Should I take vitamin D — and how much?

Vitamin D deficiency (serum 25-OH-D below 20 ng/mL) affects over 40% of American men, and functional insufficiency (below 30 ng/mL) affects a much larger proportion, supplementation to a target range of 40–60 ng/mL improves muscle function, cardiovascular risk markers, insulin sensitivity, and immune function, while the VITAL trial (2022 extension) showed benefit for vitamin D at 2,000 IU/day for cancer mortality reduction in non-Black participants (Manson et al., NEJM, 2019).

The right dose depends on your baseline level, which requires a blood test. Men in northern climates working indoors are almost universally deficient by late winter. Daily supplementation at 2,000–4,000 IU vitamin D3 (not D2) with vitamin K2 (90–120 mcg MK7 form, to direct calcium to bones rather than arteries) is reasonable for most men above 40 without testing. If you have been told you are deficient (below 20 ng/mL), a 6–8-week course of 5,000–10,000 IU/day under physician supervision to replete stores before dropping to maintenance dose is appropriate. Toxicity is possible but only at sustained doses above 10,000 IU/day, it is not a concern at 2,000–4,000 IU/day.

Honesty Scale: Solid (1) for vitamin D deficiency prevalence and muscle/immune effects of repletion. Promising (2) for vitamin D supplementation reducing cancer mortality (VITAL data). Promising (2) for K2 co-supplementation, the biological rationale is strong but RCT data is limited.

What to do: Test your 25-OH-D before supplementing if possible. Target 40–60 ng/mL. Supplement with vitamin D3 + K2 together. Retest in 3 months.

For the full picture, read The Supplementation Deep Dive