Which form of vitamin K2 is better: MK-4 or MK-7?

MK-7 is demonstrably absorbed into the blood; MK-4 is not, at least not at the amounts you get from food or ordinary supplements. After a dose of 420 µg, MK-4 could not be detected in the blood at any measurement point. MK-7 reached a measurable peak at the same dose within six hours and was still present 48 hours later. After seven days of 60 µg daily, blood levels rose in all participants.

That difference in absorption has consequences for how well your body can activate certain proteins outside the liver, including proteins involved in bone formation and the protection of blood vessel walls. Only MK-7 succeeds, at a normal dietary dose, in activating those proteins sufficiently. MK-4 and vitamin K1 fall short at the RDA level.

For bone specifically, MK-7 shows a positive effect on bone mineral density and strength across multiple studies. Part of this is because MK-7 is partly converted in the body to MK-4, so both forms ultimately play a role. For postmenopausal women, in whom bone problems are most relevant, the picture is not clear-cut: randomised studies showed no clear increase in bone mineral density at the most commonly measured skeletal sites. A possibly lower fracture risk was suggested, but those studies have methodological limitations. Vitamin K supplementation is therefore not (yet) routinely recommended for the prevention of osteoporosis.

A small study in rheumatoid arthritis patients (42 per group) found that 100 µg of MK-7 per day as a supplement to standard medication reduced disease activity. This is interesting, but it is too early to draw firm conclusions from it.