adds Dr. Lewis G. Maharam, a sports medicine expert. “The danger is exercising and raising your body temperature internally if you already have a fever because that can make you even sicker,” he says. If you have a fever higher than 101 degrees, sit it out.
The Neck Check
Whether or not you should cancel your workout routine depends a lot on the type of symptoms you have. Most experts advise that you analyze where your symptoms primarily occur -- a process known as the “neck check.”
“A neck check is a way to determine your level of activity during a respiratory illness,” says Dr. Neil Schachter, MD, medical director of respiratory care at Mount Sinai Medical Center in New York. “If your symptoms are above the neck, including a sore throat, nasal congestion, sneezing and tearing eyes, then it's OK to exercise," he says. "If your symptoms are below the neck, such as coughing, body aches, fever, and fatigue, then it's time to hang up the running shoes until these symptoms subside.”
Moderation Is Key, Say Some
Some physicians believe exercising when sick is OK in moderation. Dr. Peter Katona, an associate professor of medicine at UCLA who specializes in infectious disease, isn't completely anti-exercise for fitness fanatics who’ve come down with a virus. However, he cautions them to listen to their bodies.
"If you feel up to it, you can do some light working out, like stretching," he tells the LA Times. "Do things that aren't going to give you a lot of sweat and fatigue. Don't push yourself, but let your body be your guide." Doing too much, he adds, may put people at risk for injury. "You might be weaker than you realize," he says.
Maharam adds: “Do what you can do, and if you can't do it, then don’t,” he says. “Most people who are fit tend to feel worse if they stop their exercise, but if you have got a bad case of the flu and can’t lift your head off the pillow, then chances are you won’t want to go run around the block.”
Increase Your Workouts as You Begin to Feel Better
Even a minor case of the flu can keep you homebound for a week or more, and it’s important to NOT hit the ground running the moment you feel fine. “When your fever's gone and the worst of the symptoms are pretty much gone, just take an easy walk and see how you respond,” Nieman says.
Ease back into your regular activities only after you’ve been symptom-free for at least a week to give your immune system a chance to bounce back. The IDEA Health and Fitness Association, an association for fitness professionals, recommends two days of low-intensity workouts for every day you were down with the flu.
All experts have one final piece of advice regarding exercise and the flu: No matter what decision you make for yourself regarding your workouts, don't exercise at the gym and risk spreading the infection to others.