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Can running too intensely make you sick?

If a little of something is good, then a lot of it must be even better. Right? For runners, this is definitely not the case. Excessive running – or any exercise beyond the body’s ability to cope – could make you sick.

According to Dr. Dale Macdonald, Sport Specialist and Clinic Director at Elite Sport Performance in Calgary, if you run moderately, you’ll probably be just fine. Overdo it, however, and the opposite could be your experience.

“Running is one of the healthiest things we can do,” says Macdonald. “Countless studies show a broad range of health benefits attributed to moderate distance running. It’s important to know, however, that there are stresses associated with too much running that can impact your health.”

For example, Macdonald points out that many elite endurance runners experience increased upper respiratory tract infections around the time of competitive events or during periods of high-intensity training.

In Macdonald’s experience, when a runner trains too intensively without letting the body recover, he or she runs the risk of three potentially harmful physiological results.

1. Free radicals may weaken the body

When we exercise hard, oxygen consumption increases along with certain chemical reactions in the body created by this oxygen. For example, intense exercise can produce inflammation in the body’s tissues. Oxygen also creates a byproduct called a free radical, and production of free radicals over a sustained period can be harmful.

2. Immune system suppression may put you at risk of illness

Moderate exercise of about an hour in length can actually stimulate your immune system. On the other hand, intense or prolonged exercise can cause immune cells to drop below their normal level. While these cells generally bounce back within 24 hours, intense exercise may lower a runner’s resistance until the immune system returns to normal. Having too many consecutive training sessions without a break may increase your risk of illness.

3. A constant supply of stress hormones may undermine body strength

Stress releases epinephrine, norepinephrine and cortisol into the body. When cortisol is released, our blood sugar increases to supply more energy to our muscles. Sustained cortisol release over time can contribute to bone density loss and increased risk of skeletal stress fractures.

So what’s the best approach when running? Moderation is key, in Macdonald’s view.

“Our immune system is well-designed to deal with running in moderate amounts,” he says. “Moderate intensity would include five or more days a week of aerobic exercise, with each session lasting about 45 to 60 minutes each.”

The other key to optimal running health is giving the body adequate rest and recovery time, to let the immune system bounce back. Macdonald also recommends eating foods high in antioxidants, which naturally counter free radicals. While antioxidant supplements are available, he cites research that indicates antioxidants are most easily absorbed through food.

What if you do get sick? Macdonald offers this advice for runners.

You’re good to go, if: you have a cold with no fever and only mildly depressed energy levels. Take it easy, though. Exercise at 60% or less of your VO2 max. In other words, go for a very easy run of no longer than one hour.

Skip your run, if: you have a fever, moderate-to-severe energy depletion, muscle aches or pains not related to training, or diarrhea or vomiting, or your resting heart rate is higher than normal.