Can exercise protect the brain against depression?

It's possible, according to a new study on the subject by a team at Sweden's Karolinska Institute. Alex Hutchinson over at Runner's World does a good job reviewing it, in particular his description of how exercise might serve to prevent depression: 

"Depression is very complex and multi-faceted, so there are likely lots of different factors involved. But the particular pathway identified in the new study is sort of the opposite of the chemical-produced-in-the-brain hypothesis. Instead, it appears that there's a chemical called kynurenine produced mainly in the liver in response to stress that then travels to the brain, where it's linked to (among other things) neuroinflammation, cell death, and depression. Exercise causes your muscles to increase levels of a protein called PGC-1alpha1, which in turn leads to higher levels of an enzyme that converts kynurenine into kynurenic acid – and the key difference is that kynurenic acid is unable to cross the blood-brain barrier, so (unlike kynurenine) it can't get into your brain and wreak havoc. 

So the short version is: stress produces kynurenine which may lead to depression, but exercise helps you convert kynurenine into a harmless substance, leaving you more resilient to stress-induced depression."

It's an interesting hypothesis, and not an obvious one.

The full review can be read here.