Exercise prevents brain shrinkage, says Stephen Rao, PhD. People who have the APOE epsilon4 allele (e4 gene) are at increased risk for Alzheimer’s disease. Recent finding: After 18 months, the brain scans of people with the e4 gene who exercised moderately a few times a week showed dramatically less shrinkage in the hippocampus—which is associated with Alzheimer’s—compared with people with the gene who were not physically active.