Does showering after sunbathing wash off the Vitamin D? Sarah, Oklahoma
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.8
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, if the percentage made on 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.