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.

Essay on Flatulence (First printed in the 14th Edition of THE MANUAL) Flatulence, which can cause great psychosocial distress, is unofficially described according to its salient characteristics: The “slider” (crowded elevator type), which is released slowly and noiselessly, sometimes with devastating effect

The open sphincter, or “pooh” type, which is said to be of higher temperature and more aromatic

The staccato or drumbeat type, pleasantly passed in privacy

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 nitrogen) from the bloodstream. Bacterial metabolism yields significant volumes of hydrogen, methane, and carbon dioxide.

Hydrogen 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 hydrogen. Celiac disease, tropical sprue, pancreatic insufficiency, and other causes of carbohydrate malabsorption should also be considered in cases of excess colonic gas.

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

Carbon dioxide is also produced by bacterial metabolism and generated in the reaction of bicarbonate and hydrogen ions. Hydrogen ions may come from gastric hydrochloric acid or from fatty acids released during digestion of fats—the latter sometimes produces several hundred mEq of hydrogen ions. The acid products released by bacterial fermentation of unabsorbed carbohydrates in the colon may also react with bicarbonate to produce carbon dioxide. Although bloating may occasionally occur, the rapid diffusion of carbon dioxide 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.