I understand that the skin makes vitamin D from sun exposure in the skin oils, and then it takes some time to absorb that vitamin D through the skin. So, how long should we wait after being in the sun to shower so as to avoid washing off the vitamin D prematurely?
quote from the Vitamin D Council (here):
*"Does showering after sunbathing wash off the Vitamin D? Yes, but how much of the skin's total production? Agnes Helmer and Cornelius Jensen published a remarkable human/animal study in 1937, showing that significant amounts of Vitamin D are made on the surface of human skin. Reverend Jensen, the senior author, was a professor of biophysics as St. Thomas Aquinas, the precursor of the University of Dayton. The authors collected surface oils from young men before showering, irradiated the oils, and showed those oils contained large amounts of Vitamin D, enough to cure rickets in animals. Then, they tested a very practical question; can those oils be removed by washing? Indeed they found washing, even with plain water, removed much of the Vitamin D from the surface of human skin. Helmer AC, Jensen CH. Vitamin D precursors removed from the skin by washing. Studies Inst. Divi Thomae, 1937, 1:207–216.
Holick, et al's, landmark 1980 study showing most human Vitamin D production occurs in the deep epidermis was incomplete. It was based on surgically obtained (and assumedly surgically prepped) skin samples that had any remaining surface oils removed by washing with hot water. Indeed, to accurately address the question of where Vitamin D is made, one would need to obtain unwashed human skin, difficult to do even from cadavers. Holick MF, MacLaughlin JA, Clark MB, Holick SA, Potts JT Jr, Anderson RR, Blank IH, Parrish JA, Elias P. Photosynthesis of previtamin D3 in human skin and the physiologic consequences. Science. 1980 Oct 10;210(4466):203–5.
It appears to me that the percentage of Vitamin D made on the surface of the human skin, compared to that made inside the skin, is unknown at this time and in need of additional and careful research. Furthermore, as the percentage made of the surface is significant, studies of cutaneous Vitamin D production in modern humans—unless from skin that went unwashed for several weeks—will not give accurate estimates of Vitamin D production in early man. Thus, these studies cannot give an accurate estimate of the "natural" 25(OH)D levels present when the human genome evolved in Northeast Africa."*
On a lighter note, the timing of your shower would probably also depend on wether you're dirty and smelling, wouldn't it?
Vitamin D3, generated by sun exposure is actually Vitamin D3 sulphate, and is WATER SOLUBLE, hence is easily washed off. We still need to know how long it takes for this Vitamin D3 to be absorbed into the body from the skin's surface where the irradiated oils are concentrated - provided they were not washed off prior to sun exposure.
The absorption is probably an inversely proportional ratio, so the rate of absorption would be greatest at the time of finishing exposure, diminishing exponentionly; if we knew the "half-life" concentration, we would be in a good position to estimate the cutoff waiting time. But my guess, and it is a guess not based on measurement, would be a couple of hours, so shower/wash after two hours and get a fair amount of the Vitamin D3 sulphate into your body.
Given that salts and other water-soluble substances will wash off before oils (since they're hydrophobic), I would guess that a quick rinse to get the sweat off, but not a full "scrub everything" cleansing would be in order.
Richard at Free the Animal has a post about this: http://freetheanimal.com/2009/05/vitamin-d-and-soap.html. His opinion, which is based on Dr. Mecola's opinion is that you can wash off Vitamin D3 with soap. He particularly vilenesses chlorine and soap. Regardless of what you think about the subject, the video Richard links to on his page is very intersting.
It seems like there is a benefit in waiting a bit with washing the whole body. It would be useful to get back to old-style washing, over a basin to cover just the crucial body parts, living a lot of skin by itself (unless it got dirty). Showering the whole body every day is extremely new thing, and not that great. Not only from the perspective of vit. D, but also other beneficial oils that protect the skin from outer elements. Of course I am guilty of it as well, and b/c of that I have to cover my whole body in moisturizer each time I shower, otherwise my skin is dry like crazy. I thought that it's the soap that's the main problem, but if water is also guilty of washing off the vit. D, I will try to limit my washing as not to cover the whole body every time I wash... Will be difficult :-/
If you're stinky, just wash your pits, crotch and feet. If you've been sweating copiously, perhaps just use a damp towel to blot the sweaty areas? I like to let sweat dry off on its own, but then I'm not a guy so I don't sweat nearly as much as they do ;)
Can someone just do an experiment? Sunbathe every day and immediately shower for 10 days. Then, get your vit D level tested. Wait a few days for your vit D level to come down to where it was at the beginning of your first sunbathing session. Then sunbathe again for 10 days and wait 12 hours to shower. Get the vit D level tested.
A volunteer could probably get some chip-in funding to pay for the tests.