Nattokinase is found naturally in only one common food: natto, the sticky fermented soybeans eaten for breakfast across Japan. It is made during fermentation by the bacterium Bacillus subtilis var. natto, so tofu, soy milk, edamame and most other soy foods do not contain it. This guide explains what natto is, which other foods contain similar enzymes, why natto's vitamin K2 matters if you take warfarin, where to buy natto in the UK, and how it compares with a nattokinase supplement.
The quick answer
- Natto is the food source: nattokinase is produced when Bacillus subtilis var. natto ferments soybeans.
- Not in other soy foods: tofu, soy milk, edamame, miso, tempeh and soy sauce are not sources of nattokinase.
- Natto is also rich in vitamin K2, which can make warfarin less effective. Anyone on warfarin should not eat natto without medical advice.
- Eat it cold: natto is traditionally eaten cold or at room temperature, and enzymes are generally damaged by heat.
- In the UK you can buy frozen natto from Japanese and Asian grocers and online.
What is natto?
Natto is a traditional Japanese food made by fermenting cooked soybeans with Bacillus subtilis var. natto. Fermentation gives the beans a strong, savoury smell, a nutty and slightly bitter taste, and the sticky strings natto is known for. It is usually sold in small single-serving tubs, often with a sachet of mustard and a soy-based sauce.
The enzyme nattokinase was named after the food. It is a protein-digesting enzyme that breaks down fibrin, a protein involved in blood clotting, which is why it is now also sold as a food supplement (Weng et al., 2017).
Which foods contain nattokinase?
Nattokinase is produced by one particular bacterium during one particular fermentation, so natto is the only everyday food that is a meaningful source. Some other soybean foods fermented with Bacillus bacteria contain related enzymes. Douchi, a Chinese fermented black soybean, contains an enzyme called douchi fibrinolytic enzyme that researchers are studying (Cheng et al., 2025), but it is not the same enzyme, and its amount in food is not measured.
| Food | How it is made | Nattokinase? |
|---|---|---|
| Natto | Soybeans fermented with Bacillus subtilis var. natto | Yes, the original source |
| Douchi | Black soybeans fermented with moulds or Bacillus bacteria | A related enzyme, not nattokinase |
| Miso, soy sauce | Fermented with the mould Aspergillus oryzae (koji) | No |
| Tempeh | Fermented with a Rhizopus mould | No |
| Tofu, soy milk, edamame | Not fermented | No |
Natto and vitamin K2
Natto is one of the best-known food sources of vitamin K2, in the form menaquinone-7 (MK-7). In a food analysis by Schurgers and Vermeer, vitamin K2 was found mainly in meat, liver, butter, egg yolk, cheese and natto. When volunteers ate natto, their blood vitamin K2 levels rose to about 10 times the vitamin K1 levels seen after eating spinach (Schurgers and Vermeer, 2000).
Vitamin K contributes to normal blood clotting and to the maintenance of normal bones. That clotting role is exactly why natto matters for people on warfarin: warfarin works by blocking vitamin K, so a food rich in vitamin K2 can make it less effective. MK-7 stays in the blood for a long time, and supplements of 50 micrograms a day or more may interfere with oral anticoagulants (Schurgers et al., 2007). Even 10 micrograms a day changed how volunteers responded to a warfarin-type medicine (Theuwissen et al., 2013).
How to eat natto
In Japan, natto is most often eaten at breakfast. A simple way to start:
- Thaw it in the fridge if it is frozen.
- Stir it vigorously with chopsticks until it turns frothy and stringy.
- Add the sauce and mustard from the pack, or a little soy sauce and chopped spring onion.
- Serve it over warm rice, or with eggs, avocado or kimchi.
Natto is traditionally eaten cold or at room temperature rather than cooked. Nattokinase is an enzyme, and enzymes are generally damaged by heat, so stirring natto into very hot food may reduce its activity.
Where to buy natto in the UK
Natto is not yet a standard supermarket item in the UK, so most people buy it from:
- Japanese food shops and larger Asian supermarkets, usually in the freezer section
- Online Japanese and Asian grocers, which deliver it frozen
- Some health food shops, which may stock organic natto
Natto keeps for a few days in the fridge once thawed, and for months in the freezer. Check the label for the best before date and storage advice. If you prefer to make your own, natto starter cultures are sold online.
Natto vs a nattokinase supplement
Natto and nattokinase capsules are not the same thing. The food contains nattokinase along with vitamin K2, protein, fibre and soy, while a supplement provides the enzyme on its own.
| Natto (food) | NutritionGo nattokinase capsules | |
|---|---|---|
| Nattokinase activity | Varies by batch and is not measured on the pack | 6,000 FU per capsule |
| Vitamin K2 | Naturally high | Vitamin K2-free |
| Taste and smell | Strong and sticky; not to everyone's taste | None |
| Soy | Whole soybeans | Contains soya |
| How to take | As part of a meal, cold or at room temperature | 1 to 2 capsules a day on an empty stomach, with water |
Because the capsules are vitamin K2-free, they avoid the vitamin K issue that natto has with warfarin, but they are still not suitable for anyone on warfarin or another blood thinner, because nattokinase itself acts on clotting. See our guide to nattokinase side effects for the full safety picture, and our nattokinase dosage guide and best time to take nattokinase for how to take it.
Who should be careful with natto and nattokinase
- People on warfarin: natto's vitamin K2 can affect your INR, and nattokinase acts on clotting.
- People on other blood thinners or aspirin, or with a bleeding disorder: speak to your doctor before taking nattokinase in any form.
- People with a soya allergy: avoid both natto and nattokinase supplements.
- Anyone with surgery planned: stop nattokinase supplements at least 2 weeks before and tell your surgeon.
- Pregnant or breastfeeding women: natto is eaten as a normal food in Japan, but there is not enough safety research on nattokinase supplements, so speak to your GP or midwife.
The bottom line on nattokinase foods
Natto is the food that nattokinase comes from, and it is the only everyday food that contains it. It is also rich in vitamin K2, which is the reason people on warfarin need to avoid it unless their clinic agrees. If you enjoy natto, eat it cold, as part of a varied diet. If you would rather not eat it, a vitamin K2-free nattokinase supplement with a stated FU strength is the alternative, as long as you do not take blood thinners and you check with your GP if you take any regular medicine.
Vanessa holds a Level 3 Diploma in Nutrition and co-founded NutritionGo, where she leads editorial content. She researches and writes our guides, covering ingredient composition, traditional use and nutrition information, and fact-checks them against UK advertising rules and the GB Nutrition and Health Claims register. Every factual statement is verified against sources such as the NHS, the GB Nutrition and Health Claims register or peer-reviewed research on PubMed before publication. Vanessa is not a registered dietitian or healthcare professional, so this guide is for general information and is not a substitute for personal medical advice.
References
View all 6 references and sources
- Weng Y, Yao J, Sparks S, Wang KY. Nattokinase: an oral antithrombotic agent for the prevention of cardiovascular disease. Int J Mol Sci, 2017;18(3):523. What nattokinase is and how it is made.
- Cheng M, Wang P, Li M, et al. Research progress on douchi fibrinolytic enzyme. Protein J, 2025;44(2):162-174.
- Schurgers LJ, Vermeer C. Determination of phylloquinone and menaquinones in food. Effect of food matrix on circulating vitamin K concentrations. Haemostasis, 2000;30(6):298-307.
- Schurgers LJ, Teunissen KJ, Hamulyák K, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood, 2007;109(8):3279-83.
- Theuwissen E, Teunissen KJ, Spronk HM, et al. Effect of low-dose supplements of menaquinone-7 (vitamin K2) on the stability of oral anticoagulant treatment: dose-response relationship in healthy volunteers. J Thromb Haemost, 2013;11:1085-92.
- Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Authorised vitamin K claims; nattokinase has none.