Natto

Japan's fermented soybean superfood — dense in Vitamin K2, nattokinase, and probiotics with evidence for bone, heart, and vascular health

Natto is a traditional Japanese food made by fermenting soybeans with the bacterium Bacillus subtilis var. natto. It has a sticky, stringy texture and a pungent, earthy flavor that takes some getting used to — but it is one of the most nutritionally concentrated foods in the world. It is the richest dietary source of Vitamin K2 (as MK-7), contains a unique enzyme called nattokinase, and delivers billions of beneficial bacteria per serving. Research links regular natto consumption to stronger bones [4][5], healthier blood pressure [1][2], and reduced cardiovascular risk [3].

What Makes Natto Different From Other Fermented Foods

Most fermented foods — yogurt, kefir, sauerkraut — deliver value primarily through their probiotic content and preservation of nutrients. Natto does all that, but it also produces two compounds during fermentation that are found almost nowhere else in the food supply: nattokinase and Vitamin K2 as MK-7.

Nattokinase is a serine protease enzyme secreted by B. subtilis during fermentation. Unlike most food-derived enzymes that are destroyed in digestion, nattokinase survives stomach acid in meaningful quantities and is absorbed into circulation. Once there, it acts directly on fibrin — the protein scaffold of blood clots — dissolving existing clots and inhibiting new ones from forming. It also inactivates plasminogen activator inhibitor-1 (PAI-1), a compound that normally puts the brakes on your body's own clot-dissolving system [3].

Vitamin K2 as MK-7 is found in fermented foods and some animal products, but natto is far and away the richest source: a 100g serving contains roughly 1,000 micrograms of MK-7 — around 100 times more than most cheeses [7]. MK-7 has a long half-life (72 hours vs. a few hours for K1) and excellent bioavailability, meaning a small daily serving maintains elevated K2 levels continuously. K2's critical role is directing calcium away from arteries and soft tissues and into bones and teeth via the proteins osteocalcin and Matrix Gla Protein (MGP).

Bone Health

Studies conducted in Japan — where natto is commonly eaten — show a consistent association between regular natto consumption and reduced bone loss. In the JPOS cohort study of 944 women followed for three years, higher natto intake was associated with significantly less bone mineral density loss at the femoral neck and radius in postmenopausal women. The effect was not seen with other soy products like tofu, isolating natto's unique fermentation-derived compounds as the active factor [4]. A parallel study in 1,662 elderly men found that natto consumers had higher BMD at the total hip and femoral neck — and when researchers adjusted for undercarboxylated osteocalcin (a direct measure of K2 insufficiency), the benefit disappeared, confirming MK-7 as the mediating mechanism [5].

A three-year RCT supplementing 244 postmenopausal women with 180 mcg of MK-7 daily — the amount found in roughly one to two tablespoons of natto — found significantly reduced age-related decline in bone mineral content and density at the lumbar spine and femoral neck, and less vertebral height loss compared to placebo [6].

Cardiovascular Health

The evidence for nattokinase on blood pressure is more recent but promising. An 8-week double-blind RCT in 86 people with prehypertension or stage 1 hypertension found that nattokinase supplementation reduced systolic blood pressure by 5.55 mmHg and diastolic by 2.84 mmHg. Renin activity — a marker of how hard the kidneys are signaling to raise blood pressure — also decreased, suggesting a mechanism through the renin-angiotensin system [1].

A second multicenter North American RCT in 79 hypertensive subjects found that 100mg of nattokinase daily for 8 weeks reduced diastolic blood pressure and von Willebrand factor, a glycoprotein involved in blood clot formation and a recognized marker of cardiovascular risk [2].

Practical Tips

A typical serving is 40–50g (one small package). Even 2–3 servings per week delivers meaningful MK-7 benefits.
Eat it over rice, with a raw egg, or as a topping on avocado toast. The traditional way includes mustard and soy sauce.
Start small if you are new to the taste and texture — it is an acquired flavor.
Warfarin/Coumadin interaction: Natto's high K2 content can interfere with warfarin therapy. If you take blood thinners, consult your doctor before adding natto.
Nattokinase supplements are available for those who cannot tolerate the food, typically standardized to 2,000 FU (fibrinolytic units) per capsule.
Natto is not the same as edamame, miso, or tempeh — all are soy-based but natto's specific fermentation process is what creates nattokinase and high MK-7.

See our Fermented Foods overview for context on how fermentation transforms food nutrition. For more on Vitamin K2 and calcium routing, see Vitamin K2.

Evidence Review

Nattokinase and Cardiovascular Endpoints

The primary clinical trial base for nattokinase comes from small-to-medium RCTs focused on hypertension and thrombosis markers.

Kim et al. (2008) conducted an 8-week double-blind RCT in 86 participants with prehypertension or stage 1 hypertension randomized to nattokinase (2,000 FU/day) or placebo. The nattokinase group showed significant reductions: systolic BP −5.55 mmHg (p=0.04), diastolic BP −2.84 mmHg (p=0.02), and plasma renin activity −1.17 ng/mL/h (p=0.03) [1]. The renin finding suggests that nattokinase may act partly through the renin-angiotensin-aldosterone system, not only through direct fibrinolytic activity.

Jensen et al. (2016) replicated and extended these findings in a multicenter North American RCT with 79 hypertensive subjects receiving 100mg nattokinase daily for 8 weeks. Diastolic BP fell from 87 to 84 mmHg in the treatment group (statistically significant vs. placebo). In female participants specifically, von Willebrand factor — a recognized independent predictor of cardiovascular events — was significantly reduced, pointing to an antithrombotic effect beyond blood pressure alone [2].

A mechanistic review by Weng et al. (2017) compiled evidence for nattokinase's antithrombotic mechanisms: direct fibrin degradation (nattokinase cleaves fibrin chains at multiple sites), degradation of PAI-1 (which normally inhibits endogenous tissue plasminogen activator), and conversion of prourokinase to active urokinase. The review notes that nattokinase was in Phase II clinical trials in the US for atherothrombotic prevention as of 2017 [3]. A 2024 systematic review and meta-analysis of RCTs (PMID 39076715) pooled data finding nattokinase reduces systolic BP by −3.45 mmHg and diastolic BP by −2.32 mmHg across multiple trials.

Limitations: Most cardiovascular trials are short-duration (8–12 weeks) and small (n<100). Long-term endpoint data (myocardial infarction, stroke) are lacking. Most nattokinase trials use isolated supplements, not the whole food — it is unclear whether digestion alters activity compared to supplement formulations.

Natto and Bone Mineral Density

The most compelling bone evidence comes from two large Japanese epidemiological studies measuring BMD.

Ikeda et al. (2006) followed 944 women from the Japanese Population-Based Osteoporosis Study for 3 years, assessing habitual natto consumption via food frequency questionnaire and measuring BMD at the femoral neck, distal radius, and lumbar spine. Higher natto intake was significantly associated with less femoral neck and radius bone loss in postmenopausal — but not premenopausal — women. Crucially, this association was not present for tofu or isoflavone intake alone, isolating the fermentation-derived products (nattokinase and MK-7) as the relevant factors [4].

Fujita et al. (2012) examined 1,662 elderly men in the FORMEN cohort, finding that habitual natto consumers had significantly higher total hip BMD (p<0.05) and femoral neck BMD (p<0.05). After adjustment for undercarboxylated osteocalcin — a serum biomarker that rises when K2 is insufficient — the BMD association became non-significant, providing strong mediation evidence that MK-7 is the operative mechanism [5].

The MK-7 RCT by Knapen et al. (2013) provides the highest-level evidence: 244 postmenopausal women randomized to 180 mcg MK-7 daily or placebo for 3 years. The MK-7 group showed significantly less decline in bone mineral content at L1–L4 (p=0.014) and BMD at L1–L4 (p=0.004), along with significantly reduced vertebral height loss at thoracic vertebrae T3 and T10. The dose used (180 mcg MK-7) is readily achievable from two to three weekly servings of natto [6].

Nutritional Composition and Broader Properties

A 2022 critical review in Biochemistry Research International catalogued natto's full nutritional and bioactive profile: nattokinase (≥2,000 FU/g), MK-7 (~800–1,100 mcg/100g), isoflavones (genistein, daidzein), gamma-polyglutamic acid (a prebiotic fiber), B. subtilis probiotics, and high protein content (~18g/100g) [7]. The review notes antibacterial activity of natto extracts against H. pylori, Staphylococcus aureus, and antibiotic-resistant strains, though these effects are based primarily on in vitro work.

Warfarin interaction: The high MK-7 content in natto is clinically significant for anticoagulant users. Even a single serving of natto can substantially elevate INR variability in warfarin patients. This is a well-documented drug-food interaction that warrants medical guidance.

Overall evidence quality: The bone evidence is strong — consistent across population studies and supported by a mechanistic RCT using the same MK-7 compound. The cardiovascular evidence is promising but limited by short trial durations and small sample sizes. Gut microbiome effects are plausible given B. subtilis content but require direct clinical investigation.

References

Effects of nattokinase on blood pressure: a randomized, controlled trialKim JY, Gum SN, Paik JK. Hypertension Research, 2008. PubMed 18971533 →
Consumption of nattokinase is associated with reduced blood pressure and von Willebrand factor, a cardiovascular risk marker: results from a randomized, double-blind, placebo-controlled, multicenter North American clinical trialJensen GS, Lenninger M, Ero MP, Benson KF. Integrative Blood Pressure Control, 2016. PubMed 27785095 →
Nattokinase: An Oral Antithrombotic Agent for the Prevention of Cardiovascular DiseaseWeng Y, Yao J, Sparks S, Wang KY. International Journal of Molecular Sciences, 2017. PubMed 28264497 →
Intake of fermented soybeans, natto, is associated with reduced bone loss in postmenopausal women: Japanese Population-Based Osteoporosis (JPOS) StudyIkeda Y, Iki M, Morita A. Journal of Nutrition, 2006. PubMed 16614424 →
Association between vitamin K intake from fermented soybeans, natto, and bone mineral density in elderly Japanese men: the Fujiwara-kyo Osteoporosis Risk in Men (FORMEN) studyFujita Y, Iki M, Tamaki J. Osteoporosis International, 2012. PubMed 21394493 →
Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal womenKnapen MH, Drummen NE, Smit E, Vermeer C, Theuwissen E. Osteoporosis International, 2013. PubMed 23525894 →
Nutritional Health Perspective of Natto: A Critical ReviewAbdelaziz S, Abdel-Hamid M, El-Shafei K. Biochemistry Research International, 2022. PubMed 36312453 →