Gas occurs throughout the gastrointestinal tract. This includes the esophagus, stomach, small intestine and large intestine. Bloating is an unpleasant sensation of fullness or swelling in the abdomen that is sometimes (but not always) due to the buildup of gas in the abdomen.

Most gas results from swallowing air (aerophagia). It is normal to swallow a small amount of air when eating or drinking. However, when people eat or drink too quickly, they tend to swallow more air. Much of the air ends up in the stomach in the form of gas. A small amount may also enter the small intestine.

Gas also develops in the large intestine due to the presence of harmless bacteria. The bacteria consume undigested food (e.g., carbohydrates) and gas is the byproduct.

Although the exact composition of intestinal gas varies among people, more than 99 percent of intestinal gas is composed of nitrogen, oxygen, carbon dioxide, hydrogen and sometimes methane. These gases are odorless. The unpleasant odor associated with intestinal gas is due to the bacteria, which release sulfur. Foods rich in sulfur include cauliflower, eggs and meat. These foods often cause flatus with offensive odor.

Most people produce between one and four pints (about 0.5 to 1.9 liters) of gas per day. Some gas in the digestive tract is absorbed into the blood through the gastrointestinal (GI) lining and excreted by the lungs. The remaining gas is released in one of two ways:

• Burping (belching or eructation). The expulsion of gas in the stomach through the mouth. It may be voluntary or involuntary. Belching often occurs when the stomach is full. It is normal to occasionally belch during or after meals. Some individuals may experience a tight feeling in their chest or stomach just before belching. This tightness generally goes away after the belch.
  
  
• Flatus (flatulence). The expulsion of gas in the intestines through the anus. It may be voluntary or involuntary and quiet or noisy. The expulsion causes the anal sphincter (the circular muscle that closes the anus) to vibrate. The amount of gas, force of expulsion and resistance from the sphincter determine the volume and pitch of the flatus. Sometimes, an individual may also pass a small amount of stool when passing gas.

The average person passes gas 14 to 23 times per day. Many patients who report gas-related symptoms, such as bloating, do not actually produce more gas than others, but are instead more sensitive to the normal amount of gas in their bodies.

Bloating due to excessive gas is actually rare. A number of conditions can make a person more sensitive to gas in their bodies. Some are related to the GI system, such as irritable bowel syndrome (IBS), whereas others may be psychological in origin. For instance, people with eating disorders often report bloating.

Abdominal pain often accompanies bloating. This pain may be mild and dull or intense and sharp. It may feel constant or may come and go. Jabbing pain, cramps, and/or a knotted feeling in the stomach may occur. This pain can occur anywhere in the abdomen and can change location quickly.

Abdominal pain most often occurs when gas becomes trapped in the colon. Gas is most likely to get caught in areas where the colon bends sharply (e.g., the upper right and left parts of the abdomen). When it occurs in the upper left side, this pain may be mistaken for a heart problem. When it occurs in the right side, it may be mistaken for gallstones or appendicitis. However, in most cases, the pain subsides with the flatus or a bowel movement.

According to the American College of Gastroenterology (ACG), up to 7 percent of the general population complain of excessive or bothersome belching and about 11 percent report frequent bloating. However, some populations do frequently produce more gas than others. These individuals include:

• Smokers
  
  
• Vegetarians
  
  
• People who regularly drink carbonated beverages or beverages with high levels of sulfates (e.g., beer, wine)
  
  
• People who regularly chew gum
  
  
• People who regularly eat a lot of gas-producing foods, such as cabbage, brussel sprouts and other foods containing a carbohydrate called raffinose
  
  
• People with certain chronic intestinal conditions or food intolerances (e.g., irritable bowel syndrome)