ACSM Fellow David C. Nieman, Dr. P.H. says that moderate exercise (30 minutes a day, on most, if not all, days of the week) actually lowers the risk for respiratory infections.  Prolonged, intense exercise, on the other hand, can weaken the immune system and allow viruses to gain a foothold and spread.

“The good news, for the majority of fitness enthusiasts who put in 30-60 minutes of exercise most days of the week, is that the number of sick days they’ll take during the common cold season is reduced by at least 40 percent,” said Dr. Nieman.

People who are already sick should approach exercise cautiously during their illness.  To help people decide whether to hit the gym or stay in bed, Dr. Nieman offers the following recommendations:

-DO exercise moderately if your cold symptoms are confined to your head.  If you’re dealing with a runny nose or sore throat, moderate exercise is permissible.  Intense exercise can be resumed a few days after symptoms subside (in cases of the common cold).

-DON’T “sweat out” your illness.  This is potentially dangerous myth, and there is no data to support that exercise during an illness helps cure it.

-DO stay in bed if your illness is “systemic” – that is, spread beyond your head.  Respiratory infections, fever, swollen glands and extreme aches and pains all indicate that you should rest up, not work out.

-DON’T jump back in too soon.  If you’re recovering from a more serious bout of cold or flu, gradually ease back into exercise after at least two weeks of rest.

“In general, if your symptoms are from the neck up, go ahead and take a walk,” said Dr. Nieman.  “But if you have a fever or general aches and pains, rest up and let your body get over the illness.”

Dr. Neiman also encourages people to engage in moderate-intensity exercise before getting a flu shot.  After exercise, he said, the body responds better to the vaccine and gets a boost in immunity.

Medical News Today – American College of Sports Medicine

But the other side of the energy equation isn’t that simple. When it comes to burning off calories, people who do the same activity at the same pace for the same amount of time can burn vastly different numbers of calories, depending on their size.

Larger vs. Smaller

For example, if a family of three jogs side-by-side for 30 minutes, the 175-pound father will burn 400 calories, the 130-pound mother will burn 300 calories and the 65-pound child will burn 180 calories.

“Larger people burn more calories than smaller people, particularly with activities like walking or stair climbing where they have to carry their own weight,” says Robert McMurray, professor of exercise and sports science at the University of North Carolina at Chapel Hill.

Exercisers are often misled by workout equipment or charts that don’t factor in weight when they proclaim how many calories are being burned by an activity, he says.

If an exercise machine or chart calculates the calories burned by an “average” 150-pound person, the results would be “vastly inaccurate” for much larger or smaller exercisers.

Avoid the “Death Grip”

In addition, leaning on the hand rails of a stair climber or keeping a “death grip” on the treadmill railing will greatly decrease your caloric expenditure, says exercise physiologist Steve Farrell of the Cooper Institute for Aerobics Research in Dallas.

“The machine is assuming that your legs are carrying your entire body weight,” he notes. “But if you’re supporting yourself on the handles, you’re actually burning about 20 to 30 percent fewer calories than the machine indicates.”

Know What You’re Burning

Knowing how many calories you’re burning during activity can be important if you’re trying to lose weight, since the “general rule of thumb for weight loss is to create a 300- to 500-calorie deficit each day,” Farrell says. This means consuming 300 to 500 fewer calories than you expend.

Half of this “calorie deficit” should come from eating less and half should come from exercising more. “We generally advise people who want to lose weight to expend at least 250 calories more each day than they have in the past,” he says. “Additionally, they should eat 250 fewer calories than they’ve been eating.”

But it’s important to eat enough to fuel your body, notes Farrell, who warns men not to eat less than 1,200 calories per day and women not to eat less than 1,000.

That’s because severely restricting calories prompts the body to try and conserve fat stores, so it breaks down lean tissue mass, robbing the body of muscle and bone.

“People who are overweight want to lose fat,” he says. “But many don’t realize that about half of the weight lost on extremely low-calorie diets isn’t from fat, it’s from lean tissue.”