Vitamin D Synthesis in Skin: How UVB Sunlight Enables Calcium Absorption

Cutaneous vitamin D₃ (cholecalciferol) production is the primary natural source of vitamin D for humans.

The Photobiochemical Pathway

1. UVB Penetration: 290–315 nm photons reach epidermal 7-dehydrocholesterol (7-DHC).
2. Photolysis: 7-DHC → previtamin D₃ → (thermal isomerization) → vitamin D₃.
3. Hepatic Hydroxylation: D₃ → 25(OH)D (calcidiol) – major circulating form, half-life ~2–3 weeks.
4. Renal Hydroxylation: 25(OH)D → 1,25(OH)₂D (calcitriol) – active hormone, tightly regulated by PTH, calcium, phosphate.

Calcium Homeostasis

Calcitriol binds intestinal VDR → upregulates TRPV6, calbindin-D9k, PMCA1b → active transcellular Ca²⁺ absorption (10–15% → 30–40% efficiency). Also enhances renal Ca²⁺ reabsorption and bone resorption (with PTH).

Modifying Factors

• Latitude, season, time of day (UVB zenith angle)
• Skin melanin (natural sunscreen; SPF 15 blocks 99% vitamin D synthesis)
• Sunscreen use, clothing, aging (↓ 7-DHC), obesity (sequestration in adipose)

Practical Guidance

10–30 min midday sun exposure (arms/legs) 2–3×/week suffices for many; supplement (600–2000 IU D₃) if deficient, dark-skinned, elderly, or limited sun access. Monitor 25(OH)D; target 30–50 ng/mL.