UV-B Radiation Converts 7-Dehydrocholesterol to Previtamin D3
Sunlight starts the whole pathway inside the skin.
- 290–315 nm: UV-B wavelength (Mid-range band drives the reaction.)
- Epidermis: 7-DHC location (Concentrated in deeper living skin layers.)
- 1 per molecule: Photons needed (Single photon opens the B-ring.)
- 10 am–3 pm: Peak synthesis window (Sun angle matters more than duration.)
Photolysis mechanism
UVB energy breaks a steroid ring bond, forming previtamin D3.
What blocks synthesis
Melanin, sunscreen, glass, and clouds all absorb UVB first.
Darker skin needs more sun time for equal synthesis.
Previtamin D3 Thermally Isomerizes into Vitamin D3
No more sunlight is needed for this second step.
- Body heat (37°C): Conversion driver (Purely a thermal, not photic, reaction.)
- ~24–36 hrs: Half-life (Slow gradual isomerization in skin.)
- Lumisterol / tachysterol: Overexposure fate (Excess UVB degrades previtamin D3 safely.)
- Skin lipid layer: Storage site (Vitamin D3 buffered before blood release.)
Temperature-driven rearrangement
Skin warmth reshapes previtamin D3 into stable vitamin D3.
Built-in overdose protection
Extra UVB just degrades the precursor, capping natural synthesis.
The Liver Converts Vitamin D3 into 25-Hydroxyvitamin D
This is the form doctors actually measure in blood.
- CYP2R1 (25-hydroxylase): Enzyme (Adds hydroxyl group at carbon 25.)
- ~2–3 weeks: Circulating half-life (Long-lived, reflects weeks of sun exposure.)
- Serum 25-OH-D: Blood test target (The standard vitamin D status test.)
- Vitamin D-binding protein: Carrier protein (Transports the molecule through bloodstream.)
Why the liver form is measured
Its long half-life makes it a stable status marker.
Sources feeding this step
Skin synthesis and dietary/supplement vitamin D both funnel here.
25-OH-D, not active hormone, is the standard lab test.
The Kidney Activates 25-OH-D into Calcitriol
A second hydroxylation makes the true hormone form.
- CYP27B1 (1α-hydroxylase): Enzyme (Tightly regulated by parathyroid hormone.)
- 1,25(OH)2D (calcitriol): Product (Most biologically active vitamin D form.)
- ~4–15 hours: Half-life (Short-lived, not used for status testing.)
- Calcium/phosphate balance: Main action (Regulates gut absorption and bone mineral.)
Tight hormonal control
Parathyroid hormone and phosphate levels regulate this step.
Why testing skips this form
Its short half-life makes it a poor status marker.
Serum 25-OH-D Level Determines Vitamin D Status
One lab number sorts patients into three zones.
- <20 ng/mL: Deficient (Supplementation typically recommended.)
- 20–30 ng/mL: Insufficient (Borderline, often monitored or supplemented.)
- >30 ng/mL: Sufficient (Adequate stores, no action usually needed.)
- Serum 25-OH-D assay: Test used (Standard clinical vitamin D test.)
Reading the result
The single ng/mL value maps directly to a zone.
Guiding supplementation
Lower levels prompt higher-dose vitamin D3 supplementation plans.
Sun, season, and skin tone all shift this number.