John Hawks reviews an article by Roni Caryn Rabin on the connection with glucose metabolism and age related cognitive decline.

The original authors made clear that we remember:

Previous observational studies have shown that physical activity reduces the risk of cognitive decline, and studies have also found that diabetes increases the risk of dementia. Earlier studies had also found a link between Type 2 diabetes and dysfunction in the dentate gyrus.

Here the causality is not necessarily clear. Maybe people who have healthy metabolic profiles are more likely to be active and less likely to exhibit cognitive declines. In that scenario, you wouldn’t necessarily benefit from changing your activity pattern.

I disagree with him here.  In our society people do not (generally) exercise because they find it fun, or because it’s something they are naturally good at.  People exercise because they believe they have to.  There is a strong cultural pressure that leads people to feel like they should work out regardless of how natural it feels.

I'm working my way through grad-school as a personal trainer and fitness coach (actually, I've been a coach now for nearly a decade).  Nearly all of my clients come to me wanting to change how they look.  They know they need help from me, a trainer, precisely because they don’t find exercise natural. 

I think that the causal link is rather robust.  Most exercising Americans are far from athletes with great natural metabolic profiles.  But, exercising does improve their metabolic profiles, and can bring them up to the level of those lucky few (very few) who have it naturally without working out.