Excellent review looking at brain atrophy and exercise

Neuroscientist. 2012 Feb;18(1):82-97. Epub 2011 Apr 29.

The aging hippocampus: interactions between exercise, depression, and BDNF.

Erickson KI, Miller DL, Roecklein KA.

Source

Department of Psychology, University of Pittsburgh, Pittsburgh, PA, USA. kiericks@pitt.edu

"In summary, we conclude from this review that there is mounting evidence that BDNF protein expression
plays an important role in age-related hippocampal atrophy and that geriatric depression magnifies hippocampal atrophy and risk for AD through BDNF pathways while exercise enhances hippocampal morphology and physiology by elevating the production of BDNF. Future research is needed to determine the extent to which BDNF mediates hippocampal atrophy and memory impairment or is a biomarker for other cellular and molecular changes."