Natto, the sticky fermented soybean dish from Japan, has several properties that look genuinely helpful for people with diabetes or at risk of developing it. Human crossover studies show it can blunt post-meal blood sugar spikes by roughly 15 to 20 percent, and a meta-analysis of soy consumption links regular intake to a meaningfully lower risk of type 2 diabetes. Natto also delivers a concentrated package of vitamin K2, nattokinase, and beneficial bacteria that individually show anti-diabetic effects in laboratory and animal research. The picture is encouraging but lopsided: the human evidence is still thin, and most of the mechanistic work has been done in rodents.
How Natto Affects Blood Sugar After a Meal
The most direct evidence for natto’s diabetes relevance comes from studies measuring what happens to blood glucose in the hours after eating. In a randomized crossover trial, participants who ate white rice paired with natto rich in gamma-polyglutamic acid (a sticky compound produced during fermentation) had a blood glucose response about 20 percent lower over two hours compared to eating white rice alone, with the difference reaching statistical significance. The effect was strongest in the first 15 to 45 minutes after eating, suggesting natto slows the initial rush of glucose into the bloodstream.1PubMed Central. The Possibility of Suppression of Increased Postprandial Blood Glucose Levels by Gamma-Polyglutamic Acid-Rich Natto in the Early Phase after Eating: A Randomized Crossover Pilot Study
A separate trial found that when natto was part of a Japanese-style meal with viscous vegetables and white rice, peak blood glucose dropped from about 6.8 to 6.0 mmol/L, and the total glucose response over two hours fell by roughly 28 percent compared to a control meal. Insulin levels dropped by a similar amount, meaning the body needed less insulin to manage the incoming sugar.2PubMed. Natto and viscous vegetables in a Japanese style meal suppress postprandial glucose and insulin responses That matters because chronically high insulin output is one of the forces that wears down the pancreas over time and pushes people toward type 2 diabetes.
Researchers have identified at least two mechanisms behind this effect. Natto appears to physically slow glucose absorption in the intestine, and it inhibits dipeptidyl peptidase IV, an enzyme whose suppression is the basis of an entire class of diabetes medications (the “gliptins”).3PubMed Central. Fermented soybean foods and diabetes The fact that natto naturally does something pharmaceuticals are designed to do is interesting, though the potency is almost certainly much lower than a prescription drug.
The Broader Soy-Diabetes Connection
Natto exists within a larger family of soy foods, and the epidemiological case for soy reducing diabetes risk is reasonably strong. A systematic review and meta-analysis of observational studies found that people who ate the most soy had about a 23 percent lower risk of type 2 diabetes compared to those who ate the least. Soy protein and isoflavone intake specifically were linked to about a 12 percent lower risk. The protective effect was particularly pronounced in women, who showed a 35 percent lower risk, and in Asian populations, where the reduction was around 27 percent.4Diabetes Research and Clinical Practice. Soy and the risk of type 2 diabetes mellitus: A systematic review and meta-analysis of observational studies
These are observational findings, so they cannot prove soy itself caused the lower risk. People who eat soy regularly may also have other dietary and lifestyle habits that protect them. And the stronger results in Asian populations raise a fair question: is it the soy, or is it everything else that tends to go with a traditional Asian diet? Still, the association held up across multiple study designs and populations, which gives it more weight than a single trial would.
Natto may offer advantages beyond what unfermented soy provides, because the fermentation process transforms the bean in ways that matter for diabetes specifically. But the meta-analysis lumped all soy foods together, so it cannot tell us whether natto is better or worse than, say, tofu or soy milk for diabetes prevention.
What Fermentation Brings to the Table
Soybeans on their own are nutritious, but fermenting them with Bacillus subtilis creates compounds that do not exist in the raw bean. The fermentation process generates nattokinase (a fibrin-dissolving enzyme), large amounts of vitamin K2 in the menaquinone-7 form, gamma-polyglutamic acid, and a population of live Bacillus bacteria. Each of these has its own line of diabetes-relevant research.
A review of fermented soybean studies found that natto and similar foods modify gut bacteria and their metabolites, producing antioxidant, anti-inflammatory, and anti-diabetic effects that go beyond what the soybean contributes by itself.3PubMed Central. Fermented soybean foods and diabetes Animal work also found that natto suppresses fatty liver, a condition tightly linked to insulin resistance and type 2 diabetes progression. These fermentation-specific benefits are part of why researchers have started studying natto as a distinct food rather than grouping it with all soy products.
Vitamin K2 and Insulin Sensitivity
Natto is one of the richest dietary sources of vitamin K2, specifically the long-chain form known as menaquinone-7 (MK-7). A single serving can contain hundreds of micrograms, far more than most other foods. This matters for diabetes because vitamin K2 appears to influence how the body handles blood sugar.
In diabetic rats, vitamin K2 administration produced a dose-dependent drop in fasting blood glucose, hemoglobin A1c, and insulin resistance scores. At the same time, fasting insulin levels and measures of pancreatic beta-cell function improved, suggesting the vitamin helped the insulin-producing cells work better.5PubMed. Vitamin K(2) alleviates type 2 diabetes in rats by induction of osteocalcin gene expression The proposed pathway involves osteocalcin, a bone-derived hormone that vitamin K2 helps activate. Active osteocalcin has been shown to boost insulin secretion and improve insulin sensitivity in other research.
A hypothesis paper has proposed that vitamin K2 may work synergistically with magnesium and vitamin D to improve insulin sensitivity by stimulating osteocalcin expression and modulating inflammatory pathways.6Medical Hypotheses. Plausible mechanisms behind synergism between magnesium, vitamin D and vitamin K in management of type 2 diabetes That remains a hypothesis, not a confirmed finding, but it is consistent with the animal data.
One important caveat: positive results in animal studies of vitamin K2 have not always translated to humans. A six-month randomized trial in people with type 2 diabetes found that vitamin K supplementation did not reduce arterial calcification or slow bone mineral density decline compared to placebo.7PubMed Central. Six months vitamin K treatment does not affect systemic arterial calcification or bone mineral density in diabetes mellitus 2 That trial was looking at vascular and bone outcomes rather than blood sugar control directly, but it is a reminder that the leap from rodent data to human benefit is never guaranteed.
Nattokinase and Diabetic Complications
Even if natto only modestly affects blood sugar levels, it may still help people with diabetes through a different route: protecting against the vascular and organ damage that diabetes causes over time. Nattokinase, the enzyme unique to natto, has drawn attention here because diabetes alters how blood clots form and break down, contributing to elevated cardiovascular risk.
Clinical and pre-clinical studies have demonstrated that oral consumption of mixtures containing nattokinase improves glucose metabolism and insulin resistance, though an important limitation is that those mixtures also contained other active components, making it hard to isolate nattokinase’s individual contribution. Researchers have proposed that nattokinase combined with standard diabetes medications could be a strategy for managing both the metabolic disease and its cardiovascular consequences.8Food Science and Human Wellness. Nattokinase as a functional food ingredient: therapeutic applications and mechanisms in age-related diseases
Animal studies offer more specific data on kidney protection. In diabetic rats, high-dose nattokinase significantly reduced glycogen deposition in the kidney tubules and lowered circulating levels of advanced glycation end-products (AGEs), which are molecules that accumulate when blood sugar is chronically high and drive much of diabetes’s organ damage. The high-dose group’s AGE levels dropped to about 1.3 micrograms per deciliter, well below the 4.2 seen in untreated diabetic controls. Nattokinase also lowered C-reactive protein (a marker of inflammation) in the first week of treatment.9PubMed Central. Effect of nattokinase on the pathological conditions in streptozotocin induced diabetic rats What makes this finding particularly intriguing is that the kidney protection occurred without nattokinase lowering blood glucose itself, suggesting the enzyme may protect organs through anti-inflammatory and anti-glycation pathways rather than by controlling sugar.
Separately, a study of fermented red bean extract (produced through a similar Bacillus-driven fermentation) found that it improved kidney function, reduced oxidative stress, and inhibited AGE formation in diabetic rats, with a moderate dose preventing diabetic kidney disease by improving antioxidant status and reducing kidney inflammation.10PubMed Central. Effects of fermented red bean extract on nephropathy in streptozocin-induced diabetic rats While this was not natto itself, the overlapping fermentation chemistry and similar results strengthen the case that Bacillus-fermented legumes have kidney-protective properties in the context of diabetes.
Visceral Fat and Gut Bacteria
Excess visceral fat, the deep abdominal fat that wraps around organs, is one of the strongest drivers of insulin resistance. Mouse studies suggest natto may help here. One found that natto intake significantly reduced visceral fat accumulation in a dose-dependent manner, shrank swollen fat cells, improved carbohydrate metabolism, and reduced oxidative stress.11PubMed. Fermented Soybean Suppresses Visceral Fat Accumulation in Mice Another showed that Bacillus natto supplementation in rats on a high-fat diet reduced body weight, lowered cholesterol and triglycerides, raised HDL cholesterol, and improved insulin resistance.12Biochemical and Biophysical Research Communications. Bacillus natto ameliorates obesity by regulating PI3K/AKT pathways in rats
Part of the mechanism may involve the gut microbiome. In obese mice, Bacillus natto consumption altered the composition of intestinal bacteria and may have changed the production of short-chain fatty acids, which are bacterial metabolites that influence insulin sensitivity and appetite signaling.13Journal of Functional Foods. Bacillus natto regulates gut microbiota and adipose tissue accumulation in a high-fat diet mouse model of obesity The researchers noted that further study was needed to confirm the short-chain fatty acid connection, but the microbiome shift itself was clear.
These are rodent results, and mice are not humans. People metabolize fat differently, and the doses used in animal studies often do not scale neatly to what someone would eat at the dinner table. But the consistency of the findings across multiple animal models, showing improvements in visceral fat, lipid profiles, and insulin sensitivity, makes this a credible area for future human trials.
The Warfarin Problem
If you take warfarin (a common blood thinner), natto requires serious caution. Warfarin works by blocking vitamin K’s role in the blood-clotting cascade, and natto is loaded with vitamin K2 in the MK-7 form. Eating natto can measurably shift your clotting balance and interfere with warfarin’s effectiveness.14Nutrition Reviews. Influence of Vitamin K on Anticoagulant Therapy Depends on Vitamin K Status and the Source and Chemical Forms of Vitamin K For people on stable warfarin doses, this is not a minor concern; it is a food-drug interaction that cardiologists and pharmacists routinely warn about.
There is some interesting work on developing low-MK-7 natto that might be safe for warfarin users. A clinical trial found that a specially fermented natto with reduced vitamin K2 content did not change clotting values at doses up to 20 grams per day (roughly a tablespoon), and no adverse events occurred.15PubMed. First Clinical Trial of a Newly Developed, Low Menaquinone-7, Fermented Soybean Natto in Warfarin-Dependent Patients That said, 20 grams is a small portion, and the product is not widely available outside of research settings. If you take warfarin or another vitamin K-sensitive anticoagulant, talk to your doctor before adding natto to your diet.
People with diabetes are disproportionately likely to be on blood thinners because the disease raises cardiovascular risk. So this interaction is not a niche issue; it applies to a meaningful fraction of the people who might benefit most from natto’s other properties.
Practical Considerations for Adding Natto to a Diabetic Diet
Natto is typically sold in small styrofoam packs of about 40 to 50 grams at Japanese and Asian grocery stores, often in the frozen section. A standard serving contains around 8 to 9 grams of protein, a modest amount of carbohydrate (mostly from the soybeans themselves, not added sugar), and a generous amount of fiber. It is low on the glycemic index by itself, though it is traditionally eaten over white rice, which is not.
If your goal is blood sugar management, the pairing matters. The crossover trial that found a 20 percent reduction in glucose response was specifically comparing natto eaten with white rice versus white rice alone.1PubMed Central. The Possibility of Suppression of Increased Postprandial Blood Glucose Levels by Gamma-Polyglutamic Acid-Rich Natto in the Early Phase after Eating: A Randomized Crossover Pilot Study And the meal-based study that showed a 28 percent glucose reduction included viscous vegetables alongside the natto.2PubMed. Natto and viscous vegetables in a Japanese style meal suppress postprandial glucose and insulin responses Swapping white rice for brown rice or another whole grain, or pairing natto with fibrous vegetables, would likely amplify the benefit.
Natto’s flavor and texture are famously polarizing. The stringy, slimy consistency and strong fermented smell put off many first-time tasters, including plenty of Japanese people. Some practical workarounds: mixing natto with a raw egg and soy sauce softens the flavor considerably, stirring it vigorously increases the stringiness (which some people prefer but others find worse), and adding mustard or chopped scallions shifts the flavor profile. Freeze-dried natto snacks are a less intense entry point but contain less of the live Bacillus bacteria and sticky gamma-polyglutamic acid that drive some of the metabolic effects.
How Natto Mimics a Diabetes Drug Class
One of the more surprising findings in natto research is that it inhibits dipeptidyl peptidase IV (DPP-IV), the same enzyme targeted by drugs like sitagliptin, saxagliptin, and linagliptin.3PubMed Central. Fermented soybean foods and diabetes These medications, prescribed to millions of people with type 2 diabetes worldwide, work by preventing DPP-IV from breaking down incretin hormones. Incretins tell the pancreas to release insulin when food arrives and tell the liver to slow glucose production. By keeping incretins active longer, DPP-IV inhibitors help smooth out blood sugar swings after meals.
Nobody has measured how strong natto’s DPP-IV inhibition is compared to a pharmaceutical dose of sitagliptin, and it is almost certainly far weaker. A serving of natto is not a replacement for a prescribed diabetes medication. But the parallel is worth knowing about because it provides a mechanistic explanation for why natto seems to flatten the post-meal glucose curve in human studies. It also raises the theoretical question of whether eating natto while taking a DPP-IV inhibitor drug could produce an additive effect, something that has not been studied but that a thoughtful physician might consider relevant.
The broader lesson is that natto is not just “healthy food, vaguely good for you.” Multiple independently studied components of natto, including the soy isoflavones, the vitamin K2, the nattokinase, the gamma-polyglutamic acid, and the DPP-IV inhibition, each point toward diabetes-related benefits through distinct pathways. Whether these effects add up to a clinically meaningful difference in a real person eating natto a few times a week remains the key unanswered question. The animal data are encouraging, the small human trials are promising, and the large-scale randomized clinical trial that would settle the matter has not been done.