Natto is a traditional Japanese food made from whole soybeans fermented with the bacterium, Bacillus subtilis var. natto. The process transforms boiled soybeans into a pungent, sticky, and stringy dish valued for its nutritional profile. While its unique texture and aroma are often the first things people notice, many new consumers quickly wonder about a common side effect: gas and bloating. This digestive reaction stems directly from the unique chemical composition of the fermented soybeans.
The Science Behind Natto’s Gas Production
The primary reason natto can cause noticeable gas production is a combination of three factors related to its complex carbohydrate content. Like all legumes, the soybeans used to make natto contain a high concentration of dietary fiber, which the human small intestine cannot break down. This fiber content is substantial, with a typical 100-gram serving of natto containing around 5.4 grams of fiber.
This indigestible fiber then travels into the large intestine, where resident gut bacteria begin to ferment it. This fermentation process generates gases such as hydrogen, carbon dioxide, and sometimes methane, leading to bloating and flatulence. The process is further fueled by specific fermentable sugars known as oligosaccharides, particularly raffinose and stachyose, which are abundant in soybeans.
Although the fermentation process that creates natto is beneficial—it reduces anti-nutrients and makes the protein more digestible—it does not eliminate all gas-producing compounds. The Bacillus subtilis bacteria break down some complex starches into simpler oligosaccharides, which remain largely indigestible by human enzymes. Since these water-soluble sugars pass into the colon unchanged, they become a direct food source for intestinal microbes, increasing the amount of gas produced as a metabolic byproduct.
This mechanism explains why some individuals find natto more gas-producing than other fermented foods. The combination of high fiber and specific fermentable oligosaccharides means a significant amount of material reaches the large intestine for metabolism by the gut flora. The resulting gases expand the colon, which is perceived as bloating and discomfort.
Adapting to Natto: Minimizing Digestive Side Effects
The most effective strategy for minimizing the digestive side effects of natto is to allow the gut microbiome time to adjust to its unique composition. New consumers should begin with a small portion, such as a single tablespoon, rather than a full serving. This gradual introduction prevents a sudden influx of fermentable fibers and oligosaccharides, which can overwhelm the system and cause acute discomfort.
Consistency in consumption is another way to build tolerance over time. Regularly eating small amounts of natto encourages the growth of specific gut bacteria better equipped to process the complex carbohydrates found in soybeans. As the gut flora adapts, the amount of gas produced during fermentation should begin to decrease.
Strategic food pairing can help manage potential digestive upset. Serving natto over a staple like plain rice provides a low-fiber base that dilutes the concentration of fermentable material. Mixing natto with toppings like chopped green onions, kimchi, or a raw egg can help distribute the load of the high-fiber food.
Proper chewing and hydration are beneficial for digestion when consuming high-fiber foods. Chewing thoroughly mechanically breaks down the soybeans, assisting the initial stages of digestion. Drinking adequate water alongside the meal helps the fiber move smoothly through the intestines, reducing the sensation of heaviness or stagnation.