Vitamin D: The Complete Guide

What Is Vitamin D?

Vitamin D is a fat-soluble secosteroid that functions more like a hormone than a vitamin. Your skin synthesizes it from UVB sunlight, or you obtain it from fatty fish, egg yolks, and fortified foods. It's essential for calcium absorption, immune regulation, and expression of over 200 genes.

Deficiency is epidemic in northern latitudes, indoor workers, and people with darker skin (melanin reduces UVB penetration). Symptoms are subtle: fatigue, frequent illness, bone pain, mood changes, and poor recovery.

The Science

Vitamin D receptors (VDR) exist in virtually every tissue. Beyond bone health, adequate D status supports innate immunity (reduced respiratory infection risk in deficient individuals), mood regulation (seasonal affective disorder link), and muscle function.

Meta-analyses show vitamin D supplementation reduces fall risk in elderly deficient individuals and may reduce acute respiratory infections when baseline levels are low. Cancer mortality data is suggestive but not definitive.

• ·Immunity: reduces infection risk when deficient
• ·Bones: required for calcium absorption
• ·Mood: deficiency correlates with depression
• ·Muscle: supports strength and fall prevention in elderly

Testing & Dosing

Test serum 25-hydroxyvitamin D [25(OH)D], the standard blood marker. Optimal range for biohackers: 40–60 ng/mL (100–150 nmol/L). Standard lab "normal" starts at 30 ng/mL, but many longevity practitioners target higher.

Dosing depends on starting level, body weight, and sun exposure:

• ·Maintenance (sufficient levels): 1000–2000 IU D3 daily
• ·Deficiency (below 30 ng/mL): 4000–5000 IU daily for 8–12 weeks, retest
• ·Severe deficiency (below 20 ng/mL): medical supervision, often 5000–10000 IU temporarily

The Protocol

Daily D3 with K2 (MK-7 form, 100–200mcg) taken with a fat-containing meal (morning preferred). K2 activates osteocalcin (bone) and matrix GLA protein (prevents arterial calcification).

• ·D3 dose: 2000–4000 IU/day (adjust based on blood test)
• ·K2 MK-7: 100–200mcg alongside D3
• ·Sun: 15–20 min midday sun on arms/legs when UV index is above 3
• ·Retest: every 6 months until stable, then annually

Risks & Overdosing

Vitamin D toxicity is rare but real at extreme doses (above 10,000 IU/day for months without monitoring). Hypercalcemia symptoms: nausea, kidney stones, confusion. Always test before megadosing.

K2 is not optional at higher D doses, without K2, increased calcium absorption may deposit in arteries rather than bone. Magnesium is also required for vitamin D activation, ensure adequate magnesium intake.

Community Consensus

r/Supplements and r/longevity treat vitamin D + K2 as foundational tier-1 alongside omega-3 and creatine. Rhonda Patrick advocates testing and maintaining 40–60 ng/mL. Bryan Johnson includes D3+K2 in Blueprint.

Consensus: test, supplement to target, pair with K2, retest. Don't guess your dose based on generic RDAs, individual needs vary 10-fold.