However, a recent study provided evidence that a socially enriched environment can rescue memory deficits in a mouse model of AD by a BDNF-dependent mechanism.150 Some studies have reported that exercise can increase BDNF levels in the serum or plasma of human subjects,150 although the source of the circulating BDNF is unknown, and it is unclear whether there is a direct relationship between brain and blood BDNF levels. Here, BDNF is linked to Alzheimer disease.