A study found that resistance training, i.e. weightlifting, increased levels of BDNF, or brain-derived neurotrophic factor. BDNF is a growth factor that causes neurogenesis and increased synaptic connections. Training augments resistance exercise induced elevation of circulating brain derived neurotrophic factor (BDNF). The change was transient, that is, resting BDNF levels did not change, but after several weeks of training, “the change in serum BDNF from rest to immediately post-exercise was 98% greater at post-intervention than at baseline.”
Our study is the first to demonstrate that resistance exercise induces a robust, yet transient, elevation of circulating BDNF and that progressive resistance training augments this response; perhaps demonstrating one mechanism through which exercise influences brain health.
Another study which was much touted at the time of publication found that exercise increases brain volume, specifically that of the hippocampus. Exercise training increases size of hippocampus and improves memory.
The hippocampus shrinks in late adulthood, leading to impaired memory and increased risk for dementia. Hippocampal and medial temporal lobe volumes are larger in higher-fit adults, and physical activity training increases hippocampal perfusion, but the extent to which aerobic exercise training can modify hippocampal volume in late adulthood remains unknown. Here we show, in a randomized controlled trial with 120 older adults, that aerobic exercise training increases the size of the anterior hippocampus, leading to improvements in spatial memory. Exercise training increased hippocampal volume by 2%, effectively reversing age-related loss in volume by 1 to 2 y. We also demonstrate that increased hippocampal volume is associated with greater serum levels of BDNF, a mediator of neurogenesis in the dentate gyrus. Hippocampal volume declined in the control group, but higher preintervention fitness partially attenuated the decline, suggesting that fitness protects against volume loss. …These theoretically important findings indicate that aerobic exercise training is effective at reversing hippocampal volume loss in late adulthood, which is accompanied by improved memory function.
Note that both of these studies were trials and not associational.
So, exercise looks to be a potent way to retain brain volume with aging, or really at any age for that matter.