Exercise prompts cells to turn unwanted proteins and cellular junk into energy.

Muscle cells experience increased autophagy (signified by glowing green dots) during exercise.

Exercise is clearly beneficial, and now there’s scientific evidence that explains why time in the gym can help fend off diabetes—and potentially other diseases. According to HHMI investigator Beth Levine, cells break down cellular junk to get extra energy, thereby cleaning house while you exercise.

Cells use a process called autophagy to recycle unwanted proteins and cellular structures. During this process, a double membrane forms around the cellular garbage. An organelle called a lysosome then fuses with the membrane and its enzymes rush in to break up the unwanted cargo, yielding raw materials for producing new proteins or energy for the cell.

Scientists have long known that stress can trigger a boost in autophagy, as the process helps cells adapt to changing nutritional and energy demands. Levine, a physician at the University of Texas Southwestern Medical Center, suspected that exercise might have a similar effect because it also increases cells’ energy demands. To test this idea, she and her colleagues used transgenic mice whose cells produce a green glowing signal when autophagy occurs. After 30 minutes of running on treadmills, the mice showed increased autophagy in their heart and skeletal muscle cells as well as in their liver and pancreatic cells.

Next, the scientists created mice that could experience autophagy under normal conditions but were unable to ramp it up during exercise or starvation. In a paper published January 18, 2012, in Nature, the researchers report that these mice were unable to increase their muscle glucose uptake and had decreased endurance. And, unlike normal mice, exercise did not protect them against diabetes induced by a high-fat diet. During exertion in normal mice, an enzyme called AMP kinase helps cells take in more sugar from the bloodstream. However, this enzyme wasn’t activated in Levine’s mice. Several oral drugs used to treat type 2 diabetes work by activating AMP kinase, and it appears that autophagy induced by exercise does the same thing.

These findings suggest that increased autophagy may be the reason exercise protects against type 2 diabetes and other metabolic disorders. Levine also thinks it’s possible that activation of autophagy may contribute to other health benefits of exercise, including protection against cancer, neurodegenerative diseases, and
aging.

Photo: Congcong He / University of Texas Southwestern Medical Center