Why Do Legumes Such as Chickpeas and Beans Cause Gas?

Legumes cause gas because the complex sugars they contain, called oligosaccharides, cannot be broken down in the human digestive system. These sugars pass into the large intestine without being absorbed in the small intestine, and gas is released as the bacteria there ferment them.

What Primarily Increases Gas?
These sugars, known as the raffinose family, are found in almost all legumes, including chickpeas, beans, and lentils. Because the human body lacks the enzyme needed to break them down, unlike other easily digested carbohydrates, these sugars go straight to the large intestine. Cooking dried legumes without soaking them also increases gas; soaking water dissolves some of these sugars and separates them from the legumes. If that water is not discarded, most of the sugars remain in the dish.

How Can Gas Be Reduced?
Soaking dried legumes in water for at least 8–12 hours before cooking and discarding that water removes a significant portion of the oligosaccharides; cook the legumes in completely fresh water, not in the soaking water. Bringing the cooking water to a boil one time and discarding it, then continuing to cook the legumes in a second batch of fresh water, can strengthen the same effect; this additional step removes some more of the gas-producing compounds. It is also important for the body to get used to eating legumes gradually; the gut flora of someone who eats legumes infrequently is not used to processing these sugars, and gas symptoms generally decrease over time as the amount consumed is increased gradually.

Some spices, such as cumin, fennel, or bay leaf, are traditionally added to the cooking water to ease gas; exactly how these spices produce this effect is not clear, but it is a common practice. If using canned legumes, pouring off the liquid in the can and rinsing the legumes with clean water may also help remove some of the gas-producing compounds.

There are also differences among types of legumes: peeled, small legumes such as red lentils contain fewer oligosaccharides than chickpeas and dried beans and generally cause less gas, so lentils may feel easier for people with digestive sensitivity. Some supplements containing the enzyme alpha-galactosidase, taken before a meal, are also available on the market; this enzyme performs the very sugar-breaking function that the body lacks and may reduce gas symptoms in some people.

The cooking method can also make some difference: legumes cooked at high pressure and temperature in a pressure cooker may break down some gas-producing compounds somewhat more than ordinary boiling, but this difference is not as pronounced as the effect of soaking. Letting cooked legumes rest briefly and cool may also make them somewhat easier to digest for some people; freshly cooked legumes served hot may cause somewhat more discomfort in some people than cooled legumes. Chewing legumes thoroughly and eating slowly can also ease the burden on the digestive system; large pieces swallowed quickly are less broken down in the mouth and stomach and can make digestion somewhat more difficult.