Discovery Fit & Health
While you exercise, your body produces antibodies. These antibodies are a type of protein the immune system produces that react to antigens (such as bacteria or viruses) by hunting them down and destroying them. According to David C. Nieman in his book, "Nutrition and Exercise Immunology," exercise increases antibody production by 300 percent, and increases the number of infection-killing T-cells circulating in the body. Once these antibodies form, they hang around in your body, ready to be called upon in case of further germ invasion.
Two recent experiments help buttress this claim. In one experiment, mice were split into two groups: those that hung out and relaxed, and those that ran on treadmills for long period. The mice were then exposed to a flu virus and the mice that hung out and rested fared much better than did the mice who jogged themselves to exhaustion on a treadmill. Another experiment recorded similar findings. Mice were infected with a bug that's nasty or mice to handle. Then they split into three groups: mice that rested, mice that did light workouts and mice that trained exhaustively. Once the flu bug began to hit the mice, more than 50 percent of the resting mice died, 70 percent of the hard-training mice died and just 12 percent of the light-workout mice died [source: Reynolds].
So, while it might seem a bit counter-intuitive, excessive exercising may weaken the immune system but moderate workouts may help it. As in so many things in life, moderation seems to rule the day. If you don't exercise enough, it's not healthy; if you exercise too much, that's not the ticket either. Unfortunately, our on-the-go, image-conscious culture can make us feel the need to push beyond reasonable boundaries. In short, we can push our workouts too hard, too often. This doesn't make much sense, when you think about how the body reacts to exercise. When the body is in a state of exercise, it's a kind of stress to which the body must adapt. In doing so, the body leans on the best friend it has in this case: the immune system. Later, when, and if, the body has had a chance to rest and marshal its forces, it's that much stronger going forward. But if the body never gets enough down time to rest, or when another form of stress hits, it can't mount an effective defense [source: McGonigal].
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