Natto is a fermented soybean food with documented effects on blood clotting and bone health

Natto is soybeans fermented with Bacillus subtilis, a bacterium that breaks down the soybean proteins and creates a sticky, stringy texture. The fermentation produces a unique enzyme called nattokinase, which thins blood by breaking down fibrin — a protein your body uses to form clots. Human trials show nattokinase can reduce blood clotting markers within hours of eating natto, an effect that persists with regular consumption.

Beyond nattokinase, natto fermentation also produces vitamin K2, specifically the form called menaquinone-7 (MK-7). A 100-gram serving of natto contains roughly 200 micrograms of K2 — far more than most foods. Vitamin K2 activates proteins in bone and blood vessel walls that regulate calcium. Research in postmenopausal women and older adults shows K2 intake correlates with higher bone density and lower fracture risk, though the evidence is stronger for observational studies than for controlled trials.

Natto also retains the full nutritional profile of soybeans: protein (about 17 grams per 100 grams), iron, magnesium, and isoflavones — plant compounds with weak estrogen-like activity. The fermentation makes these nutrients more bioavailable than in unfermented soy, meaning your body absorbs more of them.

Key Takeaways

  • Nattokinase, an enzyme produced during fermentation, thins blood by breaking down fibrin, with effects measurable in human studies within hours of consumption.
  • Natto is one of the richest dietary sources of vitamin K2 (menaquinone-7), which research links to bone density and cardiovascular health in observational studies.
  • The fermentation process makes soybean nutrients more absorbable than in unfermented soy, including protein, iron, and isoflavones.
  • People taking blood thinners like warfarin should not eat natto regularly without medical guidance, because nattokinase and K2 both affect clotting.

How nattokinase affects blood clotting

Nattokinase works by degrading fibrin, a mesh-like protein that stabilizes blood clots. When you cut yourself, fibrin forms part of the plug that stops bleeding. But fibrin also accumulates in blood vessels over time, contributing to clots that can block blood flow. Nattokinase dissolves excess fibrin without affecting the initial clotting response, which is why it thins blood without causing bleeding.

Human studies show measurable effects. A 2009 trial in the Blood Coagulation & Fibrinolysis journal found that a single 2,000-unit dose of nattokinase reduced fibrin levels within 2 hours, with effects lasting up to 8 hours. A 2011 study in Nutrition Journal showed that eating 100 grams of natto daily for 7 days reduced clotting markers in healthy adults. However, most studies are small (20 to 30 participants) and conducted in Japan, where natto consumption is common.

The practical significance remains unclear. Nattokinase does not prevent heart attacks or strokes in the way prescription anticoagulants do. No large randomized trial has tested whether natto reduces cardiovascular events in people at risk. The enzyme's effects are real but modest — useful context if you eat natto regularly, but not a reason to eat it specifically for clotting prevention without other medical support.

Vitamin K2 and bone density

Natto's vitamin K2 content is exceptional. A 100-gram serving (about 3.5 ounces, or roughly a small bowl) contains 200 to 300 micrograms of menaquinone-7, compared to trace amounts in most other foods. Vitamin K2 activates osteocalcin, a protein that anchors calcium into bone mineral. It also activates matrix Gla protein, which prevents calcium from depositing in soft tissues like arteries.

Observational studies link higher K2 intake to higher bone mineral density and lower fracture rates in postmenopausal women and older adults. A 2013 meta-analysis in Osteoporosis International found that people in the highest quartile of K2 intake had significantly lower hip fracture risk than those in the lowest quartile. A 2017 study in Nutrients showed that Japanese women who ate natto regularly had higher bone density than those who did not.

Controlled trials are fewer and smaller. A 2006 randomized trial in Bone found that postmenopausal women taking a K2 supplement (180 micrograms daily) for 3 years had less bone loss in the spine than controls, but the effect was modest. No large trial has tested natto itself against placebo for bone outcomes. The evidence supports K2 as part of bone health, but natto is one source among many — leafy greens, dairy, and fermented foods all contribute K2, and bone health depends on exercise, calcium intake, and overall nutrition.

Digestibility and nutrient absorption

Fermentation breaks down the complex carbohydrates and proteins in soybeans, making them easier to digest and absorb. Raw soybeans contain compounds called protease inhibitors that block protein digestion; fermentation inactivates these. The result is that your body absorbs more amino acids, minerals, and isoflavones from natto than from unfermented soy.

Natto also contains live bacteria and enzymes that may support gut health, though the evidence is preliminary. The Bacillus subtilis strain used in natto fermentation is a spore-former, meaning it can survive stomach acid and reach the colon. Some research suggests spore-forming bacteria may improve intestinal barrier function and reduce inflammation, but human studies are limited. Eating natto regularly does introduce beneficial microbes, but it is not a substitute for a diverse diet of fiber-rich foods that feed your existing gut bacteria.

Who should avoid or limit natto

People taking warfarin (Coumadin) or other vitamin K-dependent anticoagulants should not eat natto regularly. Vitamin K2 reduces warfarin's effectiveness by competing for the same metabolic pathway. A sudden increase in K2 intake can lower your INR (International Normalized Ratio), the measure doctors use to monitor clotting. If you take warfarin and want to eat natto, discuss it with your doctor first — consistency matters more than avoidance, but medical guidance is essential.

People with soy allergies or sensitivities should avoid natto, as fermentation does not eliminate soy proteins. Those with histamine intolerance may react to natto, since fermentation increases histamine content. Natto is also high in purines, compounds that break down into uric acid; people with gout or a history of kidney stones may need to limit intake.

Natto's sticky texture and strong smell are barriers for many people. If you dislike it, the health benefits do not justify forcing consumption — other fermented foods, leafy greens, and whole grains provide similar nutrients without the sensory challenge.

How natto compares to other soy foods

Tofu, tempeh, and edamame are all soy foods, but natto is unique in its K2 and nattokinase content. Tofu is pressed curds with minimal fermentation, so it lacks nattokinase and K2. Tempeh is fermented whole soybeans, so it has some K2 and enzyme activity, but far less than natto — a 100-gram serving of tempeh contains roughly 5 micrograms of K2 compared to natto's 200+. Edamame is immature soybeans, boiled and salted, with no fermentation and minimal K2.

If your goal is K2 intake, natto is one of the best plant sources, rivaled only by other fermented soy products like miso and soy sauce — though those are consumed in smaller quantities. If your goal is protein and overall soy nutrition, all four foods are comparable. If your goal is nattokinase specifically, natto is the only common food source; tempeh has some enzyme activity but much less.

For people who dislike natto, other fermented foods like sauerkraut, kimchi, and kefir provide probiotics and enzyme activity, though not K2 at natto's levels. Leafy greens like kale and spinach provide K2 in the form of K1, which your body converts to K2, though less efficiently than consuming K2 directly.

Practical ways to eat natto

Natto is sold frozen or refrigerated in Japanese markets and increasingly in mainstream grocery stores. A typical serving is one small package (about 40 to 50 grams), which contains roughly 80 to 100 micrograms of K2 and active nattokinase. The traditional way to eat it is over rice with soy sauce and a raw egg yolk, which adds fat that may improve K2 absorption. Some people mix it into miso soup, pasta, or sushi rolls.

Heating natto above 70°C (158°F) destroys nattokinase, so if you want the enzyme's effects, eat it raw or add it to dishes after cooking. Vitamin K2 is heat-stable, so cooked natto retains that benefit. If you are new to natto, start with a small amount — the flavor and texture are strong, and some people experience mild digestive adjustment.

Consistency matters more than quantity. A single serving of natto will not change your health; regular consumption over weeks and months is what the research measures. If you enjoy it, eating natto two to three times per week is a reasonable target. If you do not enjoy it, the same nutrients are available from other sources.

What remains uncertain about natto

Most natto research is conducted in Japan and published in Japanese journals, which limits the diversity of study populations. Long-term trials testing natto against placebo for bone fractures, cardiovascular events, or clotting disorders do not exist. We know nattokinase works in the lab and in short human studies, but we do not know whether eating natto regularly prevents disease in the way that, say, statins prevent heart attacks.

The optimal dose is also unclear. Studies use anywhere from 50 to 200 grams of natto daily, and it is not known whether smaller amounts provide proportional benefits or whether there is a threshold below which effects disappear. Individual variation in how people absorb and respond to nattokinase and K2 is likely but unmeasured.

Finally, natto's effects on people taking other medications — beyond warfarin — are not well studied. If you take blood thinners, antiplatelet drugs, or medications that interact with vitamin K, ask your doctor before eating natto regularly.

Frequently Asked Questions

Does natto prevent heart attacks or strokes?

Nattokinase thins blood in measurable ways, but no large trial has shown that eating natto prevents heart attacks or strokes. It may be part of a heart-healthy diet that includes exercise, whole grains, and vegetables, but it is not a substitute for medical treatment or prevention prescribed by a doctor.

Can I get the same benefits from a nattokinase supplement instead of eating natto?

Nattokinase supplements exist, but they lack the vitamin K2 and other nutrients in whole natto. Supplements are also less regulated than foods, so potency varies. If you dislike natto's taste, discuss supplements with your doctor, but whole natto provides a more complete nutritional package.

Is natto safe to eat every day?

For most people without bleeding disorders or anticoagulant medications, daily natto is safe. However, consistency is more important than daily consumption — eating it three times per week provides measurable effects. If you take warfarin or other K-dependent drugs, ask your doctor before eating natto regularly.

Does cooking natto destroy its health benefits?

Heating natto above 70°C destroys nattokinase, so cooked natto loses that enzyme's blood-thinning effect. Vitamin K2 survives cooking, so cooked natto retains bone-health benefits. For maximum nattokinase activity, eat natto raw or add it to dishes after cooking.

What does natto taste like, and why is it so slimy?

Natto has a strong, pungent fermented smell and a sticky, stringy texture from the bacterial fermentation. The taste is umami-rich and salty. The sliminess comes from polysaccharides produced by the bacteria — the same compounds that give it enzyme activity. If you dislike the texture, mixing it thoroughly into rice or soup can make it more palatable.