Vitamin K2

Vitamin K exists in two families. K1, phylloquinone, comes from green leafy vegetables and is primarily used by the liver for blood clotting. K2, the menaquinones, comes from fermented foods and animal products and is used more widely in other tissues.

Two K2 forms dominate supplements. MK-7, derived from natto, has a long half-life of several days, giving stable blood levels from once-daily dosing. MK-4 has a half-life of only hours, requiring divided dosing, and is used at much higher amounts in research.

For MK-7, the trans isomer is the active one, and cis content is a recognised quality issue in cheaper material. A product declaring trans content is preferable. K2 is commonly paired with vitamin D3, which has a coherent rationale rather than being a marketing convenience.

Vitamin K acts as a cofactor for gamma-glutamyl carboxylase, the enzyme that carboxylates specific proteins, activating them. Without adequate vitamin K these proteins remain in an inactive, uncarboxylated state.

Two of these matter here. Osteocalcin, produced by bone-building cells, binds calcium into the bone matrix once carboxylated. Matrix Gla protein, found in vascular tissue, inhibits calcium deposition in arterial walls once carboxylated.

Together these underpin the calcium-directing rationale: vitamin D increases calcium absorption, and vitamin K influences where that calcium ends up. This is why the D3 and K2 pairing makes physiological sense, though the clinical significance in people with adequate status is less firmly established than the mechanism suggests.

Bone research is the more developed area. Japanese studies using high-dose MK-4 have reported reductions in fracture risk, though these findings have not consistently replicated elsewhere. Trials using MK-7 have reported improvements in osteocalcin carboxylation and modest effects on bone mineral density.

Vascular research is interesting but earlier. Observational work has associated higher K2 intake with lower arterial calcification, and some trials have reported improvements in arterial stiffness measures over several years. Outcome data remains limited.

The mechanism is well characterised and the biomarker effects are reproducible. What is less clear is how much this matters for someone with a reasonable diet and no deficiency. It is a sound addition to a D3 supplement rather than a transformative intervention on its own.

Vitamin K2 has a good safety record. No tolerable upper intake level has been set, and adverse effects at supplemental doses are uncommon.

One thing worth checking before you start: if you take anything that affects blood clotting, speak to your prescriber first.