Miso, a traditional Japanese paste made by fermenting soybeans, grain, and salt, is recognized for its savory taste and health benefits. When people consume miso, they sometimes experience intestinal gas, bloating, or flatulence. Miso can cause gas because of its main ingredient, the soybean, and the complex biological changes that occur during its production. This article explains the specific soybean components responsible for this effect and how fermentation alters the potential for gas production.
Why Soybeans are the Main Cause
The primary source of gas production from miso can be traced back to the carbohydrates naturally present in the raw soybean. Soybeans contain significant amounts of non-digestible sugars known as oligosaccharides, specifically raffinose and stachyose. These compounds, known as the raffinose family of oligosaccharides (RFOs), are common in legumes.
Humans lack a specific digestive enzyme called alpha-galactosidase in their small intestine, which is needed to break the chemical bonds in these RFOs. Because they cannot be properly digested or absorbed, these complex sugars pass largely intact into the large intestine. Once there, the dense population of gut bacteria begins to rapidly ferment them.
This bacterial fermentation process generates gases, predominantly hydrogen, carbon dioxide, and sometimes methane. The buildup of these gases within the colon leads to the symptoms of flatulence and abdominal discomfort. Studies have shown that the presence of these RFOs is directly linked to increased gas production and flatulence frequency.
How Fermentation Changes the Equation
The process of fermenting soybeans into miso offers a notable difference in gas potential compared to eating unfermented soy products. Miso is made by mixing soybeans with koji—rice, barley, or soybeans inoculated with the mold Aspergillus oryzae—and then fermenting the mixture with salt. This fermentation introduces beneficial microbes, such as Lactobacillus plantarum, which produce the very enzyme that humans lack: alpha-galactosidase.
The action of these microbial enzymes actively breaks down the gas-causing oligosaccharides, like raffinose and stachyose, into simpler, more digestible sugars before consumption. This partial pre-digestion significantly reduces the concentration of RFOs, potentially making miso less gas-inducing than eating unfermented soybeans. Fermentation lasting 120 hours with Lactobacillus plantarum has been shown to reduce the total RFO content in soybeans substantially.
The second factor is the introduction of probiotics, the live bacteria and yeasts that develop during the miso-making process. While RFO breakdown reduces gas, the introduction of new probiotic strains and remaining prebiotics can still cause temporary digestive upset. The new bacteria feeding on undigested fibers produce gas as a byproduct, especially as the gut microbiome adjusts. This means that even highly fermented miso can cause gas in sensitive individuals or those new to fermented foods.
Practical Ways to Reduce Miso-Related Gas
To enjoy the distinct flavor and benefits of miso while minimizing gas production, there are several practical steps to follow. The most effective approach is to begin with very small servings and gradually increase the amount over time. This allows the gut microbiome a chance to adapt to the new probiotic strains and the residual fermentable carbohydrates.
It is advised to avoid boiling miso, as excessive heat can destroy the beneficial, live bacteria and enzymes that help with digestion. Instead, add miso paste to liquids or soups after they have been removed from the heat source and cooled slightly. This preserves the cultures that assist in breaking down some of the remaining compounds. Proper hydration is also important, as drinking enough water can help flush the digestive system and prevent constipation, which contributes to gas buildup. Pairing miso with easily digestible foods can also assist the digestive process.