Vitamin D isn't really a vitamin, it's a steroid hormone. It's ESSENTIAL to health. Basically no one in the west has enough. Everyone knows about testosterone. No one knows about vitamin D - not really at least. It's not just some random shit you can take a bit of to feel better. It does HEAPS of stuff in your entire body. No one in the west has enough vitamin K either.

Here's a generic image of a man running in the sun to hammer the point home.