A recent study, published in the Proceedings of the National Academy of Sciences, has made some headway in answering this question. The researchers were interested in the role of sex hormones in the process of neurogenesis. It has long been known that the female sex hormone estrogen is produced in the brain (and not only in the brains of females), and that this hormone is suspected of being one of the triggers of new brain cell growth. What was not known was whether the male sex hormone testosterone (which again is produced in both males and females, but more of it in males) was also produced in the brain, and whether it too was a trigger for neurogenesis.
In males, testosterone is produced mainly in the gonads. In the recent study, performed on rats, the researchers believed that the only way to determine whether testosterone can also be produced in the brain was to shut off production of testosterone elsewhere. As a result, half of the male rats were castrated, while the other half were given a fake operation to ensure that all of the test animals had gone through the same somewhat traumatic experience. Some of the rats from each group were then also given a drug that keeps male sex hormones from binding to receptors for those hormones in the brain, so that if even if their brains were able to produce testosterone, it would not have any effect on new brain cell growth.
Now comes the exercise
After recovering from their operations, whether real or fake, half of the rats ran on treadmills at a leisurely pace (similar to jogging in humans) for two weeks, while the other rats remained sedentary. After this two-week period, the researchers examined their brains. What they found is that the rats that had exercised had significantly more dihydrotestosterone (DHT) in their brains. DHT is a potent form of testosterone, and was present in the brains of the exercising rats even if they had been castrated. That pretty much settled the question of whether testosterone could be produced solely in the brain.
Improve your fitness
What was even more interesting, however, is what happened in the brains of the rats who had been given the drug that blocked the binding ability of male sex hormones. In these rats, although their brains were producing DHT, no neurogenesis had taken place; their brains were the same size. But in the rats that both exercised and had not received the blocker drugs, the increase of DHT had stimulated significant new growth in the hippocampus, an area of the brain linked to memory and learning ability.
The researchers concluded from this that increased amounts of DHT in the brain was indeed one of the triggers for neurogenesis, and that exercise, even in castrated rats, caused the production of more DHT. Therefore, exercise causes the growth of new brain cells.
Even though this research was performed only on rats – and I don't expect it to be replicated on humans any time soon for what should be obvious reasons – the researchers point out that it adds to our knowledge about the mechanisms of how exercise helps our brains to grow and stay healthy. A direct link was found between exercise and increased brain cell growth, and a new trigger mechanism was identified that may someday help scientists to regenerate brain cells in people suffering from Alzheimer's disease.
One additional encouraging thing pointed out by the researchers is that the exercise done by the rats in this experiment was rather mild, the counterpart of jogging at a leisurely pace at which one could continue to chat with a running companion. That means that if we're seeking to improve our brain health by adding more exercise to our lives, it doesn't have to be extreme exercise. So if one of your goals is to improve your memory and ability to think clearly, you don't have to become a champion marathoner to accomplish this; jogging will work just as well.