Vitamin K2 supplements: bone health, evidence and interactions

Vitamin K covers several fat-soluble molecules. Phylloquinone, or vitamin K1, mainly comes from leafy green vegetables, cabbages, and certain oils. Menaquinones, or vitamin K2, notably include MK-4 and MK-7. They're found in fermented foods, especially certain cheeses, and in animal products. Natto, a Japanese fermented soybean dish, is exceptionally rich in MK-7.

Vitamin K activates clotting proteins and others linked to bone, such as osteocalcin, or to blood vessels, such as MGP protein. This role explains the scientific interest, but doesn't prove that a tablet prevents a fracture or a heart attack. MK-4 and MK-7 don't behave the same way in the body, and trials have used very different forms and doses.

A 2022 meta-analysis pooled 16 randomized trials and 6,425 postmenopausal women. K2 improved lumbar spine bone mineral density. On fractures, the overall analysis of six trials showed no difference (relative risk 0.96). The authors only obtained a reduction after removing one trial judged heterogeneous, which weakens the conclusion. Many studies came from Japan, where high-dose MK-4 is an osteoporosis medication.

Bone density is an intermediate outcome: it can improve without fracture risk decreasing to the same extent. Likewise, a drop in "undercarboxylated" osteocalcin is a biological change, not a demonstrated clinical benefit.

A fracture after a minor fall, prolonged corticosteroid therapy, or menopause with risk factors call for medical evaluation, not the choice of a supplement.

The link between MGP protein and vascular calcification has been studied for a long time. But a biological hypothesis isn't enough: no trial has shown that a K2 supplement prevents a heart attack, a stroke, or coronary calcification. A 2022 review concludes that current knowledge doesn't allow a firm conclusion on preventing osteoporosis or cardiovascular disease.

EFSA sets a satisfactory intake of 70 µg of vitamin K1 per day for adults. It hasn't been able to define a specific need for K2, for lack of sufficient data. Green vegetables, cabbages, certain oils, and fermented cheeses generally cover needs. Deficiency is rare in healthy adults, but possible with celiac disease, inflammatory bowel disease, bariatric surgery, or poor fat absorption. In these situations, it's the underlying cause that needs care. For other bone-related nutrients, see also vitamin D and calcium supplements.

Warfarin, acenocoumarol, and fluindione work precisely by blocking the recycling of vitamin K. A sudden rise or drop in intake can therefore change the INR and the treatment's effectiveness. The point isn't to cut out green vegetables, but to keep intake consistent. A K2 supplement, on the other hand, represents a concentrated change that must be reported before starting or stopping it.

Other medications can reduce vitamin K absorption: orlistat, resins such as cholestyramine, and long-term antibiotics. Direct oral anticoagulants don't depend on vitamin K, but any supplement should still be reported to the doctor or pharmacist.

MK-7 stays in the blood longer than MK-4, but that doesn't prove it protects better against fractures. No universal dose is validated for general supplementation.