Despite its familiarity, vitamin D is pretty puzzling. Not really a vitamin at all, it is actually a fat-soluble nutrient that is a prohormone (a precursor substance to a hormone). Milk is fortified with it, but it does not come in most other dairy products like cheese, yogurt, and ice cream, and it occurs in significant levels naturally in only a few foods like fatty fish and fish oils. To fulfill the recommended daily intake with milk alone may take as much as 6 to 8 glasses, far more than most people drink, and so most people get the majority of their vitamin D from sun exposure. But wearing sunscreen significantly impairs the sun’s ability to activate vitamin D — wearing SPF 8 can reduce the skin’s ability to produce vitamin D by about 95 percent.

Until recently, nobody thought much about whether they got enough vitamin D from drinking… More…