Do we not understand how a plant needs sunlight in various dimensions, depending upon its nativity? Check out British dermatologist Richard Weller.
The takeaway? Well, you can raise your vitamin D levels through supplementation. But, are we just working from an association fraught with the confounding variable of sunshine? It's probably true that higher vitamin D levels are better than low. You can also get high vitamin D levels by basking in the sun, and isn't it so odd that humans tend to have a fundamental, base, plant-like love of that?
I think it's far more interesting to examine the associations present where, at equally high levels of vitamin D, for one individual it's because of supplementation, and for another, due to sun exposure. This is the next thing to begin examining. In other words, vitamin D ought to be at good levels, say 50 ng/ml thereabouts. But how did you get there?
Is "cheating" really allowed?