Being fit is negatively correlated with lowered brain function with age… a lot.
The best thing you can do for yourself for your brain health is to do the stuff that you’d normally think to do for your heart. Move regularly. Avoid Chronic Inactivity. Don’t eat too much sugar. It’s not simply cardiovascular exercise that helps, but instead a varied fitness regiment including cardiovascular endurance, interval training, weightlifting, and mobility/flexibility training.
People who stay consistently fit for life, who meet and even exceed the recommended 150 minutes/week recommendation are the least likely to suffer cognitive declines into their later years.
Overly sugary diets cause the blood to be too saturated with blood sugar. This causes our cells to be less receptive to the sugar in our blood which basically throws our whole system out of whack.
Tribal example
An indigenous tribe in the rainforests of the Amazon river has no evidence of heart disease, diabetes, or dementia. They eat about 70% carbs (unprocessed) 15% fat, 15% protein. They walk about 17,000 steps a day (not run). They sleep and wake with the sun.