Vitamin D: Synthesis, Metabolism, and Endocrine Functions

Core Idea

Vitamin D functions as both a nutrient and a hormone: skin synthesis from 7-dehydrocholesterol is regulated by UV-B exposure, latitude, and season; hepatic 25-hydroxylation creates the circulating storage form; renal 1α-hydroxylation produces the active hormone 1,25-dihydroxyvitamin D. The active form regulates calcium-phosphorus homeostasis, immune cell differentiation, and cellular proliferation. Vitamin D insufficiency is common, particularly in northern latitudes and those with limited sun exposure or darker skin pigmentation.

Explainer

Vitamin D occupies an unusual position in human physiology: it is the only nutrient the body can synthesize in adequate quantities from sunlight alone, and its active form behaves not as a vitamin but as a steroid hormone. You already know from cholesterol synthesis that 7-dehydrocholesterol sits on the cholesterol biosynthetic pathway as an intermediate. In skin keratinocytes, UV-B radiation (wavelengths 290–315 nm) converts 7-dehydrocholesterol to previtamin D₃, which isomerizes thermally to vitamin D₃ (cholecalciferol). The amount produced depends on skin pigmentation (melanin absorbs UV-B, competing with the synthesis reaction), sun angle (latitude and season determine UV-B intensity), and surface area exposed. Dietary vitamin D₂ and D₃ supplement or replace skin synthesis when sunlight is insufficient.

The freshly made or ingested vitamin D₃ is biologically inert. It requires two sequential hydroxylation reactions — both involving cytochrome P450 enzymes — to become active. First, the liver adds a hydroxyl group at carbon-25 to produce 25-hydroxyvitamin D (calcidiol), the major circulating form used to assess vitamin D status (serum 25(OH)D). This hepatic step is largely unregulated, so circulating calcidiol reflects total vitamin D availability from all sources. Second, the kidney adds a second hydroxyl group at carbon-1α to produce 1,25-dihydroxyvitamin D (calcitriol), the fully active hormone. This renal step is tightly regulated: parathyroid hormone (PTH) and low serum phosphate upregulate the renal 1α-hydroxylase; calcitriol itself and FGF-23 downregulate it. The kidney thus controls active hormone output based on calcium-phosphorus status.

From your background in steroid hormone synthesis and signaling, you know the molecular logic: calcitriol diffuses into target cells, binds the vitamin D receptor (VDR) — a nuclear receptor — and the VDR-calcitriol complex pairs with the retinoid X receptor (RXR) to form a heterodimer that binds vitamin D response elements (VDREs) in DNA, activating or repressing target genes. In the gut, the primary targets are calcium transport proteins (TRPV6, calbindin-D9k) that increase intestinal calcium absorption from ~10–15% (deficient state) to ~30–40% (replete state). In bone, calcitriol stimulates osteoblasts and, via RANKL signaling, indirectly promotes osteoclast activity — maintaining calcium availability from skeletal stores when dietary supply is insufficient.

Beyond mineral homeostasis, VDR is expressed in immune cells, pancreatic beta cells, cardiac muscle, and many other tissues, explaining the association between vitamin D insufficiency and conditions ranging from autoimmune disease to cardiovascular risk. The practical challenge is that insufficiency is extraordinarily common — 40–80% prevalence depending on population — because modern indoor lifestyles decouple the body from its primary synthesis pathway, and foods naturally rich in vitamin D₃ are few (fatty fish, egg yolks). This makes vitamin D the most clinically relevant example of a nutrient-hormone whose insufficiency is structural rather than individual.