Reading the Number in Context: Melanin and Vitamin D Testing

Key Takeaways:

• A single dose of ultraviolet radiation equal to one minimal erythemal dose raised circulating vitamin D as much as 60-fold within 24 to 48 hours in lightly pigmented volunteers and produced no significant change at all in heavily pigmented volunteers; six times that dose was needed to reproduce the response.
• In a 2,085-person cohort of Black and White American adults, mean total 25-hydroxyvitamin D was 15.6 versus 25.8 ng/mL and mean vitamin-D-binding protein was 168 versus 337 µg/mL, with two GC-gene polymorphisms appearing to explain 79.4 percent of the variation in binding protein — while bone mineral density ran higher, 1.05 versus 0.94 g/cm².
• The conclusion that bioavailable vitamin D is equivalent across ancestry is assay-dependent: a monoclonal binding-protein immunoassay reads roughly 50 percent lower in people of African ancestry, while polyclonal antibody assays and mass-spectrometry proteomics show no such difference.

July is UV Safety Month, which makes it a good moment to examine the one common lab test where sunlight, skin biology, and a printed reference range collide. The link between melanin and vitamin D testing runs through a single photochemical step. Ultraviolet B photons in the 290 to 315 nanometer band strike 7-dehydrocholesterol in the epidermis and convert it to previtamin D3, which then isomerizes thermally to vitamin D3. Eumelanin sits in those same layers and absorbs strongly across precisely that band, so it competes for the identical photons and raises the minimal erythema dose several-fold. A small landmark experiment in five volunteers made the magnitude concrete: after a single standard exposure equal to one minimal erythemal dose, lightly pigmented subjects raised serum vitamin D as much as 60-fold within 24 to 48 hours, while heavily pigmented subjects showed no significant change whatsoever. Re-exposing one deeply pigmented volunteer to six times that dose produced concentrations comparable to what the lighter-skinned subjects reached at the lower dose. The pigment is not a defect in the system. It is an efficient UV filter doing exactly what it evolved to do, and the vitamin D consequence is a downstream cost of that protection.

That is the input side of the equation. The reporting side is where interpretation tends to go wrong. Circulating 25-hydroxyvitamin D is almost entirely protein-bound — the great majority carried by vitamin-D-binding protein, most of the remainder held loosely by albumin, with well under one percent circulating free — and the standard assay reports the total, bound and unbound together, as a single number. Vitamin-D-binding protein is encoded by the GC gene, and two common polymorphisms, rs7041 and rs4588, define the haplotypes that largely set its circulating concentration; their frequencies differ substantially by ancestry. In a cohort of 2,085 Black and White American adults, mean total 25-hydroxyvitamin D was 15.6 ng/mL versus 25.8 ng/mL, and mean vitamin-D-binding protein was 168 µg/mL versus 337 µg/mL, with those two polymorphisms alone appearing to account for 79.4 percent of the variation in binding-protein levels. Parathyroid hormone behaved as expected, rising as 25-hydroxyvitamin D fell. Bone mineral density did not: it ran higher, 1.05 g/cm² versus 0.94 g/cm². A value that reads as deficiency against a reference range built largely on one distribution of binding-protein genotypes was sitting alongside denser bone.

What does melanin and vitamin D testing actually measure?

Total 25-hydroxyvitamin D — and how much of that total is biologically available is a genuinely contested question rather than a settled one. The tidy version of the story, in which a low total in darker-skinned patients is fully offset by a low binding protein so that bioavailable 25-hydroxyvitamin D comes out equal, turns out to depend on which binding-protein assay was used, and that dependence is not a footnote. In 1,057 men sampled across the United States, the United Kingdom, and The Gambia, a monoclonal antibody immunoassay measured binding protein roughly 50 percent lower in men of African ancestry, while polyclonal antibody assays and mass-spectrometry-based proteomic quantification found no such difference. Directly measured free 25-hydroxyvitamin D correlated tightly with total 25-hydroxyvitamin D at a Spearman coefficient of 0.84, and in that study it was genuinely lower in African American men rather than equivalent. So the honest position is narrower than either headline. Do not skip the test, and do not wave away a low result — deficiency is real, it is common, and it is inexpensive to correct. But read the value the way any bound-analyte assay deserves to be read: alongside parathyroid hormone, calcium, dietary and supplemental intake, bone density, fracture history, and the person in front of you, rather than treating a single threshold as though it were a diagnosis on its own.

References:

1. Clemens, T. L., Adams, J. S., Henderson, S. L., & Holick, M. F. (1982). Increased skin pigment reduces the capacity of skin to synthesise vitamin D3. The Lancet, 1(8263), 74-76.
2. Powe, C. E., Evans, M. K., Wenger, J., Zonderman, A. B., Berg, A. H., Nalls, M., et al. (2013). Vitamin D-binding protein and vitamin D status of black Americans and white Americans. The New England Journal of Medicine, 369(21), 1991-2000.
3. Nielson, C. M., Jones, K. S., Chun, R. F., Jacobs, J. M., Wang, Y., Hewison, M., et al. (2016). Free 25-hydroxyvitamin D: Impact of vitamin D binding protein assays on racial-genotypic associations. The Journal of Clinical Endocrinology and Metabolism, 101(5), 2226-2234.