UV-B Radiation Strikes the Epidermis
Sunlight's UV-B photons trigger vitamin D synthesis in skin.
- 290–315: UV-B wavelength (nanometers)
- 10am–3pm: Peak synthesis window (sun angle dependent)
- Epidermis: Synthesis site (stratum basale/spinosum)
- Thousands: Photons needed (per converted molecule)
Where UV-B lands
UV-B photons penetrate only the outer skin layers.
Sun angle controls dose
Low sun angle scatters most UV-B away.
This pipeline starts every downstream stage from this one photon event.
Barriers block the signal
Clothing, glass, and sunscreen absorb UV-B before skin.
Latitude, Season, and Skin Type Set the Rate
Three factors combine into one synthesis-capacity number.
- Up to 90%: Melanin effect (UV-B absorbed, not used)
- >35°: Winter cutoff latitude (no synthesis Nov–Feb)
- Fitzpatrick I–VI: Skin type range (pale to deeply pigmented)
- ~2× less: Age effect (precursor in older skin)
Melanin competes for photons
Pigment absorbs UV-B that precursor molecules would use.
Latitude and season stack
High latitude winters can flatten synthesis to near zero.
The capacity slider fuses latitude, season, and skin type into one dial.
Capacity feeds the whole pipeline
This single number now drives every later stage.
Synthesized D3 Becomes Circulating 25(OH)D
Skin output plus diet set the measurable blood marker.
- D3 → 25(OH)D: Liver conversion (hydroxylation step)
- 2–3 weeks: Serum half-life (25(OH)D storage form)
- 25(OH)D: Clinical marker (not active hormone form)
- 30–50: Reference range (ng/mL sufficient)
The liver does the math
Hepatocytes hydroxylate D3 into stable 25(OH)D.
Diet adds a baseline
The starting-level slider stands in for diet and past sun.
Serum level is synthesis capacity plus that starting baseline.
Why labs test this form
25(OH)D reflects weeks of supply, not one day.
Low 25(OH)D Triggers a Correction Need
Crossing a threshold turns a number into a diagnosis.
- <20: Deficient (ng/mL)
- 20–30: Insufficient (ng/mL)
- ≥30: Sufficient (ng/mL)
- ~40%: Adults below sufficient (US population estimate)
Tiers define severity
Severe, deficient, insufficient, and sufficient bands guide action.
Low capacity compounds risk
Poor synthesis capacity pushes serum level lower still.
The deficiency flag is what makes dosing personal, not generic.
Deficiency is often silent
Many deficient adults report no obvious symptoms.
Severity and Synthesis Capacity Set the Dose
Both slider inputs merge into one weekly IU number.
- Weeks: Correction dosing (until retest)
- 1,000–2,000: Maintenance dose (IU/week, sufficient range)
- 8–12 weeks: Retest interval (after starting correction)
- ~4,000 IU/day: Upper safe intake (long-term ceiling)
Dose scales with the gap
A bigger deficit below target calls for a bigger dose.
Low capacity raises the number
Bodies that self-produce less need more from the dose.
This is the pipeline's payoff: capacity and deficit set one dose.
Retesting closes the loop
A follow-up level confirms the correction worked.