It's been known for some time that exercise is not only good for your physical health, but it also helps to keep your mind sharp.

Specifically, it helps to prevent dementia, particularly Alzheimer's, and now scientists at Massachusetts General Hospital think they've figured out why.

A little hormone called irisin.

Exercise increases the hormone irisin, which has multiple benefits including regulating glucose and lipid metabolism, as well as aiding in turning white fat to brown fat.

It is also a hormone found in the human brain, but less so in the brains of people with Alzheimer's

Here's a little bit of the science talk:

"First, we found that irisin treatment led to a remarkable reduction of amyloid beta pathology," says Dr. Se Hoon Choi in a media release. "Second, we showed this effect of irisin was attributable to increased neprilysin activity owing to increased levels of neprilysin secreted from cells in the brain called astrocytes..."

"Our findings indicate that irisin is a major mediator of exercise-induced increases in neprilysin levels leading to reduced amyloid beta burden, suggesting a new target pathway for therapies aimed at the prevention and treatment of Alzheimer's disease," says Rudolph Tanzi, PhD, a senior author of the study and director of the Genetics and Aging Research Unit.

So, in short, hit the treadmill!

If you exercise, according to The Left, it might make you a white supremacist.

But, of course, one thing we know for sure is that it's definitely good for your overall physical health and now it's clearly good for your long-term mental health.