Fermented soybeans, for your gut bacteria
Natto
Crunchy natto or natto powder
Japan's fermented soybeans, freeze-dried so they keep their Bacillus subtilis spores and their vitamin K2. The gut research is early, and we say so.
Evidence C
What it is
Natto is soybeans fermented with Bacillus subtilis var. natto. Freeze-drying turns it into a crunchy snack or a powder to sprinkle on rice, eggs or salad, without the famous stringiness. The bacteria survive as dormant spores.
Why it's here
Two reasons. Natto is one of the richest food sources of vitamin K2 (as MK-7), which your body uses to make the proteins needed for normal blood clotting and bone. And freeze-dried, it's a fermented food that keeps its live spores and lasts in a cupboard. Early studies suggest it nudges gut bacteria such as Bifidobacterium upward; what that does for your health hasn't been tested.
What the gut research shows
- In a two-month study of 205 adults, daily freeze-dried natto powder raised Bifidobacterium, most in people who started with little of it. The study was run by the powder's maker and had no placebo.
- In an observational study, people who ate more natto carried more butyrate-making bacteria. It can't show natto caused that.
- No study yet shows these shifts change digestion or how you feel.
How to choose
- Ingredients: soybeans and natto culture. Seasoned snacks add salt and sugar.
- Plain powder is the easiest to add to everyday food.
- Nattokinase isn't natto: it's an enzyme purified from it, usually with no live bacteria. The gut research here doesn't apply to it.
- Check where the soybeans come from, and whether they're non-GMO if that matters to you.
Watch out
- If you take warfarin (Coumadin) or another vitamin K antagonist, don't add natto without talking to your doctor. Its vitamin K can change how the drug works.
- Natto's bacteria have rarely got into the bloodstream of people with weakened defences, such as those on dialysis or late in pregnancy. If your immune system or gut is compromised, or you're pregnant, ask your doctor first.
- Contains soy.
The evidence Grade C
Mostly mechanistic or observational. It's food, so that's fine; we just say so.
Natto's vitamin K2 content is well measured. Its effect on gut bacteria is less certain. Freeze-dried natto carries spores of Bacillus subtilis var. natto, a hardy bacterium. In one open-label study run by the powder's maker, 205 Japanese adults were randomly assigned to daily freeze-dried natto powder or to nothing for about two months. Bifidobacterium rose in the natto group, along with Blautia in men, and the change was largest in people who started with little Bifidobacterium. There was no placebo, and the study measured bacteria, not health. In a separate observational study of 185 Japanese adults, slim people who ate a lot of barley also ate more natto and carried more bacteria that make butyrate, a fatty acid that feeds the gut lining. That kind of study can't show cause and effect.
- NIH ODS: Vitamin K fact sheet
- Study: freeze-dried natto powder and gut bacteria in 205 adults (Kono et al., 2022)
- Observational study: natto, barley and butyrate-making gut bacteria (Maruyama et al., 2024)
- Case series: B. subtilis var. natto bloodstream infections (Ishizawa et al., 2026)
Last reviewed 2026-10-23.