The evolutionary significance of vitamin D, skin pigment, and ultraviolet light

Vitamin D is essential for normal growth, calcuim absorption, and skeletal development. Vitamin D deficiency can cause death, immobilization, or pelvic deformities which prevent normal childbirth. In the past these problems were extremely common in North America and Europe, and were only elminated by adding vitamin D to food. Prior to that, variations in available vitamin D affected health, survival and reproductive efficiency sufficiently to have evolutionary significance. Vitamin D is naturally present in few foods; most comes from the photo-conversion of 7-dehydrocholesterol in skin. The limiting factor in this conversion is the availability of ultraviolet light less than310 nm. Seasonal and geographic variations in natural ultraviolet radiation cause parallel variations in blood vitamin D levels, intestinal calcuim absorption, and clinical vitamin D deficiency. These physiological variations can be abolished by exposure to comparable artificial ultraviolet radiation, or by dietary vitamin D supplements. Ultraviolet radiation less than310 nm is absorbed by skin pigment, but is also increases skin pigmentation. This has led to the hypothesis that skin pigment regulates skin vitamin D production. Little direct evidence exists to test this reasonable hypothesis, but necessary and sufficient conditions for establishing it can be outlined. Until this hypothesis is experimentally tested, it is impossible to evaluate the corollary hypothesis: that racial variations in the efficiency of cutaneous vitamin D production restricted the evolution of dark-skinned peoples to tropical latitudes and thereby caused the geographic distribution of the races.