How much natto per day is too much?

One cup = 1 cup (6.2 oz, 175 g). USDA FoodData Central, Natto. Limits count everything you eat and drink that day, so other foods and supplements use up part of each one.

• Level E NIH sets the adult upper limit for iron at 45 mg a day and notes high-dose supplements cause stomach upset, constipation, nausea and diarrhea. NIH Office of Dietary Supplements, Iron fact sheet for health professionals
• Level C A systematic review of warfarin users put the acceptable vitamin K intake at 25 to 325 micrograms a day and named fermented soybean, and foods containing it, the only ordinary foods to be avoided entirely. Shokuhin Eiseigaku Zasshi, 2015
• Level D A case report identifies poly gamma-glutamic acid, the polymer of natto mucilage, as the allergen behind late onset anaphylaxis that began about 3 hours after eating. Allergol Int, 2011

If you have kidney disease, take blood thinners or other medicines, or are pregnant, the right amount for you is a question for your clinician.

One cup of natto, 6.2 oz (175 g), carries 15 mg of iron and 369 calories in the USDA record, so the iron limit takes just under 3 cups a day, about 1,100 calories of natto.

Warfarin is the real limit. A systematic review of warfarin users put acceptable vitamin K intake at 25 to 325 mcg a day and named fermented soybean, and foods containing it, as the only ordinary foods to avoid entirely (Level C). NIH notes that natto has high amounts of vitamin K2 and that people on warfarin need a consistent vitamin K intake (Level E). If you take warfarin, talk to your clinician before eating natto.

Natto is made from soybeans, one of the nine major food allergens FDA lists (Level E). A case report also describes anaphylaxis starting about 3 hours after eating, caused by the polymer in natto mucilage (Level D).

The bacterium that ferments natto has caused bloodstream infections. Case reports describe it in three habitual natto eaters: a man on dialysis, a woman 36 weeks pregnant and a man with undiagnosed colorectal cancer (Level D). In children on chemotherapy, natto intake was linked to 3.3 times the odds of this infection (Level C). If you are on chemotherapy or dialysis, or are pregnant, ask your clinician before eating natto.

Evidence behind this note

1. Level E NIH sets the adult upper limit for iron at 45 mg a day and notes high-dose supplements cause stomach upset, constipation, nausea and diarrhea. NIH Office of Dietary Supplements, Iron fact sheet for health professionals
2. Level C A systematic review of warfarin users put the acceptable vitamin K intake at 25 to 325 micrograms a day and named fermented soybean, and foods containing it, the only ordinary foods to be avoided entirely. Shokuhin Eiseigaku Zasshi, 2015
3. Level E NIH notes that natto, made from fermented soybeans, has high amounts of vitamin K2 (menaquinones), names soybean oil among common vitamin K sources, and advises people on warfarin to keep vitamin K intake consistent. NIH Office of Dietary Supplements, Vitamin K Fact Sheet for Health Professionals
4. Level E FDA lists soybeans among the nine major food allergens in the United States, together with milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, wheat and sesame. US FDA, Food Allergies
5. Level D A case report identifies poly gamma-glutamic acid, the polymer of natto mucilage, as the allergen behind late onset anaphylaxis that began about 3 hours after eating. Allergol Int, 2011
6. Level D Case reports describe Bacillus subtilis var. natto bloodstream infection in three habitual natto eaters with impaired gut barrier or immune status, one of them at 36 weeks of pregnancy. IDCases, 2026
7. Level C Observational data link natto intake with higher odds of Bacillus subtilis bacteremia in children receiving chemotherapy for cancer (odds ratio 3.3, 95% CI 1.1 to 9.6). J Infect Chemother, 2023

Written from the data and studies on this page, reviewed by the Bioma editorial system. How we verify.

• Natto: what the research says
• Can you get too much iron?
• Can you get too much calcium?