It's starting to look like Vitamin D isn't what it's cracked up to be. The reason it seems so is because it's a proxy for sunlight exposure. In other words, Vitamin D levels are just an indirect measurement of how much sunlight you get. Controlled clinical trials with Vitamin D supplementation show none of the benefits that are seen from people with naturally higher Vitamin D levels. It's theorized that the *real* benefit comes from nitrous oxide production in the skin as it's exposed to UV.