Vitamin K2 · Evidence
Each point below summarizes specific sources, linked by number to the reference list at the bottom. Where the research is thin or mixed, we say so.
The NIH fact sheet says vitamin K is a family of compounds: phylloquinone (K1), the main form in food, and a series of menaquinones (K2), of which MK-4, MK-7 and MK-9 are the best studied. Supplements contain K1 or K2 as MK-4 or MK-7. It notes bioavailability data are limited, and one study found both K1 and MK-7 supplements well absorbed, with MK-7 lasting longer in the body.[1]
The NIH fact sheet says vitamin K is needed to make proteins involved in blood clotting and in bone.[1]
Grouped by how much research backs each outcome. A stronger group means more consistent results across more studies, not a bigger effect.
The NIH fact sheet says it is not clear whether vitamin K supplements affect osteoporosis risk. It describes a 2006 meta-analysis in which fracture trials all used MK-4 at 15 or 45 mg a day (milligrams, not micrograms) and found fewer fractures; a 3-year trial of 180 mcg MK-7 a day in postmenopausal women that found better bone strength; and a 12-month trial of 381 postmenopausal women that found no difference in bone density. It calls the trial results mixed.[1]
The fact sheet describes a 2006 systematic review and meta-analysis by Cockayne and colleagues. Of 13 trials, 12 found that vitamin K1 or MK-4 improved bone mineral density. Seven trials reported fractures, and in those MK-4 significantly lowered hip, vertebral and all nonvertebral fractures. These were the high milligram MK-4 doses above, not the microgram MK-7 doses in most K2 supplements.[1]
The fact sheet says clinical trial results are inconclusive. In observational studies of Dutch adults, people who ate more menaquinones had less artery calcification (564 postmenopausal women) and, in 4,807 adults aged 55 and older, a 27% to 57% lower rate of death from coronary heart disease in the higher intake groups. Phylloquinone (K1) intake showed no such link. These studies can’t show cause and effect, and few trials have tested K2 supplements for this.[1]
The NIH fact sheet lists an Adequate Intake for total vitamin K of 120 mcg a day for adult men and 90 mcg for adult women, including in pregnancy and breastfeeding. There is no separate amount for K2.[1]
The NIH fact sheet says warfarin (Coumadin) and some other anticoagulants work against vitamin K, so people taking them need to keep their vitamin K intake consistent.[1]