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Natto is a traditional Japanese food of whole soybeans fermented by the bacterium Bacillus subtilis, known for its sticky strings, strong aroma and very high vitamin K2 content. USDA data show 100 grams of natto provide about 19 grams of protein, 5 grams of fiber and nearly half the Daily Value for iron, with almost no sodium. The NIH Office of Dietary Supplements lists natto as the richest common source of vitamin K2, which is why people taking warfarin need to keep their intake consistent. Below are the USDA natto nutrition facts, what natto benefits the evidence supports, the warfarin question and who should be careful.
Peptide Labs does not sell natto or natto products. This article is general nutrition information, not medical advice.
Key Takeaways
- Natto is whole soybeans fermented with Bacillus subtilis; the sticky strings come from polyglutamic acid.
- Per 100 grams, USDA lists about 211 calories, 19 grams of protein, 5 grams of fiber and 48% of the Daily Value for iron.
- NIH ODS lists natto as by far the richest food source of vitamin K2 (menaquinone-7).
- People taking warfarin should keep vitamin K intake consistent and talk to their clinician before adding or dropping natto.
- Natto is a soy food, so soy allergy rules it out; severe reactions to natto have been reported.
What is natto?
A 2022 critical review in Biochemistry Research International describes how natto is made: soybeans are soaked, steamed or cooked, inoculated with Bacillus subtilis and held warm while the bacteria grow. During fermentation the bacteria produce strands of gamma-polyglutamic acid, which give natto its slimy, stringy texture. The finished food also contains soy isoflavones, vitamin K2, biogenic amines and an enzyme called nattokinase.
In the US, natto is usually sold frozen or refrigerated in small foam or paper packs, often with packets of mustard and a soy-based sauce. Those sauce packets, not the beans, carry most of the sodium. Natto belongs to the same family of fermented soybean foods as miso and tempeh, though tempeh is made with a mold rather than a bacterium.
Natto nutrition per 100 grams and per cup
The table uses the USDA FoodData Central SR Legacy entry for natto (FDC 172443), shown per 100 grams and per the USDA cup serving of 175 grams. Percent Daily Value (%DV) uses the FDA Daily Values from Nutrition Facts labels. Packs sold in stores are usually smaller than a cup, so check the label for the serving size.
| Nutrient (natto) | Per 100 g | %DV | Per cup (175 g) | %DV |
|---|---|---|---|---|
| Calories | 211 | Not applicable | 369 | Not applicable |
| Protein | 19.4 g | 39% | 34.0 g | 68% |
| Total fat | 11.0 g | 14% | 19.3 g | 25% |
| Saturated fat | 1.6 g | 8% | 2.8 g | 14% |
| Carbohydrate | 12.7 g | 5% | 22.2 g | 8% |
| Fiber | 5.4 g | 19% | 9.5 g | 34% |
| Iron | 8.6 milligrams | 48% | 15.1 milligrams | 84% |
| Manganese | 1.53 milligrams | 66% | 2.67 milligrams | 116% |
| Copper | 0.67 milligrams | 74% | 1.17 milligrams | 130% |
| Magnesium | 115 milligrams | 27% | 201 milligrams | 48% |
| Zinc | 3.0 milligrams | 28% | 5.3 milligrams | 48% |
| Calcium | 217 milligrams | 17% | 380 milligrams | 29% |
| Potassium | 729 milligrams | 16% | 1,276 milligrams | 27% |
| Sodium | 7 milligrams | 0% | 12 milligrams | 1% |
Natto stands out among soy foods for iron, manganese, copper and magnesium. The iron in plant foods is non-heme iron, which the body absorbs less efficiently than iron from meat; our iron foods guide explains the difference. Here is how natto compares with other soybean foods in USDA data.
| Soy food per 100 g (FDC ID) | Calories | Protein | Fiber | Iron | Calcium |
|---|---|---|---|---|---|
| Natto (172443) | 211 | 19.4 g | 5.4 g | 8.6 milligrams | 217 milligrams |
| Soybeans, boiled (174271) | 172 | 18.2 g | 6.0 g | 5.1 milligrams | 102 milligrams |
| Firm tofu, calcium sulfate (172475) | 144 | 17.3 g | 2.3 g | 2.7 milligrams | 683 milligrams |
| Miso (172442) | 198 | 12.8 g | 5.4 g | 2.5 milligrams | 57 milligrams |
Natto vitamin K2: the number USDA leaves out
Vitamin K comes in two main forms. Phylloquinone (K1) comes mostly from green leafy vegetables, while menaquinones (K2) are made by bacteria and appear in fermented foods and some animal foods. The USDA SR Legacy entry for natto lists only phylloquinone, so it misses the vitamin K2 that the fermenting bacteria produce.
The NIH Office of Dietary Supplements fact sheet fills that gap: a 3-ounce serving of natto provides about 850 micrograms of vitamin K as menaquinone-7, or 708% of the Daily Value of 120 micrograms. No other food in its table comes close. Our vitamin K food guide lists the other major sources, and our vitamin D3 and K2 guide covers the supplement side of K2.
Natto and warfarin: why consistency matters
Vitamin K helps the body make proteins involved in blood clotting, and warfarin (Coumadin) works by opposing it. The Office of Dietary Supplements calls this a serious and potentially dangerous interaction and says people taking warfarin need to maintain a consistent intake of vitamin K from food and supplements, because sudden changes can raise or lower the drug’s effect. MedlinePlus gives the same message: eat consistent amounts of vitamin K foods from week to week, and talk to your doctor before changing your diet. Memorial Sloan Kettering’s herb database specifically notes that natto is high in vitamin K and may interfere with warfarin.
Because a single serving of natto carries several times the Daily Value, starting or stopping it is exactly the kind of sudden change these sources describe. Anyone on warfarin should raise natto with the clinician who manages their blood tests before making it a regular food or cutting it out. The Office of Dietary Supplements also notes that orlistat, the drug in Alli, can lower vitamin K absorption.
Natto vs enzyme supplements
Natto the food and the enzyme capsules sold online are not the same thing. Supplements isolate the enzyme from the fermentation, and some labels say the vitamin K has been removed, while the whole food delivers vitamin K2, protein, fiber and minerals. The enzyme raises its own bleeding questions, especially with blood thinners and before surgery. Our guide to the natto enzyme supplement covers that research and its risks; this article sticks to the food.
Natto benefits: what the evidence supports
Much of the research on natto looks at isolated components, such as the enzyme, vitamin K2 or isoflavones, often in lab or small human studies, and observational data from Japan cannot separate natto from the rest of the diet. What can be said plainly about natto benefits is grounded in its composition:
- It is a dense source of plant protein, with about 19 grams per 100 grams, and soy protein contains all nine essential amino acids.
- It supplies fiber and meaningful shares of the Daily Value for iron, manganese, copper, magnesium and zinc in USDA data.
- It is naturally very low in sodium before sauce is added, unlike miso and soy sauce.
- It is the richest common food source of vitamin K2, which is useful to know whether you want more of it or need to keep your intake steady.
- It is a fermented food, but fermented foods are not automatically probiotics, as our guide to foods made by fermentation explains.
Who should be careful with natto
- People with soy allergy: soybeans are one of the nine major food allergens under FDA labeling law, and Memorial Sloan Kettering notes severe allergic reactions in people allergic to natto or fermented soybeans.
- People taking warfarin: keep vitamin K intake consistent and talk to your clinician before adding or removing natto.
- People taking MAOI antidepressants: fermented soy foods can contain tyramine, and NCCIH reports dangerously high blood pressure with high-tyramine soy products and MAOIs. Ask the prescriber first.
- People taking levothyroxine: MedlinePlus notes that foods containing soybeans may affect how levothyroxine works.
- People watching sodium: the beans are low in sodium, but the included sauce packets are not. Check the label.
Frequently Asked Questions
What is natto?
Natto is whole soybeans fermented by Bacillus subtilis. The bacteria create sticky strands of polyglutamic acid and a strong aroma.
Is natto healthy?
USDA data show 100 grams have about 19 grams of protein, 5 grams of fiber and nearly half the Daily Value for iron. It is also very high in vitamin K2.
How much vitamin K2 is in natto?
The NIH Office of Dietary Supplements lists a 3-ounce serving at about 708% of the Daily Value for vitamin K, as menaquinone-7.
Can I eat natto if I take warfarin?
Talk to your clinician first. Health agencies advise consistent vitamin K intake on warfarin, and natto carries several times the Daily Value per serving.
What does natto taste like?
It has a strong, earthy aroma and a sticky, stringy texture. Many people eat it over rice with mustard and the included sauce.
Is natto the same as the enzyme supplement?
No. The supplement isolates one enzyme from natto, and some versions remove vitamin K. The food also provides protein, fiber and minerals.
Questions about a label? Contact us or read our Buyer’s Guide. Food comes first; follow the label and ask a healthcare provider before starting a supplement, especially if you take medicines, are pregnant or have a food allergy. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Last reviewed October 2026.
Related guides: Miso: USDA Nutrition, Sodium per Tablespoon, Types and Miso Soup, Nattokinase: Benefits, Side Effects and Bleeding Risks, Foods High in Vitamin K: USDA List, K1 vs K2 and Warfarin Notes.