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Food · Grains & Legumes · Legumes
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.