> High fitness in middle age reduces the chance of dementia by ninety percent
Not to be a sourpuss but I believe that a reasonable theory on this is that physical activity, especially activity that uses brain power, causes neuronal development which reroutes function around damage, without necessarily preventing or mitigating damage itself.
Physical surely helps but the theory of cognitive reserve suggests heavy brain use may build up a reserve of extra connections which insulate one somewhat from brain loss. I can't quote selectively as it's a big chunk of text, but here <https://en.wikipedia.org/wiki/Cognitive_reserve>
Rerouting is mitigating. If the end result is lack of dementia, that's what's important. The fact that some percentage of the neurons are damaged is not a problem. The elasticity of the brain is an evolved trait, rerouting is what the brain does to keep working.
Not to be a sourpuss but I believe that a reasonable theory on this is that physical activity, especially activity that uses brain power, causes neuronal development which reroutes function around damage, without necessarily preventing or mitigating damage itself.