How It Works
D3 → 25(OH)D in liver → 1,25(OH)2D (calcitriol) in kidneys. Regulates 200+ genes via VDR (vitamin D receptor). Increases intestinal calcium absorption 2-4x. Modulates immune system (Th1/Th2 balance). Supports testosterone production (Leydig cells have VDR).
Dosage & Timing
Dosage Guide
Daily Dose1000-5000 IU
TimingWith fat-containing meal (fat-soluble). Morning preferred (may disrupt sleep if taken late).
BioavailabilityD3 (cholecalciferol) ~87% more effective at raising serum levels than D2 (ergocalciferol). Requires fat for absorption. Obesity reduces bioavailability (sequestered in adipose).
Optimal Timing
Morning with breakfast. Requires fat for absorption — take with eggs, avocado, or fish oil. Morning dosing avoids potential sleep disruption (D may suppress melatonin if taken late).
Blood Markers to Monitor
- 25(OH)D (target: 40-80 ng/mL)
- PTH (should decrease as D normalizes)
- Calcium (monitor if supplementing high doses)
Deficiency Symptoms
- Low mood (seasonal)
- Frequent illness
- Bone pain
- Muscle weakness
- Fatigue
- Hair loss
Cycling & Duration
No cycling required — safe for continuous daily use.
Best Forms (Ranked)
Vitamin D3 (Cholecalciferol) — ★★★★★
The form to use. From lanolin or lichen (vegan). Always D3, not D2.
Vitamin D3 + K2 (MK-7) — ★★★★★
Best combo. K2 directs calcium to bones, away from arteries. Synergistic.
Calcitriol (active form) — ★★☆☆☆
Prescription only. For renal failure patients. Do NOT self-supplement.
Evidence-Based Benefits
- Bone density improvement
- Reduced respiratory infection risk
- Testosterone increase (if deficient)
- Mood improvement (SAD/seasonal)
- Reduced all-cause mortality (observational)
- Reduced autoimmune disease risk
Side Effects & Safety
- Hypercalcemia at very high doses (>10,000 IU/day long-term)
- Kidney stones (if with high calcium, low magnesium)
- Rare: toxicity symptoms at >50,000 IU sustained
✅ Good For
- Anyone with low sun exposure
- Dark skin in northern latitudes
- Winter months (>37° latitude)
- Athletes (bone health, immunity, T support)
- Autoimmune conditions
- Older adults (reduced skin synthesis)
⚠️ Skip If
- Hypercalcemia
- Sarcoidosis/granulomatous disease
- Primary hyperparathyroidism
- Those with adequate levels (test first: target 40-80 ng/mL)
Stacks Well With
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