A person's BDNF gene variation (Val66Met) might affect how much they benefit from exercise for memory and brain structure, with some studies showing Val/Val people get more benefit than Met carriers.

Source Excerpt

“For example, BDNF genotype can modulate the effect of physical exercise on episodic memory performance and brain volume. This is evident by the fact that only Val homozygous benefited from physical exercise with larger MTL volume and hippocampal gray matter, whereas in Met carriers the contrary effect was found. In another study, in a cohort aged 65 or older the strength of the association between incidence of cognitive decline and physical activity increased with the number of Met alleles, suggesting that the Met allele may confer vulnerability to dementia in elders with less physical activity. On the other hand, there are studies reporting a reduced vulnerability of Met carriers to age-related decline in executive function, pointing toward a differential effect of BDNF on cognitive function related to the areas supporting the task.”