Why do green vegetables and legumes cause gas?

The body does not digest and absorb all of the carbohydrates (sugar, starches, and fiber) that many vegetables contain. These undigested carbohydrates eventually move into the large intestine (the last part of your digestive tract). In the large intestine, harmless bacteria break down the undigested food. This breakdown of carbohydrates by bacteria produces hydrogen gas, carbon dioxide, and sometimes methane gas. Legumes and green vegetables contain large amounts of these carbohydrates (especially fiber) that is not easily absorbed by the digestive tract, which leads to higher amounts of gas production in the large intestine.