Vitamin D
Guides, research reviews, comparisons, product recommendations and FAQs for vitamin d.
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Start with Vitamin D
What the Research Says
Frequently Asked Questions
How much vitamin D should I take?
Depends on your baseline 25(OH)D level — ideally measured via blood test. For deficiency (<20 ng/mL), 4000–5000 IU daily for 8–12 weeks typically corrects levels. For maintenance at optimal levels (40–60 ng/mL), 2000–4000 IU/day is commonly used. The UL is 10,000 IU/day, though toxicity is extremely rare below 40,000 IU/day for extended periods.
Do I need vitamin K2 with vitamin D3?
It's a prudent addition for long-term supplementation. Vitamin D increases calcium absorption; K2 (particularly MK-7 form) activates proteins that direct calcium into bones rather than arteries. While toxicity from D3 alone requires very high doses, K2 co-supplementation is supported by mechanistic evidence and is now standard in quality D3 products.
When is the best time to take vitamin D?
With the largest fat-containing meal of the day — vitamin D is fat-soluble, and absorption increases 32–50% when taken with dietary fat. The time of day doesn't matter; morning and evening show equivalent absorption.
How do I know if I'm vitamin D deficient?
The only reliable method is a serum 25-hydroxyvitamin D (25(OH)D) blood test. Optimal levels are typically defined as 40–60 ng/mL (100–150 nmol/L). Deficiency is <20 ng/mL, insufficiency is 20–30 ng/mL. Risk factors: limited sun exposure, dark skin, obesity, living above 37°N latitude, and being over 65.
Can I get enough vitamin D from sunlight?
In theory yes — 10–30 minutes of midday sun on arms and legs in summer produces 10,000–20,000 IU. In practice, this is unreliable: UVB is insufficient at latitudes above 37°N from October–March, glass blocks UVB entirely, sunscreen blocks synthesis, and dark skin requires significantly longer exposure. Supplementation is more reliable and consistent.