Intake of Fermented Soybean (Natto) Increases Circulating Vitamin K2 and Osteocalcin in Elderly Japanese Women

This clinical study enrolled 142 healthy elderly Japanese women and examined the effect of regular natto consumption on circulating vitamin K2 (menaquinone-7, MK-7) levels and markers of vitamin K-dependent protein carboxylation, including osteocalcin.

Regular natto consumption substantially increased circulating MK-7 levels and improved carboxylation of osteocalcin — a vitamin K-dependent protein involved in bone mineralization. Japan’s high natto consumption is associated epidemiologically with lower hip fracture rates compared with other countries with similar calcium intake, and this study provides one biochemical explanation: MK-7 from natto supports bone metabolism through vitamin K-dependent processes.

The cardiovascular relevance of vitamin K2 through natto is speculative but plausible: matrix Gla protein (MGP) is a vitamin K-dependent inhibitor of arterial calcification, and vitamin K deficiency leads to undercarboxylated (inactive) MGP and potentially accelerated vascular calcification. Some observational studies have found associations between vitamin K2 intake and reduced coronary artery calcification, but randomized trial data on cardiovascular outcomes are limited.

This study documents the biochemical pathway (natto → MK-7 → carboxylated proteins) but is not designed to assess cardiovascular outcomes. The study population’s Japanese dietary context limits direct generalizability to Western populations with different baseline vitamin K status.

We rate the evidence limited. A clinical study establishing that natto consumption substantially elevates circulating MK-7 and improves osteocalcin carboxylation — relevant to bone metabolism and the speculative cardiovascular calcification mechanism, but not an outcome study.