Natto (納豆) is whole soybeans fermented by Bacillus subtilis var. natto: cooked beans are inoculated with the bacterium and kept warm for about a day, until they’re covered in a sticky film that pulls into long strings when stirred. It has a strong, savory, slightly ammoniac smell. Nepal’s kinema is a close relative, made from split beans. In Japan it’s most often eaten at breakfast, stirred with mustard and a soy-based sauce and spooned over rice. Unlike miso or tempeh, natto is made by bacteria rather than mold – it belongs to the same alkaline, Bacillus-fermented family as Korean cheonggukjang and West African dawadawa.
What’s in Natto
Per 100 g, Japan’s food composition tables list natto at 184 kcal, 16.5 g protein, 9.5 g fiber, 3.3 mg iron, 120 µg folate and 600 µg vitamin K (MEXT). Much of that vitamin K is menaquinone-7 (vitamin K2), made by the bacteria – a typical 40–50 g pack holds roughly 240–300 µg.
Natto and Warfarin
That vitamin K matters if you take the blood thinner warfarin, which works by opposing vitamin K. In a Japanese study, eating natto raised blood menaquinone-7 within hours and kept it raised for about two days (Homma et al., 2006), and natto is normally off-limits for warfarin patients; a special low-vitamin-K natto has been tested as an alternative (Nakajima et al., 2021). If you take warfarin, don’t start eating natto, or change how much you eat, without your doctor. Newer anticoagulants don’t interact with vitamin K in the same way, but ask anyway.
Nattokinase: What the Research Shows
In 1987 Hiroyuki Sumi and colleagues reported a clot-dissolving (fibrinolytic) enzyme in natto and named it nattokinase (Sumi et al., 1987). It’s now sold as a supplement, and the trials are of those capsules, not of eating natto:
- In an 8-week trial in Korea of 86 adults with high-normal or stage 1 high blood pressure, nattokinase capsules lowered blood pressure by about 5.5/2.8 mmHg more than placebo (Kim et al., 2008).
- In the largest and longest trial – 265 healthy older adults for a median of 3 years – nattokinase had no effect on the progression of carotid atherosclerosis or arterial stiffness, nor on blood pressure or any of the blood tests measured, including clotting factors (Hodis et al., 2021).
So natto is a nutritious, protein-rich food, but it isn’t a proven way to dissolve clots or prevent heart disease – and nattokinase supplements shouldn’t be combined with blood thinners without medical advice.
Making Natto at Home
Natto is one of the more reliable ferments to make at home because you add a pure starter and hold the beans hot (40–42°C), where B. subtilis natto outgrows most competitors. The keys are soft beans, a real natto starter (sold as spore powder), a thin layer, humidity and steady warmth for about 18–24 hours, followed by a day in the fridge (recipe card below). Traditional straw-wrapped natto relied on bacteria living on rice straw; with straw from an unknown source you can’t tell what else you’re growing, so use a starter.
For the wider family of soy ferments – miso, doenjang, tempeh – see our fermented soybean guide and how to make tempeh.
How to Eat Natto
- Natto gohan: stir the natto 30–50 times until stringy, add mustard and a little soy sauce or tare and chopped green onion, and spoon over hot rice.
- Natto maki: rolled in rice and nori – a gentler introduction.
- In soup: stir into miso soup just before serving.
- On toast or noodles: with avocado, or tossed through hot noodles with soy sauce and green onion.