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food source · 1
Natto's signature is vitamin K2 (MK-7), made by Bacillus subtilis during fermentation: ordinary natto has about 775 µg per 100 g, and none of it comes from the soybean itself. People taking warfarin should avoid natto.
Mechanism · Made by bacteria, not the bean
The MK-7 in natto is made by Bacillus subtilis during fermentation; the lactic acid bacteria in cheese mainly make MK-8 and MK-9, and the usual yogurt cultures make even less. Vitamin K comes in two groups: the liver uses K1 mainly to activate clotting factors, while K2 reaches bone and blood vessels more and helps activate osteocalcin and matrix Gla protein. Eating natto raised blood MK-7 and activated osteocalcin (Tsukamoto 2000), but that study did not measure outcomes such as fractures.
Safety · Avoid natto if you take warfarin
The ordinary natto that Tsukamoto 2000 measured had about 775 µg of MK-7 per 100 g, so one small pack (about 40–50 g) brings more K2 than all the other foods in a normal diet combined. That is on a different scale from an occasional plate of greens, and it clearly weakens the blood thinner warfarin, so people taking warfarin should avoid natto. Soybeans that have not been fermented do not have this K2.
synergy · 1
Vitamin D helps the gut absorb more calcium; K2 activates osteocalcin and matrix Gla protein, the first of which helps lay calcium into bone while the second helps keep it from settling in artery walls. One brings calcium in, the other steers where it goes. The evidence for K2's half comes mainly from mechanism studies and observational research.
cofactor · 2
→Bone
K2 adds carboxyl groups to the glutamate units of osteocalcin, and only then can osteocalcin grip calcium. Without this step, the osteocalcin that bone-building cells release is like a rope that cannot hold on to calcium.
Vitamin K2 governs where calcium goes: osteocalcin and matrix Gla protein both need gamma-carboxylation to work, the first fixing calcium into bone and the second keeping it out of artery walls. The mechanism is clear, but the evidence from randomized trials with fractures as the endpoint is still early.
regulates · 1
K2 influences where absorbed calcium ends up: it activates osteocalcin, which helps lay calcium into bone, and matrix Gla protein, which helps keep calcium from settling in artery walls. In the Rotterdam Study, people who ate more K2 had a lower risk of coronary heart disease; this is an association.
contrast · 1
Vitamins K1 and K2 share the same naphthoquinone ring and differ in their side chain. K1 comes mainly from leafy greens and is used mostly by the liver to activate clotting factors; K2, such as the MK-7 in natto, reaches tissues outside the liver more and helps activate osteocalcin and other proteins that decide where calcium goes. Their jobs overlap; the emphasis differs.