Dr. Kumar’s Take:
This cross-sectional study from Japan asked whether habitual intake of fermented soybeans, natto, tracks with better bone health in older men. Men who reported eating more natto had higher bone mineral density at the total hip and femoral neck, and a lower risk of low BMD. They also had lower serum undercarboxylated osteocalcin (ucOC), a marker of low vitamin K status. Once ucOC was accounted for, the natto association no longer held, which points to vitamin K as the mechanism behind it. I read this as supportive of vitamin K rich foods for bone health in aging men, with the caveat that this is observational: it shows an association at a single point in time, not a proven effect of eating natto.
Key Takeaways:
✔ Men who ate more natto had higher bone mineral density at the total hip and femoral neck.
✔ Greater natto intake was associated with significantly lower serum ucOC, indicating better vitamin K status.
✔ The association disappeared after adjustment for ucOC, so vitamin K appears to be the primary mediator.
Actionable Tip:
If you want to support bone health, consider making a vitamin K rich fermented food like natto a regular part of your diet. This study cannot tell you how much a supplement would do, since it measured self-reported food intake rather than supplementation.
Brief Summary:
The FORMEN study analysed 1,662 community dwelling Japanese men aged 65 and older, all of them free of diseases and medications known to affect bone metabolism. Men with greater self-reported natto intake had significantly higher BMD and a lower risk of low BMD (T-score below -1 SD) at the total hip and femoral neck. Those same men had significantly lower serum ucOC. After further adjustment for ucOC, the link between natto and BMD became insignificant, which the authors interpret as evidence that the benefit is primarily due to the vitamin K content of natto.
Study Design:
This was a cross-sectional analysis within the Fujiwara-kyo Osteoporosis Risk in Men (FORMEN) study, drawn from the baseline survey. It included 1,662 men aged 65 years and older who had no diseases or medications known to affect bone metabolism. Natto intake was self-reported. Serum ucOC was measured as a biomarker of vitamin K intake, and BMD was measured at the lumbar spine and hip. Analyses were adjusted for confounding variables, then further adjusted for ucOC level to test whether vitamin K explained the association. The authors note that they had no data on dietary nutrient intake, including vitamin K1 and K2 themselves, which limited how far they could take the analysis.
Results:
- Greater natto intake was associated with significantly higher BMD at the total hip and femoral neck after adjustment for confounding variables.
- Greater natto intake was also associated with a lower risk of low BMD, defined as a T-score below -1 SD, at those same sites.
- Men who ate more natto had significantly lower serum ucOC levels.
- After further adjustment for ucOC, the association between natto and BMD became insignificant, indicating vitamin K as the primary mediator.
How Natto (Vitamin K2) Helps Bones:
Undercarboxylated osteocalcin is the form of osteocalcin left behind when vitamin K is in short supply, and it serves as a biomarker of vitamin K intake. In this study, men eating more natto had lower ucOC and higher hip BMD, and the bone association tracked with ucOC rather than with natto itself. Natto is a concentrated dietary source of vitamin K: one pack contains 20 µg of vitamin K1 and 380 µg of vitamin K2.
Related Studies and Research
Population-based study linking habitual natto consumption to higher bone density in elderly Japanese men. Observational data on fermented soy and bone health.
Comprehensive meta-analysis of randomized trials on MK-7 supplementation and osteoporosis prevention in postmenopausal women. Summarizes K2’s efficacy across clinical trials.
Cross-sectional analysis of vitamin K intake, undercarboxylated osteocalcin, and bone mineral density. Examines the vitamin K biomarker behind the BMD association.
Examines the conversion efficiency of various dietary vitamin K forms into bioactive MK-4 in human tissues. Tracks how different Ks become MK-4.
Investigates vitamin K2’s role in cartilage protection and GPX4 activation in osteoarthritis models. Connects K2 to joint health via antioxidant pathways.
Frequently Asked Questions
What is ucOC, and why does it matter?
ucOC stands for undercarboxylated osteocalcin. It is a blood marker used here as a biomarker of vitamin K intake. In this study, men with greater natto intake had significantly lower ucOC, and adjusting for ucOC removed the association between natto and bone density.
Can I get the same benefits from vitamin K2 supplements?
This study looked at self-reported intake of natto as a food, not at supplements, so it cannot answer that question. What it does show is that the bone association tracked with vitamin K status rather than with natto itself.
Do these results apply to women?
This analysis included only men aged 65 and older, so the findings apply to elderly men. The study does not provide evidence about women.
How much natto contains how much vitamin K?
The researchers describe natto as containing 20 µg of vitamin K1 and 380 µg of vitamin K2 per pack. Intake in this study was self-reported, and men with greater intake had higher hip and femoral neck BMD.
Conclusion
In this cross-sectional analysis of 1,662 elderly Japanese men, habitual natto intake was associated with better bone health, and that association was primarily explained by the vitamin K content of natto. Because the design is observational and intake was self-reported, it establishes an association rather than cause and effect. Even so, keeping vitamin K in the diet is a reasonable part of thinking about bone health as you age.
