Beans make you fart because they contain sugars your body literally cannot digest. These sugars, called oligosaccharides, pass through your stomach and small intestine completely intact, then arrive in your colon where trillions of bacteria feast on them and produce gas as a byproduct. It’s not a flaw in your diet. It’s a predictable chemical reaction happening in your gut every time you eat legumes.
The Sugars Your Body Can’t Break Down
Most foods get broken down by enzymes in your small intestine, absorbed into your bloodstream, and used for energy. Beans contain two specific sugars, raffinose and stachyose, that skip this entire process. The reason is straightforward: the lining of your small intestine doesn’t produce the enzyme needed to split these sugars apart. That enzyme is called alpha-galactosidase, and humans simply don’t make it. Neither do dogs, pigs, or any other single-stomached animal.
So these sugars travel intact all the way to your large intestine, where hundreds of bacterial species live. Those bacteria do have the enzymes to break down raffinose and stachyose, and they go to work immediately. The fermentation process produces a mix of gases: hydrogen, carbon dioxide, and in some people, methane. Your body has about 200 ml of gas sitting in the digestive tract at any given time, but a meal high in these undigestible sugars can increase that volume substantially. The gas has to go somewhere, and it does.
Some Beans Are Worse Than Others
Not all legumes pack the same punch. The concentration of gas-producing sugars varies dramatically across bean types. Lupin seeds top the list at 11 to 16% oligosaccharide content by weight. Soybeans come in around 6 to 8%. Common beans (kidney, black, pinto) range widely from less than 1% up to 8%, depending on the variety. Chickpeas fall between 2 and 7.6%, and lentils are similar at 1.8 to 7.5%.
On the gentler end, faba beans contain only 1 to 4.5% of these sugars, and peanuts (technically a legume) are the lowest at just 0.12 to 0.76%. If you’re trying to eat more legumes without clearing the room, lentils and faba beans are a reasonable starting point. Mung beans also sit on the lower end at around 3%.
Why the Gas Isn’t All Bad
The same fermentation that produces gas also produces compounds that are genuinely good for your colon. When bacteria break down bean fiber and resistant starch, they create short-chain fatty acids, small molecules that play several important roles in your body.
One of these, called butyrate, directly fuels the cells lining your colon, reduces inflammation, and helps protect against colon cancer. Another, acetate, makes up about 65% of the short-chain fatty acids produced in your colon and lowers the pH enough to inhibit harmful bacteria while improving absorption of calcium, iron, and sodium. A third, propionate, gets absorbed into your bloodstream and travels to your liver, where it helps reduce cholesterol levels. These same fatty acids also play a role in appetite regulation and fat metabolism. Eating beans regularly fosters the growth of beneficial gut bacteria and significantly boosts production of all three. The flatulence is essentially a side effect of your gut microbiome doing useful work.
How to Reduce the Gas
The most effective strategy is proper soaking. For every pound of dried beans, use at least ten cups of boiling water. Boil for two to three minutes, then cover and let the beans sit overnight. That initial boil breaks open the cell membranes of the beans, releasing the oligosaccharides into the water. The critical step: throw out the soaking water. If you cook beans in the same liquid they soaked in, you’re just putting those sugars right back.
Cooking technique matters too. Beans need to be fully softened before you add anything acidic like tomatoes, vinegar, or molasses. Acids prevent legumes from softening properly, and firmer beans are harder to digest. Pressure cooking can also help by breaking down more of these compounds through higher temperatures.
If preparation alone isn’t enough, over-the-counter enzyme supplements (Beano is the most well-known) provide the exact enzyme your body lacks. You take it as a tablet right before eating or with your first bite, and it breaks down the oligosaccharides before they reach your colon. The evidence on how well these work is mixed, and because they’re supplements rather than regulated drugs, the actual enzyme concentration can vary between brands and batches. Still, many people find them helpful, especially when eating beans they haven’t had a chance to soak properly.
Your Gut Adapts Over Time
People who eat beans regularly tend to experience less gas than people who eat them occasionally. This isn’t just anecdotal. When you consistently feed your gut bacteria the same types of fiber, the microbial population shifts. Bacteria that efficiently process oligosaccharides grow in number, and the fermentation process becomes less chaotic and gas-heavy. If you’re adding beans back into your diet after a long break, start with small portions a few times a week and increase gradually. Your colon’s bacterial ecosystem needs a few weeks to adjust, but it does adjust.