Natto is one of the most nutrient-dense foods you can eat, packing high-quality protein, rare vitamins, and a unique enzyme that actively breaks down blood clots. A 100-gram serving delivers 19 grams of protein, 5.4 grams of fiber, and over 130% of your daily manganese needs. But what makes natto genuinely unusual isn’t just its nutritional profile. It’s the combination of vitamin K2, a clot-dissolving enzyme called nattokinase, and beneficial bacteria that you won’t find in this concentration in any other single food.
A Nutritional Powerhouse per Serving
Natto is made from whole soybeans fermented with a specific bacterium, and fermentation transforms those beans into something far more potent than the sum of their parts. At 211 calories per 100 grams, you get 19 grams of complete plant protein, which puts it on par with many animal sources. It also provides 5.4 grams of fiber and a surprisingly rich mineral profile: 2.7 mg of manganese (about 130% of your daily value), 1 mg of copper (58% of daily needs), and a substantial amount of iron (84% of daily needs).
Those mineral numbers are striking for a single food. Manganese supports bone formation and metabolism. Copper plays a role in immune function and red blood cell production. And iron in a plant-based, fermented form can be especially valuable for people who limit or avoid meat.
The Richest Natural Source of Vitamin K2
Natto contains approximately 775 micrograms of vitamin K2 (specifically the MK-7 form) per 100 grams. That makes it, by a wide margin, the most concentrated natural source of this vitamin. Other fermented foods like cheese and miso contain various forms of vitamin K2, but in significantly lower quantities and with less bioactivity in humans.
Vitamin K2 matters because it activates proteins that direct calcium where your body actually needs it: into bones and teeth, and away from arteries and soft tissue. When you eat natto, your blood levels of MK-7 rise measurably, and a bone-building protein called osteocalcin becomes more active. Research on healthy individuals found that serum MK-7 and activated osteocalcin concentrations were significantly elevated after regular natto intake. At the same time, the inactive form of osteocalcin (which signals poor calcium utilization) decreased.
This is why natto consumption in Japan has been linked to better bone density, particularly in postmenopausal women who are most vulnerable to bone loss. Most people in Western countries get very little K2 from their diets, which makes natto an unusually efficient way to close that gap.
Nattokinase and Cardiovascular Protection
The enzyme nattokinase, produced during fermentation, is what sets natto apart from virtually every other food when it comes to heart health. It works through several mechanisms at once. It directly dissolves fibrin, the protein mesh that forms the structural backbone of blood clots. It also boosts your body’s own clot-dissolving system by increasing the release of tissue plasminogen activator (tPA) and converting inactive clot-dissolving agents into their active forms. On top of that, nattokinase blocks the formation of a compound that causes platelets to clump together, reducing clot risk without the bleeding side effects seen with some medications.
Even a single oral dose has measurable effects. In clinical testing, markers of clot breakdown were significantly elevated within four hours of taking nattokinase. Over longer periods, the benefits compound. In one randomized, double-blind, placebo-controlled trial, eight weeks of nattokinase supplementation reduced systolic blood pressure by about 5.5 mmHg and diastolic pressure by about 2.8 mmHg in people with mildly elevated blood pressure. Subsequent studies have confirmed these blood pressure benefits in patients with hypertension.
The reduction in clotting factors goes further. After two months of regular intake, levels of fibrinogen and other clotting proteins (factor VII and factor VIII) decreased significantly, suggesting a broad protective effect on the cardiovascular system. The Takayama study, a large longitudinal study following Japanese adults, found that people in the highest quartile of natto consumption had a 25% lower risk of dying from cardiovascular disease compared to those who ate the least.
Probiotic Benefits From a Resilient Bacterium
Natto is fermented by Bacillus subtilis, a spore-forming bacterium that behaves differently from the lactobacillus strains found in yogurt or kimchi. Because it forms protective spores, Bacillus subtilis survives stomach acid and heat far better than most probiotic strains, meaning more of the bacteria reach your intestines alive and functional. The 5.4 grams of fiber per serving also act as a prebiotic, feeding the beneficial bacteria already living in your gut.
This combination of a hardy probiotic and built-in prebiotic fiber makes natto a particularly effective food for supporting digestive health. Regular consumption helps maintain microbial diversity in the gut, which is linked to stronger immune function, better nutrient absorption, and reduced inflammation throughout the body.
How Much to Eat
In Japan, natto is typically sold in small styrofoam containers of about 40 to 50 grams, and many people eat one pack daily, often at breakfast over rice. This serving size is consistent with the intake levels associated with health benefits in population studies. If you’re new to natto, its sticky, stringy texture and strong fermented smell take some getting used to. Mixing it with soy sauce, mustard, rice, or chopped green onions makes it more approachable.
You can find natto in the frozen section of most Asian grocery stores and increasingly in well-stocked supermarkets. Thaw it in the refrigerator and eat it within a few days. Some people prefer to stir it vigorously before eating, which increases the stringy threads and actually enhances the flavor for many palates.
Who Should Be Cautious
The same properties that make natto beneficial for most people can be dangerous for a specific group: anyone taking warfarin or similar blood-thinning medications. The high vitamin K2 content in natto directly counteracts warfarin’s mechanism, which works by blocking vitamin K. Clinical observations in patients on anticoagulant therapy after heart valve replacement found that natto intake significantly disrupted their therapeutic levels. If you take blood thinners, natto is a food to avoid entirely, not moderate, because even a single serving can shift your clotting balance unpredictably.
People with soy allergies should also steer clear, as natto is a whole soy product. The fermentation process does break down some of the proteins that trigger reactions, but not enough to make it reliably safe for anyone with a confirmed soy allergy.