Flatulence or the passing of flatus or gas in common terminology is called by swallowed air (aerophagia) or by eating some foods. If the gas passed out is analysed in the laboratory we can judge it originated from aerophagia or from the gastro-intestinal tract. If it has originated from Aerophagia it will contain mostly nitrogen along with oxygen and carbon dioxide. If it has originated from the gastrointestinal tract it will contain mainly carbon monoxide, hydrogen and methane.
Our body does not digest some foods like sugars, starches and fibers because of shortage or absence or certain enzymes. These foods move on to the large intestine where some harmless bacteria break down the food producing hydrogen, carbon dioxide and sometimes methane. These are passed out as flatus through the rectum.
Some foods that cause gas in one person may not do so in another. This is due to the presence of different kinds of bacteria. Some bacteria produce gas by breaking down the food. While other bacteria present there destroy the gas produced by the former. The number of bacteria of both kinds present in the large intestine can tell us if the person produces gas with a foul smell or not. Carbohydrate rich foods produce more gas while protein and fats produce lesser gas.
Beans, cabbage, broccoli, Brussels, sprouts, asparagus and while grains contain a complex sugar raffinose that causes gas.
Starches containing foods like potatoes, corn, noodles and wheat also produce gas in the large intestine. Onions, artichokes, pears and wheat also contain fructose which causes gas. This fructose is also present in some soft drinks. Sorbitol, a sugar found in apples, pears, peaches and prunes, as well as dark bear and red wine are also among the foods that cause flatulence. Soluble fiber that is present in oat bran beans, peas and most fruits in digested in the large intestine and causes gas whereas insoluble fiber passed out unchanged through the rectum.