"What do we need to do to be sure we have enough vitamin D? We need adequate levels of cholesterol, from which we make that vitamin. Cholesterol is a waxy substance found only in our diets in animal fats (no plants have cholesterol). Eating animal fats, particularly from animals that eat green grass and spend their lives outside in the sun, is one way to get adequate vitamin D in the diet. Second, we need to maintain the oily coating on our skin, which contains the substances that interact with the sun to create the active form of vitamin D. And, finally, we need sun exposure, at least 20 minutes per day with as much of our skin exposed as possible. Thus, adequate vitamin D strategy is a high animal fat diet, from animals that live outside and graze green grass, limited bathing (bathing or showering washes the skin oils off, which is perhaps why traditional people rarely bathed), and adequate sun exposure. Oddly, the American way of life as prescribed by the “experts” is to eat a low-fat diet, bathe every day, and avoid the sun because it is going to kill you. Americans are severely vitamin D deficient."
(Emphasis mine.) Here's the whole article: The Metabolic Syndrome