01/27/2012 08:31 am ET Updated Mar 28, 2012

How Exercise Works: Is This Why Workouts Are So Good For Us?

As anyone who has sweated and wheezed their way through a treadmill run (only to find you've burned fewer calories than you'd eaten at lunch) knows: the health benefits of working out aren't always a simple formula of calories in, calories out.

We've long known that exercise isn't just good for our waistlines -- it reduces our risk of heart disease, diabetes and dementia, increases stamina, and improves sleep and focus. Few single acts are as healthful. But until now, the molecular process that makes exercise so good for us has been a bit of a mystery.

Now, a team of biomedical scientists from Texas Southwestern Medical Centre, led by Dr. Beth Levine, think they may be close to an understanding. Physical exercise may promote autophagy, a process in which the body preserves the health of cells by replacing damaged cellular components with fresh ones -- effectively breaking down cells for their component parts and "recycling" the healthy bits. And that, in turn, could help our bodies stave off diabetes and other chronic diseases.

Explained The Economist:

Autophagy is an ancient mechanism, shared by all eukaryotic organisms (those which, unlike bacteria, keep their DNA in a membrane-bound nucleus within their cells). It probably arose as an adaptation to scarcity of nutrients. Critters that can recycle parts of themselves for fuel are better able to cope with lean times than those that cannot. But over the past couple of decades, autophagy has also been shown to be involved in things as diverse as fighting bacterial infections and slowing the onset of neurological conditions like Alzheimer's and Huntington's diseases.

Levine and her team tested exercise's effect on autophagy by observing mice. They found that just 30 minutes of activity on a treadmill-like device increased the number of autophagosomes -- a collection of molecules that form around cellular parts that are available for recycling -- in mouse muscle tissue. And that number continued to increase for up to 80 minutes of exercise.

"During stress, increased levels of autophagy permit cells to adapt to changing nutritional and energy demands through protein catabolism," explained the researchers in their paper, published in Nature. "Moreover, in animal models, autophagy protects against diseases such as cancer, neurodegenerative disorders, infections, inflammatory diseases, aging and insulin resistance."

To test the effects of increased autophagy, the researchers created a control group of genetically modified mice whose bodies were unable to perform the function. Then they performed tests on both groups, finding that the group with disabled autophagy developed less endurance over time, despite exercise, and also wasn't able to metabolize sugar as efficiently.

It's not news that exercise is good for you -- but here's one more piece in the puzzle of how it works. Next time you're having trouble motivating off the couch, just think of what that brisk jog will do to replenish your cells -- and how much that means for your long-term health.