intestinal gas

The gut contains < 200 mL of gas, whereas daily gas expulsion averages 600 to 700 mL after consuming a standard diet plus 200 g of baked beans. About 75% of flatus is derived from colonic bacterial fermentation of ingested nutrients and endogenous glycoproteins. Gases include hydrogen (H2), methane (CH4), and carbon dioxide (CO2). Flatus odor correlates with H2 sulphide concentrations. Swallowed air (aerophagia) and diffusion from the blood into the lumen also contribute to intestinal gas. Gas diffuses between the lumen and the blood in a direction that depends on the difference in partial pressures. Thus, most nitrogen (N2) in the lumen originates from the bloodstream, and most H2 in the bloodstream originates from the lumen.


There are 3 main gas-related complaints: excessive belching, distention (bloating), and excessive flatus, each with a number of causes (see Table 11: Symptoms of GI Disorders: Some Causes of Gas-Related Complaints). Infants 2 to 4 mo of age with recurrent crying spells often appear to observers to be in pain, which in the past has been attributed to abdominal cramping or gas and termed colic. However, studies show no increase in H2 production or in mouth-to-cecum transit times in colicky infants. Hence, the cause of infantile colic remains unclear (see Approach to the Care of Normal Infants and Children: Colic).

Excessive belching: Belching (eructation) results from swallowed air or from gas generated by carbonated beverages. Aerophagia occurs normally in small amounts during eating and drinking, but some people unconsciously swallow air repeatedly while eating or smoking and at other times, especially when anxious or in an attempt to induce belching. Excessive salivation increases aerophagia and may be associated with various GI disorders (eg, gastroesophageal reflux disease), ill-fitting dentures, certain drugs, gum chewing, or nausea of any cause.

Most swallowed air is eructated. Only a small amount of swallowed air passes into the small bowel; the amount is apparently influenced by position. In an upright person, air is readily belched; in a supine person, air trapped above the stomach fluid tends to be propelled into the duodenum. Excessive eructation may also be voluntary; patients who belch after taking antacids may attribute the relief of symptoms to belching rather than to antacids and may intentionally belch to relieve distress.

Distention (bloating): Abdominal bloating may occur in isolation or along with other GI symptoms in patients with functional disorders (eg, aerophagia, nonulcer dyspepsia, gastroparesis, irritable bowel syndrome) or organic disorders (eg, ovarian cancer, colon cancer). Gastroparesis (and consequent bloating) also has many nonfunctional causes, the most important of which is autonomic visceral neuropathy due to diabetes; other causes include postviral infection, drugs with anticholinergic properties, and long-term opiate use. However, excessive intestinal gas is not clearly linked to these complaints. In most healthy people, 1 L/h of gas can be infused into the gut with minimal symptoms. It is likely that many symptoms are incorrectly attributed to “too much gas.”

On the other hand, some patients with recurrent GI symptoms often cannot tolerate small quantities of gas: Retrograde colonic distention by balloon inflation or air instillation during colonoscopy often elicits severe discomfort in some patients (eg, those with irritable bowel syndrome) but minimal symptoms in others. Similarly, patients with eating disorders (eg, anorexia nervosa, bulimia) often misperceive and are particularly stressed by symptoms such as bloating. Thus, the basic abnormality in patients with gas-related symptoms may be a hypersensitive intestine. Altered motility may contribute further to symptoms.

Excessive flatus: There is great variability in the quantity and frequency of rectal gas passage. As with stool frequency, people who complain of flatulence often have a misconception of what is normal. The average number of gas passages is about 13 to 21/day. Objectively recording flatus frequency (using a diary kept by the patient) is a first step in evaluation.
Sidebar 1

Essay on Flatulence

(First printed in the 14th Edition of The Merck Manual)

Flatulence, which can cause great psychosocial distress, is unofficially described according to its salient characteristics: (1) the “slider” (crowded elevator type), which is released slowly and noiselessly, sometimes with devastating effect; (2) the open sphincter, or “pooh” type, which is said to be of higher temperature and more aromatic; (3) the staccato or drumbeat type, pleasantly passed in privacy; and (4) the “bark” type (described in a personal communication) is characterized by a sharp exclamatory eruption that effectively interrupts (and often concludes) conversation. Aromaticity is not a prominent feature. Rarely, this usually distressing symptom has been turned to advantage, as with a Frenchman referred to as “Le Petomane,” who became affluent as an effluent performer who played tunes with the gas from his rectum on the Moulin Rouge stage.

Flatus is a metabolic byproduct of intestinal bacteria; almost none originates from swallowed air or back-diffusion of gases (primarily N2) from the bloodstream. Bacterial metabolism yields significant volumes of H2, CH4, and CO2.

H2 is produced in large quantities in patients with malabsorption syndromes and after ingestion of certain fruits and vegetables containing indigestible carbohydrates (eg, baked beans), sugars (eg, fructose), or sugar alcohols (eg, sorbitol). In patients with disaccharidase deficiencies (most commonly lactase deficiency), large amounts of disaccharides pass into the colon and are fermented to H2. Celiac disease, tropical sprue, pancreatic insufficiency, and other causes of carbohydrate malabsorption should also be considered in cases of excess colonic gas.

CH4 is also produced by colonic bacterial metabolism of the same foods (eg, dietary fiber). However, about 10% of people have bacteria that produce CH4 but not H2.

CO2 is also produced by bacterial metabolism and generated in the reaction of HCO3– and H+. H+ may come from gastric HCl or from fatty acids released during digestion of fats—the latter sometimes produces several hundred mEq of H+. The acid products released by bacterial fermentation of unabsorbed carbohydrates in the colon may also react with HCO3– to produce CO2. Although bloating may occasionally occur, the rapid diffusion of CO2 into the blood generally prevents distention.

Diet accounts for much of the variation in flatus production among individuals, but poorly understood factors (eg, differences in colonic flora and motility) may also play a role.

Despite the flammable nature of the H2 and CH4 in flatulence, working near open flames is not hazardous. However, gas explosion, even with fatal outcome, has been reported during jejunal and colonic surgery and colonoscopy, when diathermy was used during procedures in patients with incomplete bowel cleaning.

Table 11

Some Causes of Gas-Related Complaints

Suggestive Findings
Diagnostic Approach


Aerophagia (swallowing air)
With or without awareness of swallowing air

Sometimes in patients who smoke or chew gum excessively

Sometimes in patients who have esophageal reflux or ill-fitting dentures
Clinical evaluation

Gas from carbonated beverages
Beverage consumption usually obvious based on history
Clinical evaluation

Patient usually admits when questioned
Clinical evaluation

Distention or bloating

See Belching
Clinical evaluation

Irritable bowel syndrome
Chronic, recurrent bloating or distention associated with a change in frequency of bowel movements or consistency of stool

No red flag findings

Typically beginning during the teens and 20s
Clinical evaluation

Examination of stool

Blood tests

Nausea, abdominal pain, sometimes vomiting

Early satiety

Sometimes in patients known to have a causative disorder
Upper endoscopy and/or nuclear scanning that evaluates stomach emptying

Eating disorders
Long-standing symptoms

In patients who are thin but still very concerned about excess body weight, particularly young women
Clinical evaluation

Constipation if chronic
A long history of hard, infrequent bowel movements
Clinical evaluation

Non-GI disorders (eg, ovarian or colon cancer)
New, persistent bloating in middle-aged or older patients

For colon cancer, sometimes blood in stool (blood may be visible or detected during a doctor's examination)
For ovarian cancer, pelvic ultrasonography

For colon cancer, colonoscopy


Dietary substances, including beans, dairy products, vegetables, onions, celery, carrots, Brussels sprouts, fruits (eg, raisins, bananas, apricots, prune juice), and complex carbohydrates (eg, pretzels, bagels, wheat germ)
Symptoms that develop mainly after consuming food that can cause gas
Clinical evaluation

Trial of elimination

Disaccharidase deficiency
Bloating, cramps, and diarrhea after consuming milk products
Breath tests

Celiac disease (eg, celiac sprue, tropical sprue)
Symptoms of anemia, steatorrhea, loss of appetite, diarrhea

For celiac sprue,weakness, symptoms that often begin during childhood

For tropical sprue, nausea, abdominal cramps, weight loss
Blood tests

Biopsy of the small intestine

Pancreatic insufficiency
Diarrhea, steatorrhea

Usually a known history of pancreatic disease
Abdominal CT

Sometimes MRCP, endoscopic ultrasonography, or ERCP

MRCP = magnetic resonance cholangiopancreatography.


History: History of present illness in patients with belching should be directed at finding the cause of aerophagia, especially dietary causes.

In patients complaining of gas, bloating, or flatus, the relationship between symptoms and meals (both timing and type and amount of food), bowel movements, and exertion should be explored. Certain patients, particularly in the acute setting, may use the term "gas" to describe their symptoms of coronary ischemia. Changes in frequency and color and consistency of stool are sought. History of weight loss is noted.

Review of systems should seek symptoms of possible causes, including diarrhea and steatorrhea (malabsorption syndromes such as celiac sprue. tropical sprue, disaccharidase deficiency, and pancreatic insufficiency) and weight loss (cancer, chronic malabsorption).

Past medical history should review all components of the diet for possible causes (see Symptoms of GI Disorders: Some Causes of Gas-Related Complaints).

Physical examination: The examination is generally normal, but in patients with bloating or flatus, signs of an underlying organic disorder should be sought on abdominal, rectal, and (for women) pelvic examination.

Red flags: The following findings are of concern:

Weight loss

Blood in stool (occult or gross)

"Gas" sensation in chest

Interpretation of findings: Chronic, recurrent bloating or distention relieved by defecation and associated with change in frequency or consistency of stool but without red flag findings suggests irritable bowel syndrome.

Long-standing symptoms in an otherwise well young person who has not lost weight are unlikely to be caused by serious physiologic disease, although an eating disorder should be considered, particularly in young women. Bloating accompanied by diarrhea, weight loss, or both (or only after ingestion of certain foods) suggests a malabsorption syndrome.

Testing: Testing is not indicated for belching unless other symptoms suggest a particular disorder. Testing for carbohydrate intolerance (eg, lactose, fructose) with breath tests should be considered particularly when the history suggests significant consumption of these sugars. Testing for small-bowel bacterial overgrowth should also be considered, particularly in patients who also have diarrhea, weight loss, or both, preferably by aerobic and anaerobic culture of small-bowel aspirates obtained during upper GI endoscopy. Testing for bacterial overgrowth with H2 breath tests, generally glucose-H2 breath tests, is prone to false-positive (ie, with rapid transit) and false-negative (ie, when there are no H2-producing bacteria) results. New, persistent bloating in middle-aged or older women (or those with an abnormal pelvic examination) should prompt pelvic ultrasonography to rule out ovarian cancer.


Belching and bloating are difficult to relieve because they are usually caused by unconscious aerophagia or increased sensitivity to normal amounts of gas. Aerophagia may be reduced by eliminating gum and carbonated beverages, cognitive behavioral techniques to prevent air swallowing, and management of associated upper GI diseases (eg, peptic ulcer). Foods containing unabsorbable carbohydrates should be avoided. Even lactose-intolerant patients generally tolerate up to 1 glass of milk drunk in small amounts throughout the day. The mechanism of repeated belching should be explained and demonstrated. When aerophagia is troublesome, behavioral therapy to encourage open-mouth, diaphragmatic breathing and minimize swallowing may be effective.

Drugs provide little benefit. Results with simethicone

, an agent that breaks up small gas
bubbles, and various anticholinergics are poor. Some patients with dyspepsia and postprandial upper abdominal fullness benefit from antacids, a low dose of tricyclic antidepressants (eg, nortriptyline

10 to 50 mg po once/day), or both to reduce visceral

Complaints of excess flatus are treated with avoidance of triggering substances (see Table 11: Symptoms of GI Disorders: Some Causes of Gas-Related Complaints). Roughage (eg, bran, psyllium seed) may be added to the diet to try to increase colonic transit; however, in some patients, worsening of symptoms may result. Activated charcoal can sometimes help reduce gas and unpleasant odor; however, it stains clothing and the oral mucosa. Charcoal-lined undergarments are available. Probiotics (eg, VSL#3) may also reduce bloating and flatulence by modulating intestinal bacterial flora. Antibiotics are useful in patients with documented bacterial overgrowth.

Functional bloating, distention, and flatus may run an intermittent, chronic course that is only partially relieved by therapy. When appropriate, reassurance that these problems are not detrimental to health is important.

Key Points

Testing should be guided by the clinical features.

Be wary of new-onset, persistent symptoms in older patients.

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