Researchers have known for many years that regular exercise helps relieve some of the symptoms of depression, but there has been little cellular or biochemical data on why that might be so, other than the generalization that exercising just makes us feel better about ourselves. There are some new and interesting findings that shed some light on what may be happening. The graphic above, from this article, illustrates the details.

Tryptophan is an amino acid, one of the building blocks of proteins. It is normally metabolized, broken down, in the tissues, including muscle. The system that does this is called the kynurenine pathway because the product of this process is called kynurenine. This substance penetrates the brain, is itself broken down, yielding some molecules that have been implicated in several brain disorders, including depression. This is shown on the left side of the graphic.

On the right side of the graphic you see what happens with exercise. Muscle that is being actively exercised produces enzymes called KATs that take the kynurenine and, before it can penetrate into the brain, change into a similar substance called kynurenic acid. The latter substance does not go into the brain and activate depression-causing pathways.

I think this is quite interesting. It is always fascinating when we find biochemical and cellular explanations for something we’ve observed before but for which we had no explanation.