Vitamin D in the form of D3, the active form, is acquired from sun exposure and animal fat in foods such as fish and eggs.

Because it’s difficult to achieve optimal D3 levels from diet alone, sun exposure of several hours per day around solar noon is required. If this is not possible, 10,000IU of a liquid Vitamin D3 supplement should be taken daily, preferably topically.

Books

The Vitamin D Solution by Michael F. Holick