Natto is a high protein traditional Japanese food made by fermenting cooked soybeans with Bacillus subtilis. It has been a staple of the Japanese diet for generations, with 77% of people in a 2005 survey reporting that they ate natto at least once a week.
Of course, natto has one small challenge for newcomers: it doesn’t exactly smell or taste like a bowl of strawberries. Its strong aroma and famously slimy, stringy texture can take some getting used to. Fortunately, there are plenty of ways to make it more enjoyable and palatable, such as mixing it with soy sauce and mustard, or pairing it with avocado, spicy kimchi, green onions, sesame seeds, or rice.
One of natto's most distinctive nutritional characteristics is its exceptionally high concentration of protein and vitamin K2, particularly menaquinone-7 (MK-7). Fermentation also produces nattokinase, a powerful enzyme.
Natto and Bone Health
Natto has received considerable attention for its potential role in maintaining bone health because MK-7 helps activate osteocalcin, a protein involved in bone mineralization.
A 2000 human study found that eating natto increased blood levels of MK-7 and increased osteocalcin, indicating that the vitamin K2 from natto was biologically active.
Other Japanese observational research has found associations between regular natto consumption and higher bone mineral density. For example, a study of 1,662 older Japanese men found that greater natto intake was associated with higher bone density at the hip and femoral neck.
A 2025 meta-analysis combining six studies and 2,327 Japanese participants found that habitual natto consumption was associated with substantially higher serum MK-7, improved osteocalcin activation, and modestly higher bone mineral density.
In a randomized trial of 334 early postmenopausal women, one year of 360 μg/day of MK-7 from natto capsules improved osteocalcin activation but did not significantly slow bone loss.
Cardiovascular Health
Natto is also interesting because it contains nattokinase, an enzyme produced during fermentation that has fibrinolytic activity (dissolves blood clots).
A large Japanese cohort study followed 29,079 adults for approximately 16 years. People in the highest natto consumption had a 25% lower risk of cardiovascular-disease mortality than those with the lowest. Natto consumption was also associated with lower stroke mortality.
In a 2015 randomized, double-blind crossover study involving 12 healthy men, a single 2,000-fibrinolytic-unit (FU) dose of nattokinase appeared to increase the breakdown of blood clots. It raised blood markers showing that fibrin (the main protein that forms blood clots) was being broken down. It also reduced the activity of clotting factor VIII and slightly slowed the blood-clotting process.
Another human study followed 45 participants who took 2,000 FU of nattokinase twice daily for two months. Fibrinogen decreased by approximately 7–10%, factor VII by 7–14%, and factor VIII by 17–19% (proteins that clot blood), depending on the participant group.
A 2019 randomized trial involving 100 people with elevated cholesterol also found that eight weeks of nattokinase increased measures of clotting time, compared with placebo.
Natto and the Immune System
Natto may support immune health through its fermentation with Bacillus subtilis var. natto, a bacterium that can interact with intestinal and immune cells.
A 2026 review of 115 studies concluded that B. subtilis and B. natto can influence intestinal immunity, including the gut barrier, mucus production, immune signaling, and the gut microbiome.
Research specifically examining bacteria from natto has also produced interesting findings. A 2025 study using human macrophage cells found that a B. subtilis strain isolated from natto increased the expression of antiviral and anti-inflammatory cytokines (messengers used by the immune system) and activated the part of the immune system that recognizes invaders.
Earlier research found that B. subtilis var. natto could stimulate cytokine responses in human intestinal cells, while a 2023 study found that compounds produced by natto bacteria could increase the anti-inflammatory cells.
Natto and Digestion
Research suggests that the fermentation process used to make natto may make soybeans easier to digest.
A 2021 Scientific Reports study compared soaked, boiled, and fermented soybeans using a simulated gastrointestinal digestion model. Protein digestibility increased from about 20.6% in soaked soybeans to 50.2% after fermentation, with fermentation breaking soybean proteins into smaller peptides and increasing the availability of essential amino acids.
Another 2019 study using fermented soybeans made with Bacillus subtilis var. natto found that fermentation improved protein digestibility by approximately 45% and increased the release of essential amino acids during simulated digestion.
In a small study of 7 healthy adults, eating 50 g of natto daily for 14 days increased Bacillus subtilis and Bifidobacterium in stool and altered the microbiome for the better. During the two weeks, the amount of Bacillus subtilis, the bacteria used to make natto, increased in the participants’ stool. Bifidobacterium, a group of bacteria commonly considered beneficial for the gut, also increased. At the same time, some less desirable bacteria, including certain Clostridia, decreased.
The researchers also found changes in substances produced in the intestines. Acetic acid and other organic acids increased, while substances such as ammonia and certain compounds associated with protein breakdown decreased. The stool also became slightly more acidic.
Natto Nutrition
Natto provides a variety of nutrients:
High-quality soy protein (19 g in 1/2 cup)
Vitamin K2/MK-7 (600-1,100 mcg in 1/2 cup)
Iron (7.5 mg iron in 1/2 cup)
Magnesium (24% RDA in 1/2 cup)
Potassium (14% of RDA in 1/2 cup)
Calcium (191 mg in 1/2 cup)
Zinc (2.6 mg in 1/2 cup)
Soy isoflavones
Fiber
Fermentation-derived compounds such as nattokinase
Fermentation-associated microorganisms