Studies on young adults have shown that the average person generates 1 to 3 pints of gas a day. Studies have shown that intestinal gas is composed of various amounts of oxygen, nitrogen, carbon dioxide, hydrogen and methane. Oxygen, nitrogen and carbon dioxide come from swallowed air while hydrogen and methane are produced in the colon by bacteria acting on food residue. Careful analysis of intestinal gas has shown that about ninety percent is ingested air and only ten percent is actually formed in the intestine. Although less than one percent of gas is odorous, intestinal bacteria produce several sulphur containing compounds that are the primary odour culprits.
Gas is released from the intestinal tract via the mouth by belching, or via the rectum as flatus.
A normal individual emits flatus about 12 to 25 times per day. In the normal situation, swallowed air remains in the stomach for a period of time. It passes out of the stomach in small amounts at regular intervals and passes rapidly through the small intestine by means of normal propulsive contractions that are totally asymptomatic. The gas enters the colon where it passes more slowly but is eventually released through the anus in small amounts. Almost no air is swallowed during sleep, but gas is passed out of the colon during sleep, thus at the beginning of the day there is minimal gas in the intestine and more later in the day.