Vitamin K Intake From Natto and Bone Mineral Density in Elderly Japanese Men (FORMEN Study)

This study examined a benefit of natto unrelated to its clot-dissolving enzyme: its exceptionally high content of vitamin K2 (menaquinone-7), and whether that translates to stronger bones.

In over 1,600 elderly Japanese men, greater natto intake was associated with higher bone mineral density at the hip and femoral neck, and with lower levels of undercarboxylated osteocalcin — a marker of vitamin K insufficiency. Tellingly, the bone benefit disappeared after adjusting for that marker, pointing to vitamin K2 as the active ingredient rather than anything unique to the fermented soybean itself.

The relevance to cardiovascular health is indirect but interesting: vitamin K2 is also implicated in regulating where calcium deposits — bone versus artery — a hypothesis still under investigation. This study, however, speaks to bone, not arteries, and is cross-sectional, so it shows association rather than cause.

We rate the evidence moderate. It is a reasonably sized observational study supporting natto’s vitamin-K2-mediated bone benefit, with no direct bearing on cardiovascular outcomes and the usual limits of cross-sectional dietary data.