Flatulence, commonly referred to as 'farting', is caused by gas in the bowel. Ordinarily, the intestines produce between 500 and 2000ml of gas, which is regularly passed out of the anus. The gas, or 'flatus', consists of a number of gases including methane, nitrogen and carbon dioxide. The varying smell depends on the ratio of gases, which is influenced by the foods eaten.
Flatus is generated by swallowed air and by digestion, notably of high-fibre food and also from the gaseous by-products of intestinal bacteria. Some digestive system disorders, such as irritable bowel syndrome, can produce excess gas.
The healthy human digestive system works with 7-10 litres of gas per day, all of which has either been swallowed or produced by fermentation. Only a fraction of that gas is expelled; most people fart between 300ml and 2 litres of gas a day, the average being half a litre. The average person releases gas ten to twenty times per day; the normal average is 13.4 farts a day.
Abnormal and unusually disagreeable flatulence may be the result of imbalanced dietary intake, irritable bowel syndrome, diverticulitis, malabsorption or, exceptionally, pancreatic disease.
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In certain cultures belching after a meal may be considered a polite indication of satisfaction with the meal.