Or even that being healthier and less frail means you're more likely to go out and enjoy being outdoors.

As I age, I increasingly see the effect that healthy people with great genetics also have all the indicators of being healthy. I think this effect is conflated to an extent even for things like exercise: for people that feel good and strong, exercise feels great. For people that aren't, they go downhill faster anyway.

I have terrible genetics for cardiovascular disease. I've spent my life as an athlete, eating healthy, training, etc. But now in my 30's, I can feel the slow down a lot despite being more disciplined in my training and diet than ever and yet I know people my age that haven't trained in years that can just go for a run and absolutely smoke me.

I'm sure at some point my genetics is going to force me to slow down a bit on the training despite my best efforts and then I'll just end up as someone contributing to the statistics that those who exercise more are healthier.

I know that exercise has benefits nevertheless, but I also think we very often just make the mistake of concluding "healthier people are healthier".

Life is not really fair.

> I'm starting to wonder if Vitamin D and Omega 3s are indicators, not drivers.

That is not true. AFAIK there have been several studies using supplements of Vitamin D3 & Omega-3 that showed that it improved biomarkers of various diseases.

> I'm starting to wonder if Vitamin D and Omega 3s are indicators, not drivers

most cells have vitamin D receptors, so I would think it is actually a driver

No, ample evidence shows that vitamin D (or vitamin D deficiency at least) is causally related to multiple diseases. The evidence is pretty overwhelming. Eg here's a just published review on D deficiency causality for Covid based on applying Hill's Criteria for causality https://www.mdpi.com/2072-6643/17/3/599 and a prior 2022 paper on the same subject https://pubmed.ncbi.nlm.nih.gov/35308241/

Some vitamin D skeptics claim tat there's insufficient evidence from RCTs but few people question the causality of smoking's influence on cancer despite no RCTs for that---that relationship was established also via Hill's Criteria.