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Vitamin D3 is synthesized in human skin upon exposure to sunlight, when ultraviolet (UV) rays convert 7-dehydrocholesterol to previtamin D3 (which is subsequently modified in the liver and kidneys to form the biologically active compound). However, not everyone synthesizes vitamin D3 identically when exposed to sunlight—skin tone plays a large role in determining how well pre-vitamin D3 is synthesized in the skin. Research has shown that skin with greater amounts of the pigment melanin (responsible for giving skin color, with larger amounts producing darker colors) tends to be less effective at synthesizing the important vitamin.

Melanin exists as three subtypes, the most common of which is called eumelanin, a compound that confers a “black” or “brown” hue. Melanocytes (melanin-producing cells) that reside in the bottom layer of the skin produce this pigment upon exposure to UV radiation, causing the skin to darken or tan. The production of melanin, which is recognized for its ability to effectively absorb the sun’s UV rays, is the body’s primary defense mechanisms against UV radiation, one of the most common causes of melanoma (cancer of the melanocytes) when enjoyed in excess.

Increased production of melanin darkens the skin and forms a “natural sunscreen,” protecting the hypodermis, a vascular layer just below the skin. In fact, studies have shown that melanin is able to dissipate over 99.9% of UV radiation absorbed through the top layer of skin.

Putting It All Together

Increased levels of melanin–seen in those who constantly tan or in those of a race characterized by darker skin–effectively protect from increased UV-induced DNA damage, but severely impact the production of Vitamin D3. In fact, clinical and epidemiological studies have shown lower incidence of skin cancer in those with more concentrated melanin, but also conclude that those populations are more likely to be vitamin D-deficient.

For example, according to a 2012 CDC report, 65% of African American women were vitamin D deficient, compared to just 20% of Caucasian women. It was suggested that skin color is the primary contributor to the vitamin D disparity (darker skin requires longer sun exposure to synthesize vitamin D). Identical logic is applied to differing “shades” of skin, suggesting that pale-skinned people require less sunlight than less pale, but white-skinned people to produce equivalent amounts of vitamin D3. Poor levels of vitamin D may increase the risk for bone-related disease such as osteomalacia and osteoporosis, cardiovascular disease, diabetes and other chronic disorders.

Sources

  1. Header Image: Lostintheredwoods (Flickr)
  2. Radiative Relaxation Quantum Yields for Synthetic Eumelanin - Photochemistry and Photobiology
  3. The Protective Role of Melanin Against UV Damage in Human Skin - Photochemistry and Photobiology
  4. Melanogenesis: a Photoprotective Response to DNA Damage? - Mutation Research/Fundamentals and Molecular Mechanisms of Mutagenesis
  5. Vitamin D - Office of Dietary Supplements of the National Institutes of Health
  6. Vitamin D, Race, and Excessive Daytime Sleepiness - Journal of Clinical Sleep Medicine
  7. Vitamin D and African Americans - The Journal of Nutrition
  8. Vitamin D in Caucasian and African American Women - Vitamin D Council
     

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