After my previous posts on Vitamin D, there seem to be more and more articles, both saying that Vitamin D supplementation is necessary, and also that it can be linked to athletic performance.  John Post has written about the impact on muscular performance.  I also found a helpful, work-in-progress summary of the evidence for the benefits.  In short, most people seem to show benefits from Vitamin D supplementation and there doesn’t seem to be any downside, as long as you don’t take too much.