Vitamin D deficiency is very common, even among athletes who spend a lot of time outdoors. Seventy-three percent of athletes tested in a private practice were vitamin D deficient (Clinical Journal of Sport Medicine, September 2010). If you spend a lot of time in the sun and still have low vitamin D, you may have a genetic susceptibility to vitamin D deficiency, or you may just use too much soap (The Lancet, published online June 9, 2010).
Your epidermis (outer layer of skin) makes cholesterol and converts it to 7-dehydrocholesterol. Some 7-dehydrocholesterol remains in the skin, but much of it is secreted in oil to the skin's surface. There exposure to ultraviolet light converts it to previtamin D3 (cholecalciferol). Since both skin oil and vitamin D3 are fat soluble, and not water soluble, a shower does not wash away vitamin D, but using soap does. To preserve vitamin D, use soap where you need it, but don't lather it over your whole body.
Fur and feathers in some animals block UV rays from skin, but their skin oils carry 7-dehydrocholesterol to their hair and feathers where UV light converts it to previtamin D3. They get their vitamin D by licking and grooming their hair or feathers.