Natto is a traditional Japanese food made from whole soybeans fermented with a specific strain of bacteria. The result is soft, sticky beans with a pungent smell, a savory taste, and an unusually dense nutritional profile. It has been eaten in Japan for thousands of years, where it remains a breakfast staple, and it has gained international attention largely because of one enzyme it produces: nattokinase, a protein with potent blood-clot-dissolving properties.
How Natto Is Made
The process starts with whole soybeans that are soaked, then steamed or boiled until soft. The cooked beans are inoculated with Bacillus subtilis, a bacterium that drives the fermentation. Traditionally, the beans were wrapped in rice straw, which naturally harbored the bacteria. During Japan’s Taisho Period (1912 to 1926), scientists figured out how to cultivate the bacteria in a lab, making straw unnecessary for commercial production.
Once inoculated, the beans ferment at around 39°C (about 102°F) for roughly 48 hours, followed by a 24-hour ripening period. During this time, the bacteria break down the soybean proteins and produce several bioactive compounds. They also generate poly-gamma-glutamic acid, the substance responsible for natto’s signature sticky, stringy texture. When you lift a bite of natto with chopsticks, long threads stretch from the bowl. The fermentation also creates volatile organic compounds that give natto its strong, ammonia-like smell.
Taste, Texture, and How It’s Eaten
Natto is an acquired taste even for many Japanese people. The flavor is earthy, savory, and slightly bitter, with a mustard-like sharpness. The texture sits somewhere between soft cheese and sticky beans. For newcomers, the smell and sliminess can be off-putting, but the taste is milder than the aroma suggests.
The classic way to eat natto is over steamed white rice for breakfast. A typical serving involves stirring a small pack of natto vigorously (this activates more of the sticky strands and is said to improve the flavor), then spooning it over hot rice. Most single-serve packs in Japan come with a small sachet of soy-based sauce and karashi mustard. Common toppings include a raw egg cracked over the top, shredded nori seaweed, sesame seeds, a drizzle of sesame oil, and chopped green onions. Some people mix natto into miso soup, eat it with toast, or fold it into pasta.
Nutritional Profile
Natto is nutrient-dense even by fermented food standards. A 100-gram serving (roughly two of the small packs sold in Japanese grocery stores) provides about 18 grams of protein and 5 grams of fiber, along with iron, manganese, zinc, and copper. Because fermentation partially breaks down the soybeans’ proteins and anti-nutrients, the minerals in natto are more bioavailable than those in unfermented soy foods like tofu.
The standout nutrient is vitamin K2, specifically the form called MK-7. Natto contains approximately 775 micrograms of MK-7 per 100 grams, making it by far the richest dietary source of this vitamin. MK-7 plays a key role in directing calcium into bones and teeth rather than into arteries, which is why it shows up in research on both cardiovascular and bone health.
Nattokinase and Cardiovascular Effects
Nattokinase is an enzyme produced during fermentation that has drawn significant scientific interest. Despite its name, it is not a kinase. It is a serine protease, meaning it breaks down proteins, and its primary target is fibrin, the mesh-like protein that forms the structural backbone of blood clots.
Nattokinase dissolves clots through multiple pathways. It directly breaks apart fibrin strands. It also converts an inactive clot-dissolving agent in the body (prourokinase) into its active form, degrades a protein that normally suppresses clot breakdown (PAI-1), and raises levels of tissue plasminogen activator, another natural clot dissolver. In short, it attacks clots directly while also turning up the body’s own clot-clearing systems.
In a study of 12 healthy young men, a single dose of nattokinase (2,000 fibrinolytic units) produced measurable changes in clotting markers within two hours. Breakdown products of dissolved fibrin appeared in their blood within four to six hours. Animal studies have shown even more dramatic effects: in dogs given nattokinase capsules, experimentally induced blood clots in a major leg vein dissolved completely within five hours. In a rat model, nattokinase restored 62% of arterial blood flow after clot formation, far outperforming other clot-dissolving enzymes tested in the same study.
Bone Health Benefits
Natto’s high vitamin K2 content has prompted research into its effects on bone density, particularly in postmenopausal women who are at elevated risk for osteoporosis. A systematic review pooling data from three prospective studies with 1,658 participants found that regular natto intake was associated with improved bone mineral density at the femoral neck (the narrow section connecting the thigh bone to its ball-shaped head, a common fracture site). Both observational and randomized controlled studies showed statistically significant improvements at this location.
The results for the lumbar spine were more mixed. Observational studies found a significant benefit, but a randomized controlled trial did not reach statistical significance. Neither study type found that natto improved bone density at the hip joint overall. So while natto appears genuinely helpful for bone health in certain areas, it is not a blanket solution for osteoporosis.
Vitamin K2 and Calcium Regulation
Eating natto measurably raises circulating vitamin K2 levels. Research on healthy individuals showed that regular natto intake increased blood concentrations of MK-7 and simultaneously raised levels of gamma-carboxylated osteocalcin, a protein that helps bind calcium to bone tissue. This mechanism helps explain why populations with high natto consumption tend to show stronger bones: vitamin K2 activates the proteins that actually build calcium into the skeleton.
Who Should Be Cautious
Natto’s very strengths create real risks for certain people. Because nattokinase thins the blood and dissolves clots, it can be dangerous for anyone taking anticoagulant medications, antiplatelet drugs, or daily aspirin. Combining nattokinase with these treatments may increase the risk of excessive bleeding, including intracerebral hemorrhage when paired with aspirin.
The vitamin K2 content creates a separate, almost opposite problem for people on warfarin. Warfarin works by blocking vitamin K’s role in the clotting process. Natto is so rich in vitamin K that eating it can reduce warfarin’s effectiveness. Making matters more complicated, the live Bacillus subtilis bacteria in natto continue synthesizing vitamin K inside the intestine after consumption, amplifying the effect beyond what the food’s nutrient label would suggest.
People with a history of deep vein thrombosis should also avoid nattokinase supplements, because dislodging an existing clot could send it to the lungs or brain. Those with coagulation disorders of any kind are advised to steer clear of both nattokinase supplements and large amounts of natto itself.
Finding and Trying Natto
In the United States and Europe, natto is most reliably found in the frozen section of Asian grocery stores, typically sold in packs of three small styrofoam containers (about 40 to 50 grams each). Some health food stores carry it as well. It keeps for months frozen and several days refrigerated once thawed.
If you are trying natto for the first time, the most forgiving approach is the traditional one: stir it vigorously for 30 seconds or so until the threads become thick and foamy, mix in the included sauce, and spoon it over hot rice. The rice mellows the flavor and gives you a neutral base. Adding a raw or soft-boiled egg, soy sauce, sesame oil, and a sprinkle of sesame seeds rounds out the dish. Starting with small amounts and pairing natto with familiar flavors makes the texture and smell much easier to get used to.