Did you know that brain cells are being born within our brains all the time, even when we are adults? 'Brain Derived Neurotrophic Factor' or BDNF is like a fertility pill for our brain cells. We think it encourages the birth and growth of new brain cells.
Brain scan studies have shown how parts of the prefrontal cortex and hippocampus can shrink over time, with chronic stress. We also see this shrinkage in depression. It may be that this shrinkage is a result of fewer brain cells being born within those regions, since chronic stress reduces levels of BDNF within those same regions. Intriguingly, acute stress has the opposite effect - it raises the levels of BDNF in our brain.
The effect of exercise on BDNF levels is even more fascinating. We often forget this but exercise is really a form of stress and as such, its effect on our brain is very similar to the effects of stress. Exercise is extremely good for us. Just like a short bout of mild stress, moderate exercise raises levels of BDNF within the brain. However, too much exercise has the opposite effect and like chronic stress, it may reduce the levels of BDNF and shrink parts of our brain, despite improvements in fitness scores!
From the studies carried out this far, we know that intense exercise over a prolonged period of time can have this effect. However, at least in humans, we do not know exactly when we cross the boundary from raising BDNF levels to reducing them. In one study an hour of high intensity exercise carried out three times a week over a six week period improved the VO2 max scores of seventeen healthy young men by about 5%, but also resulted in varying amounts of 'shrinkage' of the hippocampus. 
The take home message from these studies is that there is no doubt that exercise is extremely good for the brain. Exercise can raise levels of BDNF which may help in defending against stress, age and depression. It is one of the best strategies against stress and burnout. Most of us don't exercise enough and every time we decide against exercising, we are missing an opportunity to create new brain cells! Just imagine, every time you are curling those biceps, your brain may be going into labour (in a manner of speaking of course)!
Like so many other things in life, too much of a good thing can be bad. Exercise can have the opposite effect if done to excess, so it is wise to be sensible and to listen to your body until we know exactly what 'too much' is!
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