A recent article by Dr Mercola on a health advocacy web site makes a number of claims about Vitamin D production in your skin. The most surprising to me is the claim that you can wash off a meaningful fraction of the Vitamin D created due to sun exposure if you wash with soap within two days of sun exposure.
It's important to understand that vitamin D3 is an oil soluble steroid hormone. It's formed when your skin is exposed to ultraviolet B (UVB) radiation from the sun (or a safe tanning bed). When UVB strikes the surface of your skin, your skin converts a cholesterol derivative in your skin into vitamin D3.
However, the vitamin D3 that is formed is on the surface of your skin does not immediately penetrate into your bloodstream. It actually needs to be absorbed from the surface of your skin into your bloodstream. The critical question then is: how long does it take the vitamin D3 to penetrate your skin and reach your bloodstream? If you're thinking about an hour or two, like I did until recently, you're wrong. Because new evidence shows it takes up to 48 hours before you absorb the majority of the vitamin D that was generated by exposing your skin to the sun!
Therefore, if you shower with soap, you will simply wash away much of the vitamin D3 your skin generated, and decrease the benefits of your sun exposure. So to optimize your vitamin D level, you need to delay washing your body with soap for about two full days after sun exposure. Now, few are not going to bathe for two full days. However you really only need to use soap underneath your arms and your groin area, so this is not a major hygiene issue. You'll just want to avoid soaping up the larger areas of your body that were exposed to the sun.
Many will dispute this recommendation as "conventional" thinking teaches that vitamin D is formed in the skin, but this research is based on Dr. Michael Hollick's work, which is over 25 years old. New evidence suggests the current view on how vitamin D is formed is inaccurate. At this time no one has ever tested whether vitamin D is formed in human sebum, the fat that your skin produces. The only study that supports that vitamin D3 is formed in the dermal epidermal junction was done in humans where the sebum was removed from the skin.
Is this true?
According to Wikipedia (lacking a direct citation), Vitamin D3 is made in the inner layers of the epidermis, which one would expect to be insulated from the effects of washing with soap.