Evidence from many credible studies shows that exercise is potent medicine for the mind. Both strength training and cardiovascular exercise yield benefits above the shoulders, like greater endurance under stress and more mental elasticity to bounce back from difficult situations. We know that some of these benefits come from exercise elevating brain chemicals, like endorphins, that produce a sense of well-being. But new research suggests that exercise may be helping our brains in an entirely different way as well – by making our bodies more efficient at removing harmful chemicals that are triggered by stress.

Researchers from the Karolinska Institutet in Sweden wanted to discover how well-trained skeletal muscles may provide the brain with this edge over stress, and ultimately engender protection against stress-related depression. They focused on a protein called PGC-1a1 that’s found in higher levels in well-trained muscle tissue.

To control the amount of PGC-1a1 present in muscle, the researchers bred a group of mice genetically modified to have especially high levels of the protein. Another group of normal mice served as a control group.

Before you ask “why mice?” it’s worth mentioning that we and they share a few crucial characteristics.  We both react psychologically to increased stress, and, like us, mice are prone to developing depression when they suffer too much stress. Depression is a disease that affects the chemical balance of the brain, and our rodent friends are just as susceptible to having their neural health knocked off kilter as we are.

Both groups of mice in this study were subjected to five weeks of elevated stress in the form of flashing lights, loud noises and interrupted sleep. As predicted, the normal mice showed symptoms of depression after the stressful weeks. But the group with high levels of PGC-1a1 showed no symptoms of depression whatsoever.

The researchers found that in addition to higher levels of the protein in their blood, the genetically-modified mice also had higher levels of KAT enzymes that convert a stress chemical called kynurenine into a form more easily metabolized by the body.

To test the limits of this effect, the researchers injected kynurenine directly into the genetically-modified mice. Instead of their blood showing higher levels of the chemical, the mice were still able to quickly convert it into the more easily-processed form.

What these results suggest is that well-trained muscles aren’t just stronger and faster — they also catalyze chemical reactions that help our bodies metabolize byproducts of stress.

“Our initial research hypothesis was that trained muscle would produce a substance with beneficial effects on the brain,” says Jorge Ruas, principal investigator at the Department of Physiology and Pharmacology, Karolinska Institutet.

“We actually found the opposite: well-trained muscle produces an enzyme that purges the body of harmful substances. So in this context the muscle’s function is reminiscent of that of the kidney or the liver.”

Another way to think of this is that training our muscles is like improving the efficiency of a body-wide detoxification pump. If allowed to accumulate, chemical byproducts of stress have mental consequences, like depression. Exercise confers protection against those consequences by keeping the pump humming.

With so many upsides of exercise already proven out, it’s hard to imagine needing more incentive to get moving — but these results certainly add to the increasingly well-supported argument that our brains benefit in substantial ways from spending time in the gym, or anywhere else you care to sweat it out.

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