|
Vitamin D Photosynthesis and Skin Pigmentation
The 'early man' theory of low vitamin D posits that melanin pigmentation evolved in people living near the equator to prevent the excessive production of vitamin D due to constant exposure to sunlight. As people migrated away from the equatorial regions, their sunlight exposure was shortened and, in order to allow adequate production of vitamin D3 and prevent rickets, the melanin levels in their skin diminished. [1] This evolutionary rationale for low serum 25(OH)D levels is highly speculative. An analysis by Greaves concludes that early humans may have evolved black skin to protect against a very high risk of dying from ultraviolet light (UV)-induced skin cancer. [2] Rick Potts, paleoanthropologist and curator of anthropology at the Smithsonian's National Museum of Natural History, points out,
"The study of human evolution has long sought to explain major adaptations and trends that led to the origin of Homo sapiens. Environmental scenarios have played a pivotal role in this endeavor. They represent statements or, more commonly, assumptions concerning the adaptive context in which key hominin traits emerged. In many cases, however, these scenarios are based on very little if any data about the past settings in which early hominins lived." [3]
Robins disputes the idea that light skin evolved to allow more vitamin D3 production. [4] Other critics of this hypothesis point out there are no reported cases of hypercalcemia secondary to vitamin D toxicity as a sole consequence of prolonged sun exposure (i.e., photosynthesis of D3 is stopped at normal levels by a protective regulation mechanism in the skin). Many factors (e.g., protection of sweat glands, sunburn, frostbite, skin cancer, defense against microorganisms, etc.) may have played a role in the evolution of skin color. [5] One study found evidence that low vitamin D levels among Africans may be due to genetic variation tied to African ancestry but genetic studies of Caucasians have found no link to skin pigmentation. [6-8] Even prominent authorities are in seeming disagreement about the role skin pigmentation plays in vitamin D production. Believing our dark-skinned ancestors produced higher 25(OH)D levels than modern man, Heaney opined, "It is highly likely that values of 32-60 ng/ml reflect the status in which human physiology developed." [9] While Holick reports, "Increases in skin pigmentation markedly diminish the production of vitamin D.". [10]
The so-called ultraviolet light or UVD theory holds that racial differences in health outcomes between blacks and whites are associated with the ability to synthesize Vitamin D3 from ultraviolet light. A number of studies have established the relationship of skin pigmentation to the capacity to produce vitamin D3 following UVB exposure but the results are not consistent. [11, 12] One study concluded that in blacks and whites, UVB exposure produces similar elevations of serum 25(OH)D concentrations and unchanged calciotropic hormones concentrations. [13] Thayer investigated racial disparities in birth outcomes and demonstrated that contrary to the UVD theory, serum levels of 25(OH)D are unlikely to play a role in differences in preterm birth and low birth weight between African-Americans and whites. [14] Low 25(OH)D levels in Indians living in India and Chinese in China does not support the hypothesis that low levels seen in people with more pigmentation are due to lack of photosynthesis from the sun at higher latitudes. [15, 16] Other studies have shown that the response to UV dose is non-linear and also dependent on genetic factors, dose of UV exposure, and baseline serum 25(OH)D levels. [17, 18]
Melanin pigmentation is only one factor that determines the amount of vitamin D3 which is photosynthesized. As early as 1994, it was known "The skin of some vertebrates contains lower quantities of 7-dehydrocholesterol and its photochemical conversion to cholecalciferol is quite inefficient." [19] In 2010, researchers at the Department of Dermatology in Copenhagen, Denmark measured the baseline serum 25(OH)D and total cholesterol levels of 182 fair-skinned and dark-skinned subjects; and studied the effect of UV radiation on their serum 25(OH)D levels. They found the amount of serum 25(OH)D produced was determined by the amount of cholesterol in the skin, not on skin pigmentation. [20]
Most importantly, skin pigmentation doesn't negatively affect vitamin D status. [21] Persons with dark skin compensate for low 25(OH)D by rapidly converting it to the active 1,25(OH)2D metabolite, thus allowing them to maintain adequate vitamin D status. [22] Matsuoka et al. investigated the effect of racial pigmentation on vitamin D3 formation, simulating the process with a fixed dose of UVB radiation and concluded that while racial pigmentation has a photo-protective effect, it does not prevent the generation of normal levels of active vitamin D metabolites. [23]
The concern about dark skin and vitamin D deficiency appears to be misplaced.
References
1. Loomis WF. Skin-pigment regulation of vitamin-D biosynthesis in man. Science. Aug 1967;157(3788):501-6.
2. Daily S. Skin cancer risk may have driven evolution of black skin. Science Daily. Feb 25, 2014. Available at: www.sciencedaily.com/releases/2014/02/140225193412.htm. Accessed Feb 27, 2014.
3. Potts R. Environmental hypotheses of hominin evolution. Am J Phys Anthropol. 1998;Suppl 27:93-136.
4. Robins AH. The evolution of light skin color: role of vitamin D disputed. Am J Phys Anthropol. Aug 2009;139(4):447-50.
5. Juzeniene A, Setlow R, Porojnicu A, Steindal AH, Moan J. Development of different human skin colors: A review highlighting photobiological and photobiophysical aspects. J Photochem Photobio B. 2009;96(2):93-100.
6. Signorello LB, Williams SM, Zheng W, et al. Blood vitamin d levels in relation to genetic estimation of African ancestry. Cancer Epidemiol Biomarkers Prev. Sep 2010;19(9):2325-31.
7. Wang TJ, Zhang F, Richards JB, et al. Common genetic determinants of vitamin D insufficiency: a genome-wide association study. Lancet. Jul 2010;376(9736):180-8.
8. Boullion R. Genetic and environmental determinants of vitamin D status. Lancet. Jul 2010;376(9736):148-9.
9. Heaney R. Information Gathering Workshop for Dietary Reference Intakes for Vitamin D and Calcium. Institute of Medicine. Aug 4, 2009. http://www.iom.edu/~/media/Files/Activity%20Files/Nutrition/DRIVitDCalcium/2009-AUG-4/2009-AUG-4/VitaminDCalciumInteractions.pdf. Accessed May 1, 2013.
10. Holick MF. Sunlight and vitamin D: both good for cardiovascular health. J Gen Intern Med. Sep 2002;17(9):733-5.
11. Clemens TL, Adams JS, Henderson SL, Holick MF. Increased skin pigment reduces the capacity of skin to synthesise vitamin D3. Lancet. Jan 1982;1(8263):74-6.
12. Bell NH, Greene A, Epstein S, Oexmann MJ, Shaw S, Shary J. Evidence for alteration of the vitamin D-endocrine system in blacks. J Clin Invest. Aug 1985;76(2):470-3.
13. Brazerol WF, McPhee AJ, Mimouni F, Specker BL, Tsang RC. Serial ultraviolet B exposure and serum 25 hydroxyvitamin D response in young adult American blacks and whites: no racial differences. J Am Coll Nutr. Apr 1988;7(2):111-8.
14. Thayer Z. Vitamin D hypothesis challenged: Some racial disparities in childbirth more environmental than genetic. ScienceDaily. Mar 13, 2014. Available at: http://www.sciencedaily.com/releases/2014/03/140313154222.htm. Accessed Mar 21, 2014.
15. Harinarayan CV, Joshi SR. Vitamin D status in India--its implications and remedial measures. J Assoc Physicians India. Jan 2009;57:40-8.
16. Lips P. Worldwide status of vitamin D nutrition. J Steroid Biochem Mol Biol. Jul 2010;121(1-2):297-300.
17. Snellman G, Melhus H, Gedeborg R, et al. Seasonal genetic influence on serum 25-hydroxyvitamin D levels: a twin study. PLoS One. Nov 2009;4(11):e7747.
18. Bogh MK, Schmedes AV, Philipsen PA, Thieden E, Wulf HC. Vitamin D production depends on ultraviolet-B dose but not on dose rate: a randomized controlled trial. Exp Dermatol. Jan 2011;20(1):14-8.
19. How KL, Hazewinkel HA, Mol JA. Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. Gen Comp Endocrinol. Oct 1994;96(1):12-8.
20. Bogh MK, Schmedes AV, Philipsen PA, Theiden E, Wulf HC. Vitamin D production after UVB exposure depends on baseline vitamin D and total cholesterol but not on skin pigmentation. J Invest Dermatol. Feb 2010;130(2):546-53.
21. Villa M, Kelsey J, Chen J, Marcus R. Skin Pigmentation does not affect vitamin D status in community-dwelling Mexican-American women. J Bone Miner Res. 1994;9 (Suppl 1):S418.
22. Matsuoka LY, Wortsman J, Chen TC, Holick MF. Compensation for the interracial variance in the cutaneous synthesis of vitamin D. J Lab Clin Med. Nov 1995;126(5):452-7.
23. Matsuoka LY, Worstman J, Haddad JG, Kolm P, Hollis BW. Racial pigmentation and the cutaneous synthesis of vitamin D. Arch Dermatol. Apr 1991;127(4):536-8.
| 
