Moderate exercise helps the brain:

  1. For those at high genetic risk for Alzheimer’s

Alzheimer's disease study links brain health, physical activity  6/22/2017

  https://www.sciencedaily.com/releases/2017/06/170622103818.htm

“Researchers used accelerometers to measure the daily physical activity of participants, all of whom are in late middle-age and at high genetic risk for Alzheimer's disease, but presently show no cognitive impairment. Activity levels were measured for one week, quantified, and analyzed. This approach allowed scientists to determine the amount of time each subject spent engaged in light, moderate, and vigorous levels of physical activity. Light physical activity is equivalent to walking slowly, while moderate is equivalent to a brisk walk and vigorous a strenuous run. Data on the intensities of physical activity were then statistically analyzed to determine how they corresponded with glucose metabolism -- a measure of neuronal health and activity -- in areas of the brain known to have depressed glucose metabolism in people with Alzheimer's disease. To measure brain glucose metabolism, researchers used a specialized imaging technique called 18F-fluorodeoxyglucose positron emission tomography (FDG-PET)"

"Seeing a quantifiable connection between moderate physical activity and brain health is an exciting first step," said Okonkwo. He explained that ongoing research is focusing on better elucidating the neuroprotective effect of exercise against Alzheimer's disease.”

2. Above article supports the exercise benefit of the clinical study for MCI posted earlier in May below:

Walking intervention improved neural connectivity in older adults with MCI

https://www.sciencedaily.com/releases/2017/05/170503213532.htm

"Dr. Smith's research team recruited two groups -- one with 16 healthy elders and another with 16 elders diagnosed with mild cognitive impairment to participate in an exercise intervention that included walking for 30 minutes, four times a week (at 50-60 % of heart rate reserve) for three months."

 “While it is unclear yet whether the effects of exercise training can delay further cognitive decline in patients diagnosed with MCI, the neural network connectivity changes documented in this study provide hope that exercise training may stimulate brain plasticity and restore communication between brain regions that may have been lost through Alzheimer's disease. The specificity of these effects in the MCI group further suggest that exercise may be particularly useful in those who have already experienced mild memory loss.”

Thank you for posting this valuable information.  I think this is part of the explanation for why moderate exercise improves brain health.

Exercise activates the phosphatidylinositol 3-kinase pathway.

These results suggest that the exercise-induced expression of BDNF [brain derived neurotrophic factor] is associated with the increased expression of several key intermediates of the PI-3 kinase/Akt pathway, which is known for its role in enhancing neuronal survival.