Vitamin K2
What it is
Vitamin K2 (menaquinone) is a fat-soluble vitamin distinct from vitamin K1 (phylloquinone). While K1 is found in green leafy vegetables and is primarily involved in blood clotting, K2 is found mainly in fermented foods and animal products and activates proteins involved in calcium regulation, directing calcium towards bones and teeth and away from soft tissues and arterial walls.
The two principal forms studied in clinical research are MK-4 (menaquinone-4) and MK-7 (menaquinone-7). MK-7 has a substantially longer half-life (approximately 72 hours versus 1-2 hours for MK-4) and is effective at nutritional doses of 90-180mcg/day. MK-4 has been studied primarily in Japan at pharmacological doses of 45mg/day, thousands of times higher than typical MK-7 supplement doses. Evidence from MK-4 trials at these doses does not transfer to MK-7 supplementation without direct comparative data.
Low dietary K2 intake is common in Western populations, as the richest dietary sources, natto, hard cheeses, egg yolks, and liver, are not widely consumed. Clinical deficiency in the sense of a defined syndrome with a consensus threshold has not been established.
What the evidence shows
Bone mineral density in postmenopausal women is the best-evidenced application for MK-7 at nutritional doses. Supplementation at 90-180mcg/day attenuates age-related bone loss in postmenopausal women, with effect sizes that are modest, typically small reductions in the rate of BMD loss rather than meaningful increases in bone density. The evidence base is concentrated in a small number of research groups with industry links, and no large independent multi-centre replication trials are available. Activation of osteocalcin by K2 has not been shown to consistently translate into reduced clinical fracture events in adequately powered MK-7 trials.
Fracture risk reduction with demonstrated effect applies specifically to MK-4 at pharmacological doses (45mg/day) in Japanese osteoporotic women. This is a licensed pharmaceutical application in Japan. Whether MK-7 at nutritional doses produces equivalent fracture risk reduction has not been established.
Cardiovascular calcification biomarkers show a consistent directional signal from small intervention trials, including reductions in dp-ucMGP and modest improvements in arterial stiffness. This is biomarker-level evidence only, dp-ucMGP clinical threshold values have not been established, and there is no evidence from randomised controlled trials that reducing dp-ucMGP translates into reduced cardiovascular events. Observational associations between dietary K2 intake and coronary heart disease risk cannot establish causation.
Vitamin D3 co-administration has a mechanistically plausible rationale but clinical benefit has not been established in adequately powered trials. This is a secondary hypothesis rather than a primary evidence-based application.
What the evidence does not show
Fracture risk reduction with MK-7 at nutritional doses has not been demonstrated. MK-4 pharmacological dose evidence should not be used to infer this.
Cardiovascular event reduction has not been demonstrated in randomised controlled trials. Biomarker changes and observational associations do not establish clinical outcome benefit.
General supplementation in healthy adults with no specific bone or cardiovascular risk factors and adequate dietary intake is not supported by the evidence.
Form and dose considerations
MK-7 is the appropriate form for supplementation at nutritional doses, with a typical evidence-based range of 90-180mcg/day. Vitamin K2 is fat-soluble and should be taken with a meal containing fat. MK-4 at 45mg/day is a pharmaceutical dose with a distinct Japanese evidence base, it is not interchangeable with MK-7 at nutritional doses and the evidence does not transfer between them.
Who the evidence applies to
The evidence is strongest for postmenopausal women with bone density concerns, older adults, and those with low dietary intake of fermented foods and animal products. Individuals on warfarin or other vitamin K antagonist anticoagulants must not supplement without clinical supervision.
Safety and contraindications
The primary safety concern is the interaction with vitamin K antagonist anticoagulants. Any individual on warfarin must discuss K2 supplementation with their prescriber before starting. MK-7's long half-life means any effect on anticoagulant control will be sustained. A theoretical concern about hypercoagulability at very high K2 doses has been raised but has not been demonstrated clinically at standard supplemental doses.
There is no established safety concern with dietary-level K2 intake in pregnancy. High-dose supplementation has not been specifically studied in pregnant women and caution is appropriate. No upper tolerable intake limit has been established and no toxicity has been demonstrated at supplemental doses.
What can reasonably be concluded
Vitamin K2 as MK-7 has a meaningful evidence base for attenuation of age-related bone loss in postmenopausal women, with modest effect sizes and a limited independent replication base. This is the most robustly supported clinical application at nutritional doses.
The cardiovascular evidence is at the level of biomarkers and observational associations only. It does not support event-level claims.
Form specificity is central to interpreting this evidence base. MK-4 pharmacological dose fracture data does not apply to MK-7 nutritional supplementation.
Where evidence is limited or outcomes are uncertain, conclusions should be treated as provisional and subject to revision as the evidence base develops.