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Flatulence is defined in the medical literature as "flatus expelled through the anus" or the "quality or state of being flatulent", which is defined in turn as "marked by or affected with gases generated in the intestine or stomach; likely to cause digestive flatulence." The root of these words is from the Latin flatus – "a blowing, a breaking wind". Flatus is also the medical word for gas generated in the stomach or bowels. These standard definitions do not reflect the fact that a proportion of intestinal gas may be composed of swallowed environmental air, and hence flatus is not totally generated in the stomach or bowels. The scientific study of this area of medicine is termed flatology.
It is normal for humans to pass flatus through the rectum, although the amount and the frequency may vary greatly between individuals. It is also normal for intestinal gas passed through the rectum to have a characteristic feculent smell, although this too may vary in concentration. Flatus is brought to the rectum by specialised contractions of the muscles in the intestines and colon. The noises commonly associated with flatulence ("Blowing a raspberry") are caused by the vibration of the anal sphincter, and occasionally by the closed buttocks. Both the noise and smell associated with flatus leaving the anus can be sources of embarrassment or comedy in many cultures.
There are five general symptoms related to intestinal gas: pain, bloating and abdominal distension, excessive flatus volume, excessive flatus smell and gas incontinence. Furthermore, eructation ("an act or instance of belching", colloquially known as "burping") is sometimes included under the topic of flatulence.
Non-medical definitions of the term include "the uncomfortable condition of having gas in the stomach and bowels", or "a state of excessive gas in the alimentary canal". These definitions highlight that many people consider "bloating", abdominal distension or increased volume of intestinal gas to be synonymous with the term flatulence (although this is technically inaccurate). Colloquially, flatulence may be referred to as "farting", "passing gas", "breaking wind" or simply (in American English) "gas" or (British English) "wind". Derived terms include vaginal flatulence, otherwise known as a queef.
Flatus (intestinal gas) is mostly produced as a byproduct of bacterial fermentation in the gastrointestinal (GI) tract, especially the colon. There are reports of aerophagia (excessive air swallowing) causing excessive intestinal gas, but this is considered rare. Over 99% of the volume of flatus is composed of non-smelly gases. These include oxygen, nitrogen, carbon dioxide, hydrogen and methane. Nitrogen is not produced in the gut, but a component of environmental air. Patients who have excessive intestinal gas that is mostly composed of nitrogen may have aerophagia. Hydrogen, carbon dioxide and methane are all produced in the gut and contribute 74% of the volume of flatus in normal subjects. 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 faeces of nine adults, only five of the samples contained archaea capable of producing methane.
The remaining trace (<1% volume) compounds give flatus its smell. Historically, compounds such as indole, skatole, ammonia and short chain fatty acids were thought to cause the smell of flatus. More recent evidence proves that the major contribution to the smell of flatus comes from a combination of volatile sulphur compounds (VSC). It is known that hydrogen sulphide (H2S), methyl mercaptan (MM, methanethiol, MT), dimethyl sulphide (DMS), dimethyl disulphide (DMDS) and dimethyl trisulphide (DMTS) are present in flatus. The benzopyrrole volatiles indole and skatole actually have a mothball type smell, and therefore probably do not contribute greatly to the characteristic smell of flatus.
In one study, H2S concentration was shown to correlate convincingly with perceived bad smell of flatus, followed by MM and DMS. This is supported by the fact that H2S may be the most abundant VSC present. However, these results were generated from subjects who were eating a diet high in pinto beans to stimulate flatus production. Others report that MM was the greatest contributor to the smell of flatus in patients not on under any specific dietary alterations. It has now been demonstrated that MM, DMS and H2S (described as decomposing vegetables, unpleasantly sweet/wild radish and rotten eggs respectively) are all present in human flatus in concentrations above their smell perception thresholds. Furthermore, it is known that increased dietary sulphur containing amino acids significantly increases the smell of flatus. It is therefore likely that the smell of flatus is created by a combination of VSC, with minimal contribution from non sulphur volatiles.
The incidence of odoriferous compounds in flatulence emissions increases from herbivores, such as cattle, through omnivores to carnivorous species. Such smell can also be caused by the presence of large numbers of microflora bacteria and/or the presence of faeces in the rectum. Diets high in protein, especially sulphur-containing amino acids, have been demonstrated to significantly increase the smell of flatus.
Normal flatus volume range is around 476 to 1491ml per 24 hours. This variability between individuals is greatly dependent upon diet. Similarly the number of flatus episodes per day is variable, the normal range is given as 8-20 per day. The volume of flatus associated with each flatulence event again varies (5-375ml). The volume of the first flatulence upon waking in the morning is significantly larger than those during the day. This may be due to buildup of intestinal gas in the colon during sleep, the peak in peristaltic activity in the first few hours after waking and/or the strong prokinetic effect of rectal distension on the rate of transit of intestinal gas. It is now known that gas is moved along the gut independently of solids and liquids, and this transit is more efficient in the erect position compared to when supine. It is thought that large volumes of intestinal gas present low resistance, and can be propelled by subtle changes in gut tone, capacitance and proximal contraction and distal relaxation. This process is thought not to affect solid and liquid intra-lumenal contents.
Researchers investigating the role of sensory nerve endings in the anal canal did not find them to be essential for retaining fluids in the anus, and instead speculate that their role may be to distinguish between flatus and faeces, thereby helping detect a need to defecate or to signal the end of 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 flatus.
Generally speaking, there are four different types of complaint that relate to intestinal gas, which may present individually or in combination.
Patients may complain of bloating as abdominal distension, discomfort and pain from "trapped wind". In the past, functional bowel disorders such as irritable bowel syndrome (IBS) which produced symptoms of bloating were attributed to increased production of intestinal gas. However, three significant pieces of evidence refute this theory. First, in normal subjects, even very high rates of gas infusion into the small intestine (30 ml/min) are tolerated without complaints of pain or bloating and harmlessly passed as flatus per rectum. Secondly, studies aiming to quantify the total volume of gas produced by IBS patients (some including gas emitted from the mouth by eructation) have consistently failed to demonstrate increased volumes compared to healthy subjects. The proportion of hydrogen produced by IBS patients may be increased in a subset of IBS patients, but this does not effect the total volume. Thirdly, it is known that the total volume of flatus produced by IBS patients who complain of pain and abdominal distension would be tolerated in normal subjects without any complaints of pain.
Patients who complain of bloating frequently can be shown to objective increases in abdominal girth, often increased throughout the day and then resolving during sleep. The increase in girth combined with the fact that the total volume flatus is not increased lead to studies aiming to image the distribution of intestinal gas in patients with bloating. They found that gas was not distributed normally in these patients, there was segmental gas pooling and focal distension. In conclusion, abdominal distension, pain and bloating symptoms are the result of abnormal intestinal gas dynamics rather than increased flatus production.
As mentioned above, the normal range of volumes of flatus in normal individuals varies hugely (476-1491ml/24h). All intestinal gas is either swallowed environmental air, present intrinsically in foods and beverages or the result of gut fermentation. Swallowing small amounts of air occurs while eating and drinking. This is emitted from the mouth by eructation (burping) and is normal. Excessive swallowing of environmental air is called aerophagia, and has been shown in a few case reports to be responsible for increased flatus volume. This is however considered a rare cause of increased flatus volume. Gases contained in food and beverages is likewise emitted largely through eructation, e.g. carbonated beverages. Endogenously produced intestinal gasses make up 74% of flatus in normal subjects. The volume of gas produced is partially dependent upon the composition of the intestinal microbiota, which is normally very resistant to change, but is also very different in different individuals. Some patients are predisposed to increased endogenous gas production by virtue of their gut microbiota composition.
The greatest concentration of gut bacteria is in the colon, while the small intestine is normally near sterile. Fermentation occurs when unabsorbed food residues may arrive in the colon. Therefore, even more than the composition of the microbiota, diet is the primary factor which dictates the volume of flatus produced. Diets which aim to reduce the amount of undigested fermentable food residues arriving in the colon have been shown to significantly reduce the volume of flatus produced. Again, it is emphasised that increased volume of intestinal gas will not cause bloating and pain in normal subjects. Abnormal intestinal gas dynamics will create pain, distension and bloating, regardless of whether there is high or low total flatus volume.
Although flatus possesses physiological smell, this may be abnormally increased in some patients and cause social distress to the patient. Increased smell of flatus presents a distinct clinical issue from other complaints related to intestinal gas. Some patients may exhibit over-sensitivity to bad flatus smell, and in extreme forms, olfactory reference syndrome may be diagnosed.
"Gas incontinence" could be defined as loss of voluntary control over the passage of flatus. It is a recognised subtype of faecal incontinence, and is usually related to minor disruptions of the continence mechanisms. Some consider gas incontinence to be the first, sometimes only, symptom of faecal incontinence.
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, spring onions, 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 oligosaccharides (carbohydrates) that are particularly resistant to digestion by mammals, but which are readily digestible by gut flora – 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.
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, the phenomena of high altitude flatus expulsion was first recorded over two hundred years ago.
Since problems involving intestinal gas presents as different (but sometimes combined) complaints, the management is cause related.
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 faecal 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.
Other drugs including prokinetics, lubiprostone, antibiotics and probiotics are also used to treat bloating in patients with functional bowel disorders such as IBS, and there is some evidence that these measures may reduce symptoms.
One method of reducing the volume of flatus produced is dietary modification, reducing the amount of fermentable carbohydrates. This is the theory behind diets such as the low FODMAP diet (low fermentable oligosaccharide, dissacharide, monosaccharide and polyols).
Certain spices have been reported to counteract the production of intestinal gas, most notably the closely related cumin, coriander, caraway and others such as 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. 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. Some legumes also stand up to prolonged cooking, which can help break down the oligosaccharides into simple sugars. On the other hand, fermented bean products such as miso are less likely to produce as much intestinal gas. Fermentative lactic acid bacteria such as Lactobacillus casei and Lactobacillus plantarum reduce flatulence in human intestinal tract.
Probiotics (live yogurt, kefir, etc.) are reputed to reduce flatulence when used to restore balance to the normal intestinal flora. Live (bioactive) yogurt contains, among other lactic bacteria, Lactobacillus acidophilus which may be useful in reducing flatulence. L. acidophilus may make the intestinal environment more acidic, supporting a natural balance of the fermentative processes. L. acidophilus is available in supplements (some believe non-dairy is best[who?]). Prebiotics, which generally are non-digestible oligosaccharides, such as fructooligosaccharide, generally increase flatulence in a similar way as described for lactose intolerance.
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.
Smell from flatulence is commonly treated with bismuth subgallate, available over-the-counter in the US as Devrom. Bismuth subgallate is commonly used by individuals who have had ostomy surgery, bariatric surgery, faecal incontinence and irritable bowel syndrome. Bismuth suvsalicylate is a compound that binds H2S, and one study reported a dose of 524 mg four times a day for 3–7 days bismuth subsalicylate yielded a >95% reduction in faecal H2S release in both humans and rats. Another bismuth compound, bismuth subnitrate was also shown to bind with H2S. Another study showed that bismuth acted synergistically with various antibiotics to inhibit sulphate-reducing gut bacteria and sulphide production. Some authors proposed a theory that H2S was involved in the development of ulcerative colitis and that bismuth might be helpful in the management of this condition. However, bismuth administration in rats did not prevent them from developing ulcerative colitis despite reduced H2S production. Also, new evidence suggests that colonic H2S is largely present in bound forms, probably sulphides of iron and other metals. Rarely, serious bismuth toxicity may occur with higher doses.
Despite being an ancient treatment for various digestive complaints, activated charcoal did not produce reduction in both the total flatus volume nor the release of sulphur-containing gasses, and there was no reduction in abdominal symptoms (after 0.52g activated charcoal four times a day for 1 week). The authors suggested that saturation of charcoal binding sites during its passage through the gut was the reason for this. A further study concluded that activated charcoal (4g) does not influence gas formation in vitro or in vivo. Other authors reported that activated charcoal was effective. A study in 8 dogs concluded activated charcoal (unknown oral dose) reduced H2S by 71%. In combination with yucca schidigera, and zinc acetate, this was increased to an 86% H2S reduction, although total flatus volume and number of flatus events was unchanged. An early study reported activated charcoal (unknown oral dose) prevented a large increase in the number of flatus events and increased breath hydrogen concentrations that normally occur following a gas-producing meal.
Garments & external devices
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. In 2001 Mr Weimer received the Ig Nobel Prize for Biology for his invention.
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.
"Flatus incontinence" where there is involuntary passage of flatus, is a type of faecal incontinence, and is managed similarly.
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[examples needed], 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. The humorous insult "I fart in your general direction" from the comedy film Monty Python and the Holy Grail has been reprinted on several posters, t-shirts, and hats. The Whoopee cushion is a joking device invented in the early 20th century for simulating a fart. In 2008, a farting application for the iPhone earned nearly $10,000 in one day.
In January 2011, the Malawi Minister of Justice, George Chaponda, said that Air Fouling Legislation would make public farting illegal in his country. When reporting the story, the media satirised Chaponda's statement with punning headlines. Later, the minister withdrew his statement.
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.
Since New Zealand produces large amounts of agricultural products, it is in the unique position of having high methane emissions from 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. (It should be recognised, however, that this has nothing to do with flatulence. The gas in question is produced by anaerobic fermentation of cattle waste in an enclosed lagoon.)
Historical comment on the ability to fart at will is observed as early as Saint Augustine's The City of God (5th century AD). Augustine mentions men who "have such command of their bowels, that they can break wind continuously at will, so as to produce the effect of singing". The fact that mankind in general has lost this ability he attributes to the first sin of Adam and Eve and its consequences with respect to body control. Intentional passing of gas and its use as entertainment for others appear to have been somewhat well known in pre-modern Europe, according to mentions of it in medieval and later literature, including Rabelais.
Le Pétomane ("the Fartomaniac") was a famous French performer in the 19th century who, as well as many professional farters before him, did flatulence impressions and held shows. The performer Mr. Methane carries on le Pétomane's tradition today. Also, a 2002 film Thunderpants revolves around a boy named Patrick Smash who has an ongoing flatulence problem since he was a newborn baby. He eventually overcame his problems and fulfilled his dreams including being an astronaut.
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