How is gas caused?

Gas in the digestive tract, which includes the oesophagus, stomach, small intestine and large intestine, comes from two sources:

  • Ingestion of air, which is usually oxygen and nitrogen contained in the inhaled atmospheric air
  • The physiological breakdown of certain foods that have not been digested by harmless bacteria, which occurs in the large intestine and produces hydrogen, methane and carbon dioxide

Air ingestion

Air ingestion or air swallowing is a common cause of gas accumulation in the stomach. We all swallow small amounts of air when we eat or drink. However, by eating or drinking quickly, chewing gum, smoking or keeping loose dentures, some people tend to swallow more air.

Burping or belching expels from the stomach the largest amount of air, which contains nitrogen, oxygen, carbon dioxide, that one swallows. The remaining gas moves to the small intestine, where it is partially absorbed. A small amount enters the large intestine to be released through the rectum. The stomach also releases carbon dioxide when stomach acid and bicarbonate are mixed. But most of these gases are absorbed by the blood and do not enter the large intestine.

Breakdown of undigested food

The body does not digest and absorb in the small intestine some carbohydrates, such as sugar, starch and fiber found in many foods because of a deficiency or absence of certain enzymes.

This undigested food then passes from the small intestine into the large intestine, where normal, non-harmful bacteria break down the food to produce hydrogen, carbon dioxide and, for 1/3 of people, methane. These gases eventually exit the rectum.

People who produce methane do not necessarily release more gases, nor do they have unique symptoms. A person who produces methane simply expels stool that floats in the water. Research has not yet shown why some people produce methane and others do not.

Foods that produce gas in one person may not in another. Some common bacteria in the colon may destroy the hydrogen produced by other bacteria. The balance of the two types of bacteria may explain why some people have more gas than others.