Vitamin D3
Supplements · Vitamins · Supplement, Typed Supps
Your skin makes it from sunlight — but often not enough.
Stage by stage
- Intake (Softgel or drops): Fat-soluble, so it travels best dissolved in the capsule oil or in dietary fat already in your stomach.
- Digestion & absorption (Micelle transport): Stomach acid and dietary fat begin emulsifying it, and bile then packages it into micelles — the only way a fat-soluble vitamin crosses the gut lining.
- Bloodstream (Bound to protein): Rides a carrier protein through the blood, first stopping at the liver.
- Liver → kidney → bone (Becomes active hormone): Converted to its active form, helping the gut absorb calcium for bone mineralisation.
What the evidence supports
- Correcting a documented deficiency — strong evidence. Not really in question — supplementing raises blood levels in people who are low, and reverses the bone disease severe deficiency causes.
- Reducing fractures in older adults who are deficient, taken with calcium — moderate evidence. The benefit shows up fairly consistently in institutionalised and deficient older people. It largely disappears in community-dwelling adults who already have normal levels.
- Preventing fractures or falls in people who are not deficient — insufficient evidence. Large trials in people with adequate baseline levels found no reduction. Very high intermittent dosing performed worse than placebo in some.
- Reducing respiratory infections — limited evidence. Pooled analyses suggest a small benefit concentrated almost entirely in people who were deficient to begin with. Individual large trials have been null.
- Preventing cancer or cardiovascular disease — insufficient evidence. The largest placebo-controlled trial to date found no reduction in either over roughly five years.
Typical dose
1,000–2,000 IU (25–50 mcg) daily in most supplements
Upper limit: 4,000 IU (100 mcg) daily for adults
Timing
Take with your largest meal — fat improves absorption meaningfully.
Time to effect
- 8–12 wks: Blood levels normalize if deficient.
- Months: Bone/immune benefits accumulate.
Medication interactions
- Thiazide diuretics: Reduce how much calcium you excrete. Combined with vitamin D and calcium, that can push blood calcium too high. (moderate)
- Digoxin: Raised blood calcium increases the risk of digoxin toxicity and dangerous heart rhythms. (high)
- Orlistat, cholestyramine, and other fat-absorption blockers: Cut absorption of vitamin D along with dietary fat — separate doses and expect to need more. (moderate)
Common myth
“If I get sun, I don’t need to supplement.” In reality: Latitude, season, skin tone, and sunscreen all limit skin synthesis — deficiency is common even in sunny climates.
Food sources
Relevant movements
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