A podcast kept me up last night. Our middle daughter Natalie — a PhD student at Johns Hopkins Bloomberg School of Public Health, and someone who has an uncanny gift for finding things I didn’t know I needed — turned me onto a show called Boring History for Sleep. Which, despite the name, is genuinely excellent, and apparently not as soporific as advertised. The episode: “How Humans Became White: Climate, Migration, and Adaptation.” Its premise: human skin didn’t lighten because of race or culture. It lightened because, as populations migrated north into low-UV environments, the body needed a way to keep making vitamin D with less sunlight. Melanin, which protects against UV damage near the equator, became a liability at higher latitudes. Evolution dialed it back. Not for aesthetics. For survival.

That’s a remarkable thing to sit with. And it sent me down a rabbit hole that I’m now going to inflict on you.

The Two-Nutrient Tightrope

Here’s where the story gets even more interesting. Skin pigmentation isn’t just about vitamin D. It’s about two nutrients in direct tension with each other — and sunlight sits at the fulcrum between them.

The other nutrient is folate — vitamin B9. Folate is essential for DNA synthesis and repair, and critically, for closing the neural tube during early fetal development. Neural tube defects like spina bifida result from folate deficiency in the first weeks of pregnancy, often before a woman even knows she’s pregnant. This is why folate supplementation in pregnancy is one of the most evidence-based interventions in all of obstetrics.

What’s less widely known is that UV radiation degrades folate in the body. High UV exposure — the kind you’d get near the equator — rapidly destroys circulating folate. For a pregnant woman living close to the equator, unprotected sun exposure could compromise fetal neural tube closure. Melanin, in this reading, is not merely a sunscreen — it is folate armor. Dark skin evolved near the equator because the stakes of folate destruction were existential: a species that couldn’t close its neural tubes didn’t survive long enough to pass on its genes.

Move that same population to Scotland or Scandinavia, and the calculus inverts. The UV radiation is too weak to threaten folate, but also too weak to drive vitamin D synthesis through heavily pigmented skin. Now the risk flips: vitamin D deficiency, rickets, impaired immunity, failed pregnancies from a different cause entirely. Evolution’s answer was to reduce melanin — to let more UV through — accepting the modest folate risk in an environment where UV was scarce anyway.

The anthropologist Nina Jablonski formalized this framework: human skin color across the globe is essentially a map of the planet’s UV environment, with evolution threading the needle between two photosensitive nutrients at every latitude. The remarkable diversity of human skin tones isn’t incidental. It is, in a very literal sense, the solution to a biochemical optimization problem that took tens of thousands of years to solve.

That’s what kept me awake.

A Very Old Relationship with Sunlight

The earliest recorded recognition of sunlight as medicine comes from ancient Egypt, Greece, and India — civilizations that used heliotherapy to treat skin conditions, melancholy, and what we would now recognize as musculoskeletal disease. Greek physicians observed that children raised in sun-deprived environments developed soft, deformed bones. Roman soldiers stationed in northern Europe noted the same. They didn’t know the mechanism; they knew the remedy.

What we now call rickets — the dramatic bowing of the legs and softening of the skull seen in vitamin D-deficient children — was epidemic in the industrial cities of northern Europe and North America by the 17th and 18th centuries. Factory smoke blocking sunlight, children working indoors, a diet low in oily fish: the perfect storm. In 1650, the English physician Francis Glisson described the condition in meticulous anatomical detail. The cure still eluded him.

The modern story of vitamin D begins in the early 20th century. In 1919, Edward Mellanby demonstrated in dogs that rickets was a dietary deficiency disease — it could be induced by a limited diet and cured by cod liver oil. Elmer McCollum, already famous for discovering vitamin A, showed in 1922 that the anti-rachitic factor in cod liver oil was distinct from vitamin A. He named it vitamin D. Shortly after, Harry Steenbock at the University of Wisconsin demonstrated that irradiating food with ultraviolet light could confer anti-rachitic properties — a discovery so valuable he patented it and used the royalties to fund what became the Wisconsin Alumni Research Foundation (WARF). By the 1930s, vitamin D-fortified milk had largely eliminated rickets in the United States — one of the quiet public health triumphs of the 20th century.

The Molecule Behind the History

Vitamin D is not truly a vitamin in the classical sense — it is a secosteroid hormone. The skin synthesizes vitamin D3 (cholecalciferol) from 7-dehydrocholesterol under UVB radiation. It undergoes sequential hydroxylation: first in the liver to 25-hydroxyvitamin D [25(OH)D] — the form measured in clinical practice — and then in the kidney to the active form, 1,25-dihydroxyvitamin D (calcitriol). The vitamin D receptor (VDR) is expressed in virtually every tissue in the body, including immune cells, keratinocytes, fibroblasts, and endothelial cells — a distribution that should have signaled decades ago that its job description extends well beyond calcium and bone.

VDR activation modulates hundreds of genes. It regulates innate and adaptive immunity, promotes keratinocyte differentiation and migration, suppresses inflammatory cytokines including IL-6 and TNF-α, and supports angiogenesis. Each of these functions is directly relevant to wound healing — and each is compromised in diabetes.

Vitamin D and Diabetes: A Troubled Partnership

People with diabetes are disproportionately vitamin D deficient, and the reasons are multifactorial: reduced outdoor activity, obesity (which sequesters vitamin D in adipose tissue), renal insufficiency (which impairs the final activation step), and chronic inflammation that upregulates VDR degradation. Deficiency is not a peripheral finding — it is the norm in the population at highest risk for diabetic foot ulcers.

As early as 2012, we were asking on this blog whether vitamin D could play a preventive role in diabetes itself — wondering aloud whether supplementation from early life might alter disease risk. That question remains open, but the downstream consequences of deficiency have become steadily clearer.

What the Evidence Now Says About Wound Healing

The accumulation of clinical evidence over the past decade has moved the conversation from plausible hypothesis to serious therapeutic candidate.

In 2017, we highlighted intriguing data from Gupta and colleagues suggesting that vitamin D supplementation improves phagocytosis and modulates inflammatory cytokines — including IL-6 and TNF-α — in patients with diabetic foot infection. The anti-inflammatory angle connected directly to the persistent, dysregulated inflammation that drives so many DFUs toward chronicity.

In 2020, serum data made the association more concrete: lower 25-OH vitamin D levels were associated with the presence and severity of diabetic foot ulcers in patients with type 2 diabetes. Association, of course, is not causation — but the signal was consistent and biologically plausible.

By 2021, systematic review data arrived. A rigorous review of the correlation between vitamin D levels and hard-to-heal wounds concluded that the relationship was real and consistent — while honestly flagging that causal direction remained to be established. That same year brought the clearest interventional evidence yet: Halschou-Jensen and colleagues published a randomized double-blind trial demonstrating improved healing of diabetic foot ulcers after high-dose vitamin D supplementation. This was not an observational study. This was an RCT.

In 2022, a meta-analysis pooling 7,586 subjects confirmed the association between vitamin D deficiency and diabetic foot ulcer wounds, providing the statistical weight that single studies cannot.

In early 2023, molecular-level data emerged: Tang and colleagues mapped both 25-hydroxyvitamin D levels and vitamin D receptor expression in DFU tissue, connecting...

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