The answer is natto, a fermented soybean dish that’s a staple of traditional Japanese breakfasts. It’s made by fermenting whole soybeans with a specific bacterium, producing a sticky, pungent food with a flavor profile that ranges from nutty to intensely funky. Natto is one of those foods people tend to either love or tolerate out of habit, and its strong smell is the first thing newcomers notice.
What Makes Natto Smell So Strong
Natto’s distinctive aroma comes from a class of compounds called pyrazines, produced by the bacteria during fermentation. These are the same family of chemicals responsible for the smell of roasted nuts, baked potatoes, and even chocolate. The bacteria in natto, Bacillus subtilis, generates at least six different types of pyrazines during the fermentation process, creating a layered, complex smell. On top of that, the fermentation produces ammonia, which gives natto its sharper, more pungent edge. The combination is what makes it so polarizing: some people detect pleasant roasted notes, while others are hit primarily by the ammonia.
How Natto Is Made
The process starts with yellow soybeans that are soaked, steamed, and then inoculated with Bacillus subtilis natto. The beans ferment at around 39°C (about 102°F) for roughly 24 to 48 hours, followed by a ripening period. During this time, the bacteria break down proteins in the soybeans and produce a white, mucus-like coating on the surface.
That coating is where natto’s most famous trait comes from: its stickiness. When you stir natto, it pulls apart into long, thin strings. In fact, the quality of natto in Japan is partly judged by how stretchy and viscous those strings are. Good natto should have abundant mucus, long pull-apart threads, and a soft, light yellow interior.
Nutritional Profile
Natto packs a surprising nutritional punch for a small serving. Per 100 grams, it delivers about 211 calories, 19 grams of protein, 5.4 grams of fiber, and significant amounts of minerals: 8.6 mg of iron (close to a full day’s needs for most adults), 729 mg of potassium, 217 mg of calcium, and 201 mg of magnesium (about half the daily requirement). It also provides meaningful amounts of zinc, phosphorus, copper, and selenium.
But the nutrient natto is most famous for is vitamin K2, specifically a form called MK-7. Regular natto contains roughly 775 micrograms of MK-7 per 100 grams. That’s an extraordinarily high concentration. Vitamin K2 plays a key role in directing calcium into bones rather than arteries, and regular natto consumption has been shown to increase circulating K2 levels and improve markers of bone health. This is one reason natto is often cited in discussions about osteoporosis prevention in Japan.
Cardiovascular Effects
Natto produces an enzyme during fermentation that has drawn considerable scientific attention. This enzyme has stronger clot-dissolving activity than the body’s own clot-busting proteins. It works by both directly breaking down the fibrin mesh that forms blood clots and by stimulating blood vessel walls to release their own clot-dissolving agents.
A meta-analysis of randomized controlled trials found that supplementation with this enzyme significantly reduced systolic blood pressure by about 3.5 points and diastolic blood pressure by about 2.3 points compared to placebo. It also appears to lower blood pressure through the same pathway that common blood pressure medications target: blocking the enzyme that tightens blood vessels. These effects position natto as a potentially useful food for cardiovascular health, though lower doses may not meaningfully affect cholesterol levels.
One important caution: natto’s extremely high vitamin K2 content can interfere with blood-thinning medications like warfarin. Vitamin K works in the opposite direction of these drugs, promoting clotting. There’s at least one documented case of a patient substituting a natto-derived supplement for their prescribed blood thinner after heart valve surgery, with dangerous results. If you take anticoagulant medication, natto consumption needs to be discussed with your care team.
How Japanese People Actually Eat It
A typical natto breakfast starts with a small styrofoam or plastic pack containing about 40 to 50 grams of fermented beans, a packet of tsuyu (a sauce made from soy sauce, sake, mirin, and dashi), and a small packet of karashi, a hot yellow mustard. The preparation technique matters: you stir the beans vigorously before adding anything, which develops the stringy threads and changes the texture. Then you add the tsuyu, stir again, add the mustard, and stir once more. The result is a gooey, savory topping that goes directly over hot white rice.
Some people add chopped green onions, a raw egg yolk, or a drizzle of sesame oil. The rice is essential for most eaters because it tempers the intensity and gives the sticky beans something to cling to. Natto is eaten almost exclusively at breakfast in Japan, often alongside miso soup, grilled fish, pickles, and rice as part of a traditional morning spread.
Varieties You’ll Find in Stores
Japanese grocery stores carry natto in several forms. The most common distinction is bean size: small beans (kotsubu) are the most popular and have a more uniform, less intense eating experience, while large beans (ootsubu) have a chewier, more pronounced soybean flavor. The sauce packets also vary. Standard packs come with dashi-flavored tsuyu and mustard, but you’ll find versions with plum (ume) flavoring, extra-strong mustard, or bonito-forward sauces.
For people who want the nutritional benefits but struggle with the smell, some brands produce reduced-odor natto. These use modified fermentation processes to dial back the ammonia and pyrazine production while keeping the basic texture and nutritional profile intact. They’re a common gateway for first-time eaters, though natto enthusiasts generally consider the full-strength version superior in flavor.