What causes belching?

The ability to belch is almost universal. Belching, also known as burping (medically referred to as eructation), is the act of expelling gas from the stomach out through the mouth. The usual cause of belching is a distended (inflated) stomach caused by swallowed air. The distention of the stomach causes abdominal discomfort, and the belching expels the air and relieves the discomfort. The common reasons for swallowing large amounts of air (aerophagia) are gulping food or drink too rapidly, anxiety, and carbonated beverages. People are often unaware that they are swallowing air. "Burping" infants during bottle or breastfeeding is important in order to expel air in the stomach that has been swallowed with the formula or milk.

Excessive air in the stomach is not the only cause of belching. For some people, belching becomes a habit and does not reflect the amount of air in their stomachs. For others, belching is a response to any type of abdominal discomfort and not just to discomfort due to increased gas. Everyone knows that when they have mild abdominal discomfort, belching often relieves the problem. This is because excessive air in the stomach often is the cause of mild abdominal discomfort. As a result, people belch whenever mild abdominal discomfort is felt regardless of its cause.

Belching is not the simple act that many people think it is. Belching requires the coordination of several activities.

• The larynx must be closed-off so that any liquid or food that might return with the air from the stomach won't get into the lungs.
  
  
• This is accomplished by voluntarily raising the larynx as is done when swallowing.
  
  
• Raising the larynx also relaxes the upper esophageal sphincter so that air can pass more easily from the esophagus into the throat.
  
  
• The lower esophageal sphincter must open so that air can pass from the stomach into the esophagus.
  
  
• While all this is occurring, the diaphragm descends just as it does when a breath is taken.
  
  
• This increases abdominal pressure and decreases pressure in the chest.
  
  
• The changes in pressure promote the flow of air from the stomach in the abdomen to the esophagus in the chest.

One unusual type of belching has been described in aerophagic individuals who swallow air. It has been demonstrated that during some of their belches, room air enters the esophagus and is immediately expelled, giving rise to a belch. This in and out flow of air also is likely to be the explanation for the ability of many people to belch at will, even when there is little or no air in the stomach.

If the problem causing the discomfort is not excessive air in the stomach, then belching does not provide relief from the discomfort. When belching does not ease the discomfort, the belching should be taken as a sign that something may be wrong within the abdomen and the cause of the discomfort should be sought. Belching by itself, however, does not help the physician determine what may be wrong because belching can occur in virtually any abdominal disease or condition that causes abdominal discomfort.

What causes bloating?

It is important to distinguish between bloating and distention.

• Bloating is the subjective sensation (feeling) that the abdomen is larger than normal. Thus, bloating is a symptom akin to the symptom of discomfort.
  
  
• In contrast, distention is the objective determination (physical finding) that the abdomen is larger than normal. Distention can be determined by such observations as the inability to fit into clothes or looking down at the stomach and noting that it is clearly larger than normal.

In some instances, bloating may represent a mild form of distention since the abdomen does not become physically (visibly or measurably) enlarged until its volume increases by one quart. Nevertheless, bloating should never be assumed to be the same as distention. Bloating and even very mild cases of distention may be caused by relaxation of the muscles of the abdominal wall.

There are three ways in which abdominal distention can arise. The causes are an increase in air, fluid, or tissue within the abdomen. The diseases or conditions that cause an increase of any of these three factors are very different from one anther. Therefore, it is important to determine which of them is distending the abdomen.

There are two types of distention; continuous and intermittent.

• Continuous distention may be caused by the enlargement of an intra-abdominal (within the abdomen) organ, an intra-abdominal tumor, a collection of fluid around the intra-abdominal organs (ascites), or just plain obesity.
  
  
• Intermittent distention is usually due to the occasional accumulation of gas and/or fluid within the stomach, small intestine, or colon.

What causes flatulence?

Flatulence, also known as farting, is the act of passing intestinal gas from the anus. Gas in the gastrointestinal tract has only two sources. It is either swallowed air or is produced by bacteria that normally inhabit the intestines, primarily the colon. Swallowed air is rarely the cause of excessive flatulence.

The usual source is the production of excessive gas by intestinal bacteria. The bacteria produce the gas (hydrogen and/or methane) when they digest foods, primarily sugars and polysaccharides (for example, starch, cellulose), that have not been digested during passage through the small intestine. (The bacteria also produce carbon dioxide, but the carbon dioxide is so rapidly absorbed from the intestine that very little passes in flatus.)

Sugars

Sugars that are commonly poorly digested (maldigested) and malabsorbed are lactose, sorbitol, and fructose.

• Lactose is the sugar in milk. The absence of the enzyme lactase in the lining of the intestines, which is a genetic trait, causes the maldigestion. Lactase is important because it breaks apart the lactose so that it can be absorbed.
  
  
• Sorbitol is a commonly used sweetener in low calorie foods.
  
  
• Fructose is a commonly used sweetener in all types of candies and drinks.

Starches

Starches are another common source of intestinal gas. Starches are polysaccharides that are produced by plants and are composed of long chains of sugars, primarily fructose. Common sources of different types of starch include wheat, oats, potatoes, corn, and rice.

• Rice is the most easily digested starch and little undigested rice starch reaches the colon and the colonic bacteria. Accordingly, the consumption of rice produces little gas.
  
  
• In contrast, some of the starches in wheat, oats, potatoes, and, to a lesser extent, corn, all may reach the colon. These starches, therefore, may result in the production of appreciable amounts of gas.
  
  
• The starch in whole grains produces more gas than the starch in refined (purified) grains. Thus, more gas is formed after eating foods made with whole wheat flour than with refined wheat flour. This difference in gas production probably occurs because of the fiber (similar to a complex starch) present in the whole grain flour. Much of this fiber is removed during the processing of whole grains into refined flour.
  
  
• Finally, certain fruits and vegetables, for example, cabbage, also contain poorly digested starches that reach the colon and result in the formation of gas.
  
  
• Most vegetables and fruits contain cellulose, another type of polysaccharide that is not digested at all as it passes through the small intestine. However, unlike sugars and other starches, cellulose is used only very slowly by colonic bacteria. Therefore, the production of gas after the consumption of fruits and vegetables usually is not great unless the fruits and vegetables also contain sugars or polysaccharides other than cellulose.

Small amounts of air are continuously being swallowed and bacteria are constantly producing gas. Contractions of the intestinal muscles normally propel the gas through the intestines and cause the gas to be expelled. Flatulence (passing intestinal gas) prevents gas from accumulating in the intestines.

However, there are two other ways in which gas can escape the intestine.

• First, it can be absorbed across the lining of the intestine into the blood. The gas then travels in the blood and ultimately is excreted in the breath.
  
  
• Second, gas can be removed and used by certain types of bacteria within the intestine. In fact, most of the gas that is formed by bacteria in the intestines is removed by other bacteria in the intestines. (Thank goodness!)

What are the causes of intermittent abdominal bloating/distention?

Excessive production of gas

Excessive production of gas by bacteria is a common cause of intermittent abdominal bloating/distention. Bacteria can produce too much gas in three ways.

• First, the amount of gas that bacteria produce varies from individual to individual. In other words, some individuals may have bacteria that produce more gas, either because there are more of the bacteria or because their particular bacteria are better at producing gas.
  
  
• Second, there may be poor digestion and absorption of foods in the small intestine, allowing more undigested food to reach the bacteria in the colon. The more undigested food the bacteria have, the more gas they produce. Examples of diseases of that involve poor digestion and absorption include lactose intolerance, pancreatic insufficiency, and celiac disease.
  
  
• Third, bacterial overgrowth can occur in the small intestine. Under normal conditions, the bacteria that produce gas are limited to the colon. In some conditions, these bacteria spread into the small intestine. When this bacterial spread occurs, food reaches the bacteria before it can be fully digested and absorbed by the small intestine. Therefore, the bacteria in the small intestine have a lot of undigested food from which to form gas. This condition in which the gas-producing bacteria move into the small intestine is called bacterial overgrowth of the small intestine (bowel).

Excessive production of gas by bacteria usually is accompanied by flatulence. Increased flatulence may not always occur, however, since gas potentially can be eliminated in other ways - absorption into the body, utilization by other bacteria, or possibly, by elimination at night without the awareness of the gas-passer.

Physical obstruction

An obstruction (blockage) can occur virtually anywhere from the stomach to the rectum. When the blockage is temporary or partial, it can cause intermittent abdominal bloating/distention. For example, scarring of the pylorus (pyloric stenosis) can obstruct the opening from the stomach into the intestines, thereby blocking the complete emptying of the stomach. After meals, the stomach is normally filled with food and swallowed air. Then, during the next hour or two, the stomach secretes acid and fluid, which mix with the food and assist in digestion. As a result, the stomach distends further. When the obstruction is incomplete, the food, air, and fluid eventually pass into the intestines and the bloating/distention resolves.

An obstruction in the small bowel, which is most commonly due to adhesions from a previous surgery, is another cause of intermittent abdominal distention. To make matters worse, the distention that is caused by the physical obstruction stimulates both the stomach and intestines to secrete fluid, which adds to the distention.

Severe constipation or fecal impaction (hardened stool in the rectum) can also obstruct the flow of the intestinal contents and result in distention. In this case, however, the bloating/distention usually is constant and progressive and is relieved by bowel movements or removal of the impacted stool.

Functional obstruction

A functional obstruction is not caused by an actual physical blockage, but rather by the poor functioning of the muscles of the stomach or intestines that propel the intestinal contents. When these muscles are not working normally, the intestinal contents will accumulate and distend the abdomen. Examples of functional obstruction include:

• gastroparesis (paralysis of the stomach) of diabetes;
  
  
• chronic intestinal pseudo-obstruction, an unusual condition in which the muscles of the small intestine do not work normally; and
  
  
• Hirschsprung's disease, in which a small stretch of colonic muscle does not contract normally due to missing nerves.

There is accumulating scientific evidence that some patients with abdominal bloating and distention due to gas may have a functional abnormality of the intestinal muscles that prevents gas from being normally transported through the intestine and expelled. Instead, their gas accumulates in the intestine. Among patients with irritable bowel syndrome (IBS) with abdominal bloating or distention as an important symptom, the gas accumulates in the small intestine and not the colon. The gas accumulates during the day and is greatest in the evening.

Fats in food have an effect on the intestine that mimics a functional obstruction. Dietary fat reaching the small intestine causes transport of digesting food, gas, and liquid within the intestines to slow. This can promote the accumulation of food, gas, and liquid and lead to bloating and/or distention.

Dietary fiber or fiber used for treating constipation can cause bloating without increasing the production of gas in the intestine. It is believed that this sensation of bloating (and possibly even distention) is caused by the slowed passage of gas through the intestine which is caused by fiber. Of course, some types of fiber may lead to increased production of gas because they are digested to some extent by the colonic bacteria.

Intestinal hypersensitivity

Some people appear to be very sensitive (hypersensitive) to distention of their intestines, and they may feel bloated even with normal amounts of digesting food, gas, and fluid in the intestine after a meal. The bloating may be aggravated or even progress to distention if the meal contains substantial amounts of fat.

How are belching, bloating/distention, and flatulence evaluated?

Medical history

A patient's medical history is important because it directs the evaluation.

• If the bloating/distention is continuous rather than intermittent, then enlargement of abdominal organs, abdominal fluid, tumors, or obesity are probable causes.
  
  
• If the bloating/distention is associated with increased flatulence, then bacteria and excessive gas production are likely factors.
  
  
• If a diet history reveals the consumption of large amounts of milk or dairy products (lactose), sorbitol or fructose, then the maldigestion and malabsorption of these sugars may be the cause of the distention.
  
  
• When individuals complain of flatulence, it may be useful for them to count the number of times they pass gas for several days. This count can confirm the presence of excessive flatulence since the number of times gas is passed correlates well with the total amount (volume) of passed gas. As you might imagine, it is not easy to measure the amount of passed gas. It is normal to pass gas up to 20 times a day. (The average volume of gas passed daily is estimated to be about ¾ of a quart.)
  
  
• If an individual complains of excessive gas but passes gas fewer than 20 times per day, the problem is likely to be something other than too much gas. For example, the problem may be the foul odor of the gas (often due to ingestion of sulfur-containing foods), the lack of ability to control (hold back) the passing of gas, or the soiling of underwear with small amounts of stool when passing gas. All of these problems, like excessive gas, are socially embarrassing and may prompt individuals to consult a physician. These problems, however, are not due to excessive gas production, and their treatment is different.

Simple abdominal X-rays

Simple X-rays of the abdomen, particularly if they are taken during an episode of bloating or distention, can often confirm air as the cause of the distention since large amounts of air can be seen easily within the stomach and intestine. Moreover, the cause of the problem may be suggested by noting where the gas has accumulated. For example, if the air is in the stomach, emptying of the stomach is likely to be the problem.

Small intestinal X-rays

X-rays of the small intestine, in which barium is used to fill and outline the small intestine, are particularly useful for determining if there is an obstruction of the small intestine.

Gastric emptying studies

These studies measure the ability of the stomach to empty its contents. For gastric emptying studies, a test meal that is labeled with a radioactive substance is eaten and a Geiger counter-like device is placed over the abdomen to measure how rapidly the test meal empties from the stomach. A delay in emptying of the radioactivity from the stomach can be caused by any condition that reduces emptying of the stomach (for example, pyloric stenosis, gastroparesis).

Ultrasound, CT scan, and MRI

Imaging studies, including ultrasound examination, computerized tomography (CT), and magnetic resonance imaging (MRI), are particularly useful in defining the cause of distention that is due to enlargement of the abdominal organs, abdominal fluid, and tumor.

Maldigestion and malabsorption tests

Two types of tests are used to diagnose maldigestion and malabsorption; general tests and specific tests.

The best general test is a 72 hour collection of stool that measures fat in the stool; if maldigestion and/or malabsorption exist because of pancreatic insufficiency or diseases of the lining of the small intestine for example, celiac disease), the amount of fat in the stool will increase.

Specific tests can be done for maldigestion of individual sugars that are commonly maldigested, including lactose (the sugar in milk) and sorbitol (a sweetener in low calorie foods). The specific tests require ingestion of the sugars followed by hydrogen/methane breath testing. (See below.) The sugar fructose, a commonly used sweetener, like lactose and sorbitol, also may cause abdominal bloating/distention and flatulence. However, the problem that can occur with fructose is different from that with lactose or sorbitol. Thus, as already described, lactose and sorbitol may be poorly digested by the pancreatic enzymes and small intestine. On the other hand, fructose may be digested normally but may pass so rapidly through the small intestine that there is not enough time for digestion and absorption to take place.

Hydrogen/methane breath tests

The most convenient way to test for bacterial overgrowth of the small intestine is hydrogen/methane breath testing. Normally, the gas produced by the bacteria of the colon is composed of hydrogen and/or methane. For hydrogen/methane breath testing, a non-digestible sugar, lactulose, is consumed. At regular intervals following ingestion, samples of breath are taken for analysis. When the lactulose reaches the colon, the bacteria form hydrogen and/or methane. Some of the hydrogen or methane is absorbed into the blood and eliminated in the breath where it can be measured in the samples of breath.

In normal individuals, there is one peak of hydrogen or methane when the lactulose enters the colon. In individuals who have bacterial overgrowth, there are two peaks of hydrogen or methane. The first occurs when the lactulose passes and is exposed to the bacteria in the small intestine. The second occurs when the lactulose enters the colon and is exposed to the colonic bacteria. Hydrogen breath testing for overgrowth also may be done utilizing lactose, glucose, sorbitol, or fructose as the test sugar.

How is excessive intestinal gas treated?

The treatment of excessive intestinal gas depends on the cause.

• If there is maldigestion of specific sugars-lactose, sorbitol, or fructose--the offending sugars can be eliminated from the diet.
  
  
• In the case of lactose in milk, an alternative treatment is available. Enzymes that are similar to intestinal lactase can be added to the milk in order to break down the lactose prior to its ingestion so that it can be absorbed normally. Some people find that yogurt, in which the lactose has been broken down partially by bacteria, produces less gas than milk.
  
  
• There also are certain types of vegetables and fruits that contain types of starches that are poorly digested by people but well digested by bacteria. These include beans, lentils, cabbage, brussel sprouts, onions, carrots, apricots, and prunes. Reducing the intake of these vegetables and fruits, as well as foods made from whole grains, should reduce gas and flatulence. However, the list of gas-producing foods is rather long, and it may be difficult to eliminate them all without severely restricting the diet.
  
  
• When maldigestion is due to pancreatic insufficiency, then supplemental pancreatic enzymes can be ingested with meals to replace the missing enzymes.
  
  
• If maldigestion and/or malabsorption is caused by disease of the intestinal lining, the specific disease must be identified, most commonly through a small bowel biopsy. Then, treatment can be targeted for that condition. For example, if celiac disease is found on the biopsy, a gluten-free diet can be started.
  
  
• An interesting form of treatment for excessive gas is alpha-D-galactosidase, an enzyme that is produced by a mold. This enzyme, commercially available as Beano, is consumed as either a liquid or tablet with meals. This enzyme is able to break down some of the difficult-to-digest polysaccharides in vegetables so that they may be absorbed. This prevents them from reaching the colonic bacteria and causing unnecessary production of gas. Beano has been shown to be effective in decreasing the incidence of intestinal gas.
  
  
• Two other types of treatment have been promoted for the treatment of gas; simethicone (Phazyme; Flatulex; Mylicon; Gas-X; Mylanta Gas) and activated charcoal. It is unclear if simethicone has an effect on gas in the stomach. However, it has no effect on the formation of gas in the colon. Moreover, in the stomach, simethicone would be expected only to affect swallowed air, which, as previously mentioned, is an uncommon cause of excessive intestinal gas. Nevertheless, some individuals are convinced that simethicone helps them. Activated charcoal has been shown to reduce the formation of gas in the colon, though the way in which it does so is unknown.
  
  
• If there is a physical obstruction to the emptying of the stomach or passage of food, liquid, and gas through the small intestine, then surgical correction of the obstruction is required. If the obstruction is functional, medications that promote activity of the muscles of the stomach and small intestine are given. Examples of these medicines are erythromycin or metoclopramide (Reglan).
  
  
• Bacterial overgrowth of the small bowel usually is treated with antibiotics. However, this treatment is frequently only temporarily effective or not effective at all. When antibiotics provide only a temporary benefit, it may be necessary to treat patients intermittently or even continuously with antibiotics. If antibiotics are not effective, probiotics (for example, lactobacillus) can be tried although their use in bacterial overgrowth has not been studied. This condition may be difficult to treat.

Intestinal Gas At A Glance

• The usual cause of belching is excessive gas in the stomach that comes from swallowed air. However, discomfort in the abdomen for any reason may also cause belching. Therefore, belching does not always indicate the presence of excessive gas in the stomach.

• Bloating is the subjective feeling that the abdomen is enlarged but does not necessarily mean that the abdomen is, in fact, enlarged. Distention is the objective enlargement of the abdomen.

• Continuous distention of the abdomen is usually caused by fluid, tumors, enlarged organs, or fat within the abdomen.

• Intermittent distention of the abdomen may be caused by excessive formation of intestinal gas, as well as physical or functional obstruction of the intestines.

• Flatulence results from the production of gas by bacteria within the intestines when they digest sugars and polysaccharides.

• Excessive production of gas and increased flatulence may occur because of: (1) the greater ability of some bacteria to produce gas; (2) maldigestion or malabsorption of sugars and polysaccharides; and (3) bacterial overgrowth of the small intestine.

• Belching, bloating/distention, and flatulence are evaluated with a medical history, simple abdominal X-rays, small intestinal X-rays, gastric emptying studies, ultrasound examination, computerized tomography (CT), magnetic resonance imaging (MRI), tests for maldigestion and malabsorption, and hydrogen breath testing.

• The treatment of excessive intestinal gas depends on the underlying cause and may include dietary changes, medications that reduce the amount of gas, medications that stimulate the muscles of the intestine, or antibiotics.

Previous contributing medical editor: Leslie J. Schoenfield, MD, PhD

sourceLast Editorial Review: 3/10/2009

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