We've all heard many reasons to exercise, but in this Moment of Science Don tells Yael about what might be a new one to most of us.  D: (In the background, Don is breathless doing jumping jacks and counting).

Y: Don--Don, we're on the air--D: (BREATHLESS) Sorry, Yael, just getting in some jumping jacks before work. I don't want to be one of the quarter million Americans who die prematurely each year from conditions like diabetes, heart disease, and obesity.

Y: Oh brother, another segment about the benefits of exercise. What's the big deal? Isn't there medicine that takes care of these diseases?

D: Well, medication is great for stabilizing existing conditions, but it doesn't solve the underlying problems--Y: (BORED)--and everybody knows that exercising is good for you.

D: Yes, but now scientists suggest that exercise is even more than that. The theory is that humans are genetically programmed by evolution to be physically active. After all, our ancestors spent millions of years hunting and gathering and only the last few thousand years as cultivators.

Y: So cushy office jobs aren't natural?

D: Right. So today, when we think of the average person, we think of someone who doesn't exercise. And what these scientists are saying is that the biological norm is a person who is very active.

Y: I'm not sure I see your point.

D: So instead of saying that exercising decreases your risk for certain health problems, it's more correct to say that not exercising increases your risk for those problems. Your body expects you to exercise, and when you don't, your body is more likely to break down. So when you do nothing, you're actually making things worse.