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Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The mixture of gases is known as flatus, (informally) fart, or simply gas, and is expelled from the rectum in a process colloquially referred to as "passing gas" or "farting". Flatus is brought to the rectum by the same peristaltic process which causes feces to descend from the large intestine. The noises commonly associated with flatulence are caused by the vibration of the anal sphincter, and occasionally by the closed buttocks.
Composition of flatus gases
Nitrogen, the main constituent of air, is the primary gas released during flatulence, along with carbon dioxide, which is present in higher quantities in those who drink carbonated beverages regularly. The lesser component gases methane and hydrogen are flammable, and so flatus containing adequate amounts of these can be ignited. However, not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane. Similar results are found in samples of gas obtained from within the rectum.
The gas released during a flatus event frequently has an unpleasant odor. For many years, this was thought to be due to skatole and indole, which are byproducts of the digestion of meat. However, gas chromatography testing in 1984 revealed that sulfur-containing compounds, such as methanethiol, hydrogen sulfide (rotten egg smell) and dimethyl sulfide, were primarily responsible for the scent.
The incidence of odoriferous compounds in flatulence emissions increases from herbivores, such as cattle, through omnivores to carnivorous species, such as cats. Such odor can also be caused by the presence of large numbers of microflora bacteria and/or the presence of feces in the rectum.
The major components of the flatus, which are odorless, by percentage are:
Mechanism of action
Flatus is brought to the rectum by the same process which causes feces to descend from the large intestine, and may cause a similar feeling of urgency and discomfort. Nerve endings in the rectum usually enable individuals to distinguish between flatus and feces,  although loose stool can confuse the individual, occasionally resulting in accidental defecation.
The sound varies depending on the tightness of the sphincter muscle and velocity of the gas being propelled, as well as other factors, such as water and body fat. The auditory pitch (sound) of the flatulence outburst can also be affected by the anal embouchure. Among humans, flatulence occasionally happens accidentally, such as incidentally to coughing or sneezing or during orgasm; on other occasions, flatulence can be voluntarily elicited by tensing the rectum or "bearing down" on stomach or bowel muscles and subsequently relaxing the anal sphincter, resulting in the expulsion of a flatus.
Intestinal gas is composed of varying quantities of exogenous sources (air that is ingested through the nose and mouth) and endogenous sources (gas produced within the digestive tract). The exogenous gases are swallowed (aerophagia) when eating or drinking or increased swallowing during times of excessive salivation (as might occur when nauseated or as the result of gastroesophageal reflux disease). The endogenous gases are produced either as a by-product of digesting certain types of food, or of incomplete digestion. Anything that causes food to be incompletely digested by the stomach and/or small intestine may cause flatulence when the material arrives in the large intestine, due to fermentation by yeast or prokaryotes normally or abnormally present in the gastrointestinal tract.
Flatulence-producing foods are typically high in certain polysaccharides, (especially oligosaccharides such as inulin). Those foods include beans, lentils, dairy products, onions, garlic, scallions, leeks, turnips, rutabagas, radishes, sweet potatoes, potatoes, cashews, Jerusalem artichokes, oats, wheat, and yeast in breads. Cauliflower, broccoli, cabbage, Brussels sprouts and other cruciferous vegetables that belong to the genus Brassica are commonly reputed to not only increase flatulence, but to increase the pungency of the flatus. In beans, endogenous gases seem to arise from complex oligosaccharide (carbohydrates) that are particularly resistant to digestion by mammals, but which are readily digestible by microorganisms (methane-producing archaea; Methanobrevibacter smithii) that inhabit the digestive tract. These oligosaccharides pass through the upper intestine largely unchanged, and when they reach the lower intestine, bacteria feed on them, producing copious amounts of flatus. In the case of people who have lactose intolerance, intestinal bacteria feeding on lactose can give rise to excessive gas production when milk or lactose-containing substances have been consumed.
Interest in the causes of flatulence was spurred by high-altitude flight and the space program; the low atmospheric pressure, confined conditions, and stresses peculiar to those endeavours were cause for concern. In the field of mountaineering, high altitude flatus expulsion was first noticed over two hundred years ago.
Certain spices have been reported to counteract the production of intestinal gas, most notably cumin, coriander, caraway and the closely related ajwain, turmeric, asafoetida (Hing), epazote, and kombu kelp (a Japanese seaweed). Most starches, including potatoes, corn, noodles, and wheat, produce gas as they are broken down in the large intestine. Rice is the only starch that does not cause gas. The amount of water-soluble oligosaccharide in beans that may contribute to production of intestinal gas is reputed to be reduced by a long period of soaking followed by boiling, but at a cost of also leaching out other water-soluble nutrients. Also, intestinal gas can be reduced by fermenting the beans, and making them less gas-inducing, and/or by cooking them in the liquor from a previous batch.
Lactobacillus casei and Lactobacillus plantarum have recently been hypothesized as being responsible for this effect. Some legumes also stand up to prolonged cooking, which can help break down the oligosaccharides into simple sugars. Fermentation also breaks down oligosaccharides, which is why fermented bean products such as miso are less likely to produce as much intestinal gas).
Probiotics (live yogurt, kefir, etc.) are reputed to reduce flatulence when used to restore balance to the normal intestinal flora. Live yogurt contains Lactobacillus acidophilus which may be useful in reducing flatulence. L. acidophilus may make the intestines more acidic, thus maintaining the natural balance of fermentation processes. L. acidophilus is available in supplements (some believe non-dairy is best). Prebiotics, which generally are non-digestible oligosaccharides, such as fructooligosaccharide, generally increase flatulence in a similar way as described for lactose intolerance.
Medicinal activated charcoal tablets (brand name CharcoCaps) have also been reported as effective in reducing both odor and quantity of flatus when taken immediately before food that is likely to cause flatulence later.
Digestive enzyme supplements may significantly reduce the amount of flatulence caused by some components of foods not being digested by the body and thereby promoting the action of microbes in the small and large intestines. It has been suggested that alpha-galactosidase enzymes, which can digest certain complex sugars, are effective in reducing the volume and frequency of flatus. The enzymes alpha-galactosidase ), lactase, amylase, lipase, protease, cellulase, glucoamylase, invertase, malt diastase, pectinase, and bromelain are available, either individually or in combination blends, in commercial products.
While not affecting the production of the gases themselves, surfactants (agents which lower surface tension) can reduce the disagreeable sensations associated with flatulence, by aiding the dissolution of the gases into liquid and solid fecal matter.  Preparations containing simethicone reportedly operate by promoting the coalescence of smaller bubbles into larger ones more easily passed from the body, either by burping or flatulence. Such preparations do not decrease the total amount of gas generated in or passed from the colon, but make the bubbles larger and thereby allowing them to be passed more easily.
Often it may be helpful to ingest small quantities of acidic liquids with meals, such as lemon juice or vinegar, to stimulate the production of gastric hydrochloric acid. In turn, acid ingestion may increase normal gastric enzyme and acid production, facilitating normal digestion and perhaps limiting intestinal gas production. Ingestion of bromelain- or papain-containing supplements (such as raw pineapple or papaya, respectively,) may be helpful.
Odor from flatulence, caused by the intestinal bacteria called microflora in the bowel, can be treated by taking bismuth subgallate, available over-the-counter as Devrom. Bismuth subgallate is commonly used by individuals who have had ostomy surgery, bariatric surgery, fecal incontinence and irritable bowel syndrome.
In 1998, Chester "Buck" Weimer of Pueblo, Colorado received a patent for the first undergarment that contained a replaceable charcoal filter. The undergarments are air-tight and provide a pocketed escape hole in which a charcoal filter can be inserted.
A similar product was released in 2002, but rather than an entire undergarment, consumers are able to purchase an insert similar to a pantiliner that contains activated charcoal. The inventors, Myra and Brian Conant of Mililani, Hawaii still claim on their website to have discovered the undergarment product in 2002 (eight years after Chester Weimer filed for a patent for his product), but state that their tests "concluded" that they should release an insert instead.
As a normal body function, the action of flatulence is an important signal of bowel activity, and hence is often documented by nursing staff following surgical or other treatment of patients. However, symptoms of excessive flatulence can indicate the presence of irritable bowel syndrome or some other organic disease. In particular, the sudden occurrence of excessive flatulence together with the onset of new symptoms provide reason for seeking further medical examination.
Flatulence is not poisonous; it is a natural component of various intestinal contents. However, discomfort may develop from the build-up of gas pressure if an attempt is made to refrain from releasing them. In theory, pathological distension of the bowel, leading to constipation, could result if a person holds in flatulence.
Not all flatus is released from the body via the anus. When the partial pressure of any gas component of the intestinal lumen is higher than its partial pressure in the blood, that component enters into the bloodstream of the intestinal wall by the process of diffusion. As the blood passes through the lungs, this gas can diffuse back out of the blood and be exhaled. If a person holds in flatus during daytime, it will often be released during sleep involuntarily when the body is relaxed. Some flatus can become trapped within the feces during its compaction and will exit the body, still contained within the fecal matter, during the process of defecation.
Flatulence is often blamed as a significant source of greenhouse gases, owing to the erroneous belief that the methane released by livestock is in the flatus. While livestock account for around 20% of global methane emissions, 90-95% of that is released by exhaling or burping. Only 1–2% of global methane emissions come from livestock flatus.
Since New Zealand produces large amounts of agricultural product it is in a unique position of having high methane emissions livestock compared to other greenhouse gas sources. The New Zealand government is a signatory to the Kyoto Protocol and therefore, attempts are being made to reduce greenhouse emissions. To achieve this an agricultural emissions research levy was proposed, which promptly became known as a "fart tax" or "flatulence tax". It encountered opposition from farmers, farming lobby groups and opposition politicians.
In Fresno, California, a system to harvest methane by-product from dairy cattle and convert it to usable bio-gas is being used, in a partnership with Pacific Gas & Electric (PG&E) and BioEnergy Solutions, in which BioEnergy Solutions sells the methane harvested from cows to PG&E, who then converts the methane to usable bio-gas, which is very similar to natural gas.
In June 2009 Paul McCartney and other celebrities launched a "Meat Free Monday" campaign in order to reduce greenhouse gas emissions from the world's livestock.
In many cultures, human flatulence in public is regarded as embarrassing but, depending on context, can also be considered humorous . People will often strain to hold in the passing of gas when in polite company, or position themselves to conceal the noise and scent. In other cultures, it may be no more embarrassing than coughing.
While the act of passing flatus in said cultures is generally considered to be an unfortunate occurrence in public settings, flatulence may, in casual circumstances and especially among children, be used as either a humorous supplement to a joke ("pull my finger"), or as a comic activity in and of itself. The social acceptability of flatulence-based humor in entertainment and the mass media varies over the course of time and between cultures.
In 2008, a farting application for the iPhone raked in nearly $10,000 in one day.
- Aerophagia - swallowing of air
- Flatulence humor
- Vaginal flatulence
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- Conant, Brian J., Myra M. Conant Flatulence deodorizer. URL accessed on 2007-09-10.
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- ABC Southern Queensland: "Could skippy stop cows farting and end global warming?" Friday, 3 February 2006. Example of error. Although the article doesn't specify whether the methane is released by flatulence or eructation, it appears the headline-writer assumes it's through flatulence.
- Nowak, Rachel Burp vaccine cuts greenhouse gas. New Scientist.
- "Bovine belching called udderly serious gas problem - Global warming concerns spur effort to cut methane." By Gary Polakovic. Los Angeles Times, Sunday, July 13, 2003.
- "California converting cow dung into biogas." By Catherine Elsworth. Telegraph Media Group Limited, Friday, May 30, 2008.
- "Sir Paul McCartney calls for meat-free Mondays to combat climate change", The Telegraph, 15 June 2009, retrieved 16 June 2009
- iPhone Fart App Rakes in $10,000 a Day, Wired, December 24, 2008
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