Exercise increases the size and number of mitochondria in the brains of mice (American Journal of Physiology, September 2011). The mice ran on a treadmill for an hour a day, six days a week, for eight weeks.

This could explain how exercise improves memory, treats depression, and makes people feel better and helps them to think more clearly. Until now, the leading theory to explain how exercise improves memory and treats depression was that exercise causes the brain to release endorphins, morphine-like compounds that can improve mood (Journal of Applied Physiology May 1982). However, endorphins would not explain the improvement in memory and brain function associated with a regular exercise program.

Mitochondria are tiny chambers in cells that turn food into energy more efficiently than any other process in your body. Scientists have known for years that exercise enlarges and increases the number of mitochondria in muscle cells, to increase strength, speed and endurance; but this is the first research paper to offer a plausible explanation why exercise improves memory and relieves depression.

The increase in brain mitochondria could also explain how training for sports increases endurance by making the brain resistant to fatigue. It also could explain how exercise treats mental disorders, delays aging, and improves certain types of nerve damage.