Vitamin K2 Decides Where the Calcium Goes

Most people have heard of vitamin K as the one involved in blood clotting. That is K1, and it is abundant in leafy greens. K2 is a different set of compounds with a different job, and it is largely missing from how most people eat now.

The mechanism, which is unusually clean

Two proteins depend on K2 to function. Osteocalcin pulls calcium into bone. Matrix Gla protein keeps calcium out of soft tissue, including arterial wall. Both are made regardless; both are inactive until K2 modifies them.

So the same calcium intake, in the presence or absence of K2, ends up in different places. That is the entire argument, and it explains why calcium supplementation trials have sometimes shown increased arterial calcification alongside modest bone benefit.

Why it disappeared from the diet

• MK-4 comes from animal fat, and specifically from animals eating green plants — pastured butter, egg yolk, liver, hard cheese from grass-fed milk
• MK-7 comes from bacterial fermentation. Natto is by far the richest source; sauerkraut and some aged cheeses contribute
• Grain-fed animals and pasteurised, unfermented food contain very little of either

Practical guidance

• MK-7 for supplementation: it has a much longer half-life than MK-4 and a daily dose maintains a steady level
• 100–200 mcg daily covers most purposes; higher doses appear in trials but the marginal benefit is unclear
• Take it with vitamin D, and with fat
• If you take warfarin, do not add vitamin K without your doctor adjusting the dose — this interaction is real and clinically significant

The evidence for K2 on arterial calcification is suggestive rather than settled: population data is consistent, intervention trials are fewer and smaller than the confidence with which the supplement is sold.

The honest summary

The biochemistry is well described and not controversial. The dietary shortfall is real. The clinical trials showing hard outcome benefit are thinner than the marketing implies. Given that K2 is inexpensive, safe outside of anticoagulant use, and directly addresses a known consequence of high-dose vitamin D, it is one of the more defensible things to take — provided you take it for that reason rather than the ones on the label.