Vitamin D: the dose-response question almost no one gets right

The recommended dietary allowance (RDA) for vitamin D in the United States is 600 IU per day for adults under 70. Most clinicians who work on vitamin D as a research subject — not as a clinical guideline — consider that figure conservative to the point of being incorrect.

Where the RDA came from

The 600 IU figure was set by the Institute of Medicine in 2010, based on the dose required to prevent rickets and overt bone-mineralization disease. It is, by design, a deficiency-floor dose. It is not the dose at which any other vitamin D–dependent process is optimized.

The serum target everyone actually cares about

The clinically meaningful endpoint is serum 25-hydroxyvitamin D — 25(OH)D — measured in nanograms per milliliter (ng/mL) in the US or nanomoles per liter (nmol/L) elsewhere.

• < 20 ng/mL: deficient
• 20–30 ng/mL: insufficient
• 30–50 ng/mL: the consensus “sufficient” range
• 50–80 ng/mL: where most independent vitamin D researchers want their own levels
• > 100 ng/mL: approaching the toxicity threshold

The 600 IU dose will get most adults somewhere into the 20–30 ng/mL range. The 4,000–5,000 IU range is what is typically required to push a deficient adult into the 50+ ng/mL band.

Individual variation is large

Body fat sequesters vitamin D. Skin pigmentation reduces cutaneous synthesis. Latitude matters enormously above the 37th parallel. Two adults on the same 5,000 IU dose can land at 35 ng/mL and 75 ng/mL respectively.

The only way to get this right is to measure. A serum 25(OH)D test costs $30–$60 out of pocket. Run it once, adjust dose, run it again in 90 days. That is the protocol.

The K2 question

Vitamin D directs calcium absorption. Vitamin K2 directs where that calcium gets deposited. The case for pairing the two is mechanistically tight; the case for taking D3 alone above 2,000 IU/day without K2 is, in our view, weaker than it should be. We pair them in one softgel for this reason.