it’s difficult to say whether doing sports stops you getting a brain disease, but there is evidence to suggest that once you have a brain disease it may help.
Take for example Alzheimer’s disease. If you do moderate exercise, there is some evidence to suggest that your learning and memory will improve. There are lots of reasons why this might be. If you are taking part in a group exercise class for instance, the social interaction may have an effect on your Alzheumer’s disease. There is alos evidence to suggest that the area of the brain involved in learning and ememory – the hippocampus – benefits from exercise which may explain why exercise is useful in Alzheimer’s disease.
Hope this answers you question.