Vitamin D3
Functions more like a hormone than a vitamin. Critical for bone density, immune regulation, mood, and signalling in nearly every tissue in the body.
Bone density, fall prevention in older adults, immune function, and depression have extensive RCT support. The cardiovascular and cancer-prevention claims are weaker / mixed. Serum 25-OH-D levels rise within 2–4 weeks of starting; mood and immune effects follow over 4–8 weeks; bone benefits accrue over 6–12 months.
Vitamin D3 (cholecalciferol) is the form your skin produces when exposed to UVB sunlight, and the form your body actually uses. It binds nuclear receptors in nearly every tissue — bone, immune cells, brain, heart — acting as a master regulator of calcium handling, immune function, and gene expression. Deficiency is the rule rather than the exception in modern life (indoor work, sunscreen, latitudes far from the equator), and it tracks with depression, weaker immunity, slower bone healing, and higher all-cause mortality. The supplement is cheap, well-tolerated, and one of the easiest wins in this entire reference.
- Form
- D3 (cholecalciferol), not D2 (ergocalciferol — vegan but ~40% less effective at raising blood levels). Liquid drops or soft-gels in olive or coconut oil — D is fat-soluble. Almost always pair with K2 (MK-7) so the calcium D mobilises goes to bone, not arteries.
- Dose
- 1,000–5,000 IU per day depending on baseline blood level and body weight. Most adults land at 2,000–4,000 IU. Don't mega-dose blind — get a 25-OH-D blood test first if you can.
- Timing
- With the fattest meal of the day (fat-soluble — absorption drops significantly without fat). Morning is preferable — some people report sleep disruption from evening dosing.
- Schedule
- Daily. Weekly mega-doses (e.g. 50,000 IU once a week) also work but daily is gentler on the system.
- Side effects
- Uncommon at standard doses. At very high sustained doses (>10,000 IU/day for months), risk of hypercalcemia — fatigue, nausea, confusion, kidney stones. Easily detected on blood work, easily reversed by stopping.
- Interactions
- Thiazide diuretics can elevate calcium when combined with high-dose D. Some statins (especially atorvastatin) may modestly reduce D synthesis. Corticosteroids reduce D activation. Anti-seizure medications (phenobarbital, phenytoin) accelerate D breakdown — higher doses may be needed.
- Contraindications
- Sarcoidosis and other granulomatous diseases (the granulomas convert D to its active form unregulated, causing hypercalcemia). Hypercalcemia of any cause. Hyperparathyroidism (relative).
- Lab markers
- 25-hydroxyvitamin D (25-OH-D) — this is the standard marker. Mainstream target: 30–50 ng/mL. Functional / longevity-oriented practitioners often aim for 50–70 ng/mL. Below 20 ng/mL is deficient; above 100 ng/mL is too high. Retest 8–12 weeks after starting or changing dose.
- Food sources
- Fatty fish (salmon, mackerel, sardines, herring), egg yolks, beef liver, mushrooms exposed to UV light, fortified dairy and plant milks. The biggest natural source is bare skin in midday sun — 10–20 min on arms and legs produces several thousand IU, but skin pigmentation, age, latitude, and sunscreen all reduce yield significantly.
Pairs with
Vitamin K2 MK-7 (routes calcium to bone and away from arteries — essentially non-negotiable when supplementing D). Magnesium (required to convert D to its active 1,25-OH form — supplementing D without enough magnesium can paradoxically worsen symptoms). Calcium from food (not separate supplements, which carry cardiovascular risk).
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