Why is it that so many people need to take a vitamin D supplement in order to have optimal levels? We’re supposed to be able to make vitamin D after we go into the sunlight. But I have reported to you in the past that for most people, sunlight is just not enough. And here’s another question. Why do some of us need to take such high vitamin D doses in order to have optimal levels? I need to take 15,000 units a day. And many of my patients need even more than that. But why?