Natto is fermented soybeans with a distinctive smell and sticky texture that some people add to their diet for potential health benefits
Natto is made by fermenting cooked soybeans with a live bacterium called Bacillus subtilis. The fermentation process creates a sticky, stringy coating around the beans and produces a strong ammonia-like smell that takes some people time to get used to. It has been eaten in Japan for over a thousand years and is commonly served over rice, in sushi rolls, or mixed into noodle dishes.
People who eat natto regularly often do so because of compounds created during fermentation that may support certain body functions. The most studied of these is an enzyme called nattokinase, which some research suggests may affect how blood clots. Natto also contains vitamin K2, a form of vitamin K that differs from the K1 found in leafy greens, and it retains much of the protein and fiber from the original soybeans.
Key Takeaways
- Natto is fermented soybeans that contains nattokinase, an enzyme some research links to blood flow and circulation.
- One serving of natto provides vitamin K2, which your body uses differently than the vitamin K1 found in vegetables.
- Natto also supplies protein, fiber, and probiotics from the fermentation process, similar to other fermented foods.
- The taste and smell are strong and unfamiliar to most people outside Japan, so starting with a small amount is common.
What happens during fermentation and why it matters
When soybeans are fermented with Bacillus subtilis, the bacteria break down the soybean proteins and create new compounds that do not exist in plain cooked soybeans. This process is what produces nattokinase and also makes the beans easier for your body to digest. The fermentation also lowers the levels of compounds called phytates, which can bind to minerals and make them harder for your body to absorb.
The sticky texture comes from a polysaccharide (a type of carbohydrate) that the bacteria produce. This sliminess is part of what makes natto distinctive and is also what some people find off-putting. Mixing natto vigorously before eating it—a common practice in Japan—makes the texture even stickier and is thought to help distribute the beneficial compounds throughout the dish.
Nattokinase and blood circulation
Nattokinase is an enzyme that some laboratory and animal studies suggest may help break down fibrin, a protein involved in blood clotting. A few small human studies have looked at whether eating natto or taking nattokinase supplements might support circulation or reduce clot risk, but the research is limited and the results are mixed. Most of these studies involved small numbers of people and lasted only a few weeks.
If you take blood thinners like warfarin or aspirin, or if you have a bleeding disorder, you should talk with your doctor before eating natto regularly or taking nattokinase supplements. The vitamin K2 in natto can also interact with warfarin, since vitamin K affects how that medication works. This does not mean natto is unsafe for everyone, but it does mean the interaction matters for certain people.
Vitamin K2 and bone and heart health
Natto is one of the richest food sources of vitamin K2, also called menaquinone. Unlike vitamin K1, which your liver uses mainly for blood clotting, vitamin K2 appears to play a role in directing calcium to your bones and teeth and away from your arteries. Some research suggests that people who eat more vitamin K2 have stronger bones and lower heart disease risk, though it is not yet clear whether eating natto specifically causes these benefits.
A single serving of natto (about 50 grams, or roughly 3 tablespoons) contains roughly 200 micrograms of vitamin K2—far more than most other foods. For comparison, a cup of broccoli contains only a few micrograms. If you are interested in vitamin K2 for bone health, natto is one of the few foods where a small serving delivers a meaningful amount, though other fermented foods like some cheeses and sauerkraut also contain it.
Protein, fiber, and probiotics from fermentation
Because natto is made from soybeans, it retains the protein and fiber of the original bean. One serving provides roughly 9 grams of protein and 5 grams of fiber, making it a reasonably dense source of both. The fermentation process also creates live bacteria (probiotics) that may support gut health, though the amount varies depending on how the natto was made and stored.
If you are looking to add more plant-based protein to your diet, natto is one option, though it is not necessarily better than other soy foods like tofu or tempeh. The main difference is that natto's fermentation creates the unique compounds like nattokinase and the high vitamin K2 content. If you are interested in probiotics, natto does contain them, but other fermented foods like yogurt, kimchi, and miso may be more familiar starting points.
How to eat natto and where to find it
Natto is sold in most grocery stores with Japanese or Asian foods, usually in the refrigerated section near tofu. It comes in small styrofoam containers, often with a packet of sauce and a packet of mustard included. The beans should smell strong and look slimy—this is normal and expected. Once you open it, natto keeps in the refrigerator for a few days, though the flavor and texture change as it ages.
The most common way to eat natto is mixed into a bowl of warm rice, where the heat slightly warms the beans and the stickiness coats the rice. You can also add it to sushi rolls, mix it into noodle soups, or stir it into pasta. Many people start by mixing it with soy sauce and a raw egg yolk over rice, which adds richness and makes the strong flavor less intense. If the smell or texture is too much at first, try a very small amount mixed into something mild, like plain rice or avocado.
Potential downsides and who should be cautious
For most people, natto is safe to eat regularly. The main downsides are taste and texture—the strong smell and sticky consistency are genuine barriers for many people, and there is no reason to force yourself to eat something you dislike. Some people also experience mild digestive changes when they first eat fermented foods, including natto, as their gut adjusts to the new bacteria.
As mentioned earlier, if you take warfarin or other blood thinners, or if you have a bleeding disorder, talk with your doctor before making natto a regular part of your diet. People with soy allergies should avoid natto. If you are pregnant or nursing and interested in natto for its vitamin K2 content, a conversation with your healthcare provider can help you decide whether it fits your situation.
Frequently Asked Questions
Does natto really help with blood clots?
Some laboratory and small human studies suggest nattokinase may affect blood clotting, but the evidence is not strong enough to say natto prevents clots or reduces clot risk in most people. If you have concerns about blood clotting or circulation, talk with your doctor rather than relying on natto alone.
Can I get vitamin K2 from other foods?
Yes. Natto is the richest source, but vitamin K2 also appears in aged cheeses, sauerkraut, tempeh, and some other fermented foods. If you do not like natto, these alternatives may work for you, though they contain less K2 per serving.
Is natto safe if I am on warfarin?
Natto contains vitamin K2, which can interfere with how warfarin works. If you take warfarin, ask your doctor before eating natto regularly. You may be able to eat it occasionally if your doctor monitors your medication levels, but this is a conversation to have first.
What does natto taste like?
Natto has a strong, pungent smell similar to ammonia or aged cheese, and a nutty, slightly bitter taste. The texture is sticky and stringy. Most people find it takes several tries to enjoy, and some people never warm to it—that is completely normal.
Where can I buy natto?
Natto is sold in the refrigerated section of most grocery stores with Asian or Japanese foods, often near tofu and miso. You can also order it online from specialty food retailers. Look for containers labeled "natto" with a date on the package.