The recent Netherlands paper on “natural” vitamin D levels in Africa immediately brought to my mind another paper, a similar attempt to gather this vital information. If we don’t know what our vitamin D levels were during the two million years our species was evolving, then how do we know what level to achieve while we wait for science to complete its work?
Professor Binkley and his colleagues found that surfers and skate-boarders in Hawaii had levels around 30 ng/ml with the highest level being 60 ng/ml. Why would light-skinned sun-drenched outdoor sport enthusiasts in Hawaii have an average level of 30 ng/ml, while very dark-skinned hunter-gatherers in Africa have levels of 50 ng/ml?
Sunscreen is one possibility, but 40% of Binkley et al’s subjects reported never using sunscreen, and the rest were not obsessive about using it. Essentially, all of Binkley et als’ subjects had evidence of abundant sun-exposure, indicating that the author’s calculation that they obtained the equivalent of 11 hours of full-body sun exposure per week with no sunscreen was correct. If not sunscreen, then what could explain the difference between Hawaii and Africa? Genetics is a possibility, as about 40% of vitamin D levels are heritable, but if genetics was the answer, why would the highest levels be in subjects living on the equator, where sun can so easily do the job?
What about latitude? The Africans were at latitude 3 degrees while the “A’ala Park Board Shop” in Hawaii is at latitude 21 degrees. That may explain some of it, although the sun in Hawaii gets very high in the sky, even in March. Anyway, the hunter-gatherers in Africa regularly avoided activity during midday, planning their foraging and hunting trips in the morning or evening.
The only other possibility I can think of is an obscure paper from 1937, a paper that continues to be ignored. The authors obtained surface oils from young men, sebum. In the first experiment, they irradiated the oils and in a second experiment irradiated the young men. They collected the sebum and showed that irradiated sebum cured rickets in rats (showing effective treatment of rachitic rats was the only way of measuring vitamin D activity in 1937). The authors concluded, “The evidence presented in the two groups of experiments indicates that washing the human skin by the usual methods removes vitamin D and its precursors from the outer layer of the skin.”
Helmer AC, Jensen CH: Vitamin D precursors removed from the skin by washing. Studies Inst Divi Thomae 1937, 1:207-216.
Even more upsetting, they concluded, “There is definitive evidence that the secretions from the skin contains precursors of vitamin D, which after irradiation are to be reabsorbed by the body, and the removal of which tends to produce a dearth of the vitamin unless it be supplied in some other form.” I could not see evidence that they supported this statement with their research. What they showed was simple.
Humans make some vitamin D on the surface of their skin, which water washes off. How much humans make on the surface and how much inside the skin, no one knows. However, the vitamin D levels of the African tribesmen support (but do not prove) the proposition that humans living in a natural state make a significant proportion of vitamin D on the surface of their skin for later absorption. Assuming the African hunter-gatherers do not take showers twice a day that so many cosmetically brainwashed Americans do, then simple water, especially soapy water, routinely washes off oils containing vitamin D in modern humans. This means we must add soap and frequent showering to the list of things that explain why modern vitamin D levels continue to decline, decade after decade.