Dr. Kumar’s Take:
This is a systematic review and meta-analysis of six observational and quasi-experimental studies in Japanese populations, not a randomized trial and not a study limited to postmenopausal women. It pooled 2,327 participants, comparing habitual consumers of natto, a traditional Japanese fermented soybean food, with low or non-consumers. Natto is the richest natural source of menaquinone-7 (MK-7), a form of vitamin K2 involved in activating osteocalcin and in bone mineralization. Habitual eaters had markedly higher serum MK-7, more carboxylated osteocalcin, less undercarboxylated osteocalcin, and modestly greater bone mineral density.
I read this as supportive rather than conclusive. The designs are observational, so I can say natto intake tracks with better vitamin K status and better bone markers, not that it causes them. If you already like natto, this is a reasonable food to keep in the rotation.
Key Takeaways:
✔ Habitual natto eaters had substantially higher serum MK-7 (d = 2.10, 95% CI 1.55 to 2.66 after removing an outlier).
✔ Osteocalcin carboxylation was better: osteocalcin up (d = 0.26, 95% CI 0.08 to 0.43) and undercarboxylated osteocalcin down (d = -0.50, 95% CI -0.74 to -0.26).
✔ Bone mineral density was modestly greater across sites (d = 0.65, 95% CI 0.09 to 1.21; d = 0.35, 95% CI 0.21 to 0.48 on sensitivity analysis).
✔ Certainty of evidence was moderate for serum MK-7 and low or very low for the osteocalcin and BMD outcomes.
Actionable Tip:
If you want a food-based way to raise vitamin K2 status, natto is the densest natural source of MK-7 there is. Eat it habitually rather than occasionally, since the comparison in this analysis was habitual consumers versus low or non-consumers.
Brief Summary:
The authors searched PubMed, Web of Science, and Scopus through January 2025 for studies comparing habitual natto consumers with low or non-consumers, requiring outcomes for serum MK-7, carboxylated or undercarboxylated osteocalcin, or bone mineral density. Six studies totaling 2,327 participants qualified. They pooled results with random-effects models using Cohen’s d and graded certainty with the GRADE framework.
Natto intake was associated with significantly elevated serum MK-7, increased osteocalcin, decreased undercarboxylated osteocalcin, and modestly greater BMD across skeletal sites. Publication bias appeared minimal. The authors concluded that habitual natto consumption is associated with improved vitamin K status and bone metabolism markers, but that the observational nature of the evidence and its moderate to low certainty call for caution, and that generalizability beyond Japanese populations needs further study.
Study Design:
- Type: Systematic review and meta-analysis of six observational and quasi-experimental studies
- Participants: 2,327 people in Japanese populations
- Comparison: Habitual natto consumers versus low or non-consumers
- Databases: PubMed, Web of Science, Scopus, searched through January 2025
- Outcomes: Serum MK-7, carboxylated and undercarboxylated osteocalcin, bone mineral density
- Analysis: Random-effects models, Cohen’s d with 95% confidence intervals, GRADE certainty rating
Results:
- Serum MK-7: d = 2.10, 95% CI 1.55 to 2.66 after sensitivity analysis removing an outlier.
- Osteocalcin: d = 0.26, 95% CI 0.08 to 0.43.
- Undercarboxylated osteocalcin: d = -0.50, 95% CI -0.74 to -0.26.
- Bone mineral density across sites: d = 0.65, 95% CI 0.09 to 1.21, and d = 0.35, 95% CI 0.21 to 0.48 on sensitivity analysis.
- Certainty ranged from moderate for serum MK-7 to low or very low for the osteocalcin and BMD outcomes, reflecting the observational designs and remaining imprecision. Publication bias appeared minimal.
How Natto May Support Bone Health
Natto is the richest natural source of menaquinone-7, a form of vitamin K2 involved in osteocalcin activation and bone mineralization. Osteocalcin has to be carboxylated to do its job in bone, and the pooled data fit that mechanism: habitual eaters had higher serum MK-7, more carboxylated osteocalcin, and less of the undercarboxylated form that signals inadequate vitamin K status.
Related Studies and Research
- Is Vitamin K2 a Blood Thinner? No, It Does the Opposite Explores natto intake’s association with reduced bone loss rates in postmenopausal women. Links fermented soy consumption with slower bone resorption.
Meta-analysis of Japanese observational and quasi-experimental studies on habitual natto intake, vitamin K status, and bone density. Pools association data on serum MK-7, osteocalcin carboxylation, and BMD.
Three-year RCT evaluating low-dose MK-7’s impact on bone loss in healthy postmenopausal women. Details long-term MK-7 effects on bone density.
Compares bioavailability and serum retention of MK-7 versus MK-4 in healthy women. Contrasts pharmacokinetics of two K2 forms.
Investigates combined vitamin D3 and K2 supplementation effects on bone mineral density over 18 months. Reviews D+K synergy in BMD changes.
Frequently Asked Questions
What is natto?
Natto is a traditional Japanese fermented soybean food. It is the richest natural source of menaquinone-7, the MK-7 form of vitamin K2.
Does this prove natto builds bone?
No. The six pooled studies were observational and quasi-experimental, so they show association, not cause. The authors rated certainty as moderate for serum MK-7 and low or very low for the osteocalcin and bone density outcomes.
Does this apply outside Japan?
The analysis was limited to Japanese evidence, and the authors state that generalizability beyond Japanese populations warrants further investigation.
Is natto safe to eat daily?
For most people, yes. My one caution is clinical rather than from this analysis: if you take warfarin, talk to your doctor before changing your vitamin K intake.
Conclusion
Pooled across six Japanese observational and quasi-experimental studies, habitual natto consumption tracked with higher serum MK-7, better osteocalcin carboxylation, and modestly greater bone mineral density. The mechanism is plausible and the direction is consistent, but the designs cannot establish cause, and certainty for the bone outcomes was low. I treat natto as a sensible dietary source of vitamin K2 rather than a proven osteoporosis intervention.
