Michael Holick, M.D. recently came out with the book “The vitamin D solution”.
He claims that vitamin D deficiency is responsible for many chronic health problems and that 200 million Americans are vitamin D deficient. I have not read his book. However, I do agree with the essential point.
In his book he recommends to spend time in the sun without protection and even use tanning beds. Considering the high occurence of skin cancer in the US both very controversial recommendations. I think supplements work very well.