So after a five hour trip to Costa Rica I go out on the beach and spend 20 minutes there to get my dose of Vitamin D. Did I get the 10,000 iu that everyone says I should receive in that time or would another person receive that much? A lot of factors here which includes age ( greatly reduced vitamin D absorption when aged sixty or more), color of skin ( greatly reduced if not white and you are brown or black skinned), if you are tanned ( greatly decreased depending on the amount of tanning), what is the amount of skin exposed ( obviously a bikini is going to expose more skin that a suit covering over half the body), the angle of the sun ( no vitamin D if your shadow is longer than your height) and the new one that I just found out about. Soap!
What happens is the vitamin D is produced in the skin and deposited in the oil on the skin. So, if you sun bath or go to a tanning parlor then go home and take a shower you just washed off the vitamin D. It takes 48 hours for the vitamin D to be absorbed by your system from the oil. So if you take a shower use only water and if you do use soap wash only unexposed areas of your body to sunlight. Arm pits and groin for example would be fine.
I might point out that our ancestors a million years ago did not have soap. Such a good thing, soap, but it has a down side. Just another example of diseases caused by the improvements of "civilization".
They have a saying in Costa Rica which replaces hola in other Spanish speaking countries. Hola means hello in Spanish. The Costa Rican's say pura vita, or literally translated means "pure life". According to the record book they are one of the happiest people on earth and live some of the longest lives. So.............