Vitamin K2 / MK-4 / MK-7

Vitamin K allows the body to activate proteins needed for blood clotting and for directing calcium into bone rather than soft tissue. K1 comes mainly from leafy greens, K2 from fermented foods and animal products. Most interest in K2 supplements comes from bone and artery claims that remain under study.

K1 (phylloquinone) and K2 (menaquinones) share a common ring structure with different side chains. MK-7 is usually produced by bacterial fermentation, MK-4 is typically synthetic.

Vitamin K is the cofactor that switches on certain proteins, including clotting factors in the liver and matrix Gla protein and osteocalcin outside the liver.

Vitamin K's clotting role is beyond dispute. For bone, trials report changes in biochemical markers and some density findings, but fracture evidence is inconsistent and much of the strongest early data came from single-country studies with high-dose MK-4. Trials of K2 for arterial calcification and cardiovascular events have largely not shown clinically meaningful benefit.

Animal and cell studies show that vitamin K-dependent matrix Gla protein limits vascular calcification and that deficiency promotes it.

Evidence gap: Non-human evidence. It explains why the cardiovascular hypothesis was tested and does not itself demonstrate benefit in people.

Absorption needs dietary fat, and fat malabsorption or bile problems reduce it. The critical interaction is with warfarin and similar anticoagulants, where changes in vitamin K intake shift INR and bleeding risk.

Vitamin K has low direct toxicity and no established upper intake level, but it is not risk-free in context: it can make vitamin K antagonist anticoagulation less effective or unstable, which is a serious clinical problem. People with bleeding or clotting disorders, liver disease affecting clotting factors, or planned surgery should review supplements with their clinician. Combined vitamin D and K products mean people sometimes take K2 without realising it.

Softgels, capsules, tablets and combination vitamin D + K2 products are common. Trials have used MK-4 in high daily amounts and MK-7 in much smaller daily amounts.

Adequate vitamin K is part of nutrient adequacy, met through leafy greens, fermented foods and dairy. For bone health, the better-supported approach remains adequate calcium, vitamin D and protein, resistance exercise, fall prevention and, where indicated, prescription osteoporosis treatment. K2 is not a substitute for those.

Educational example: a person on warfarin starts a bone supplement containing MK-7 and their INR drifts below range at the next check. The decision factors are which anticoagulant is involved, whether intake is consistent, and whether the monitoring clinician knows about the supplement. Questions to ask: does this product contain vitamin K, does it affect my anticoagulant, and should my INR be checked sooner? Claims that K2 makes calcium supplements safe or reverses artery calcification go well beyond current human evidence.