A study involving 97 healthy older adults (65-89) has found that those with the “Alzheimer’s gene” (APOe4) who didn’t engage in much physical activity showed a decrease in hippocampal volume (3%) over 18 months. Those with the gene who did exercise showed no change in the size of their hippocampus, nor did those without the gene, regardless of exercise. Physical activity was classified as low if the participant reported two or fewer days per week of low intensity activity, such as no activity, slow walking or light chores. Physical activity was classified as high if the participant reported three or more days/week of moderate to vigorous activity
The finding suggests that those with the risky gene will benefit most from regular exercise — indeed, this is as yet the only known means to counteract hippocampal shrinkage.
 Physical activity reduces hippocampal atrophy in elders at genetic risk for Alzheimer's disease. Frontiers in Aging Neuroscience. 6,(2014).