Gas in the Digestive Tract Comes From Two Causes

• 1) Swallowing Air (aerophagia)
  Everyone swallows small amounts of air when eating and drinking, but eating or drinking rapidly, as well as chewing gum, smoking, or wearing loose dentures can cause some people to take in more air. Belching or burping, is the way most swallowed air leaves the stomach. The remaining gas is partially absorbed into the * small intestine and a small amount goes into the large intestine and is released through the rectum (flatus).
• 2) Breakdown of Certain Undigested Foods
  When we eat food and drink liquids, they are not in a form that the body can use as nourishment. Our food and drink must be changed into smaller * molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy. Digestion begins in the mouth, when we chew and swallow, and is completed in the small intestine. The chemical process varies somewhat for different kinds of food. Flatulence occurs when certain foods do not break down completely in the stomach and small intestine. The undigested food then passes into the large intestine (colon), where hundreds of harmless and normal bacteria are waiting to break down the food, producing hydrogen, carbon dioxide, and, methane (CH4) gases, which are released through the rectum (flatus).

According to the authors of the book “Intestinal Gas”, Michael D. Levitt, M.D., and John H. Bond, M.D., “only about one third of us excrete methane (CH4)”. Wondering if you are in the 1/3 category? “A person who produces methane will have stools that consistently float in water, the reason for this being methane gets trapped in the stool, making it less dense than water”.

Source: digestive.niddk.nih.gov