Even though gas is found in the normal gut at any given time, it's not the exact same gas from hour to hour. Any individual bubble dissipates, and is replenished - so that some is always there. Where does it go? Although a tiny fraction gets absorbed through the lining of the GI tract, the great majority gets passed along and out: a bit burped up from the stomach, the lion's share passed out the anus. Passing gas either "up" or down and out" of the body is called flatulence.

Where does gas come from? It's the air we swallow allong with the gasses made in the process of food digestion.

Gas is normal yes, but excess gas can be a problem for people, either an embarrasment kind of problem, or a cause of physical pain and distress. In fact, the production and passage of gastrointestinal gas is part and parcel of several medical conditions.

Further, when there is a shut-down of gastrointestinal function (a lack of gut mobility called ileus) , one of the first things that happens is gas passage stops. That's a danger sign.

What about the discomfort called 'gas'? Gas pain happens because our stomach and bowels hurt if they expand too much. That's true even though both can be cut without causing pain.

• The walls of our internal gastrointestinal tract are different than our skin and muscles when it comes to pain. A sharp cut or stab isn't even felt- but pressure, enough pressure to stretch the wall, is a very big deal- it causes an intense sensation. That's one reason that a large amount of gas in any of these organs- stomach, small or large intestine, can be very distressing.
• The amount of gas that each organ of the digestive tract can hold before it stretches to the point that pain receptors are triggered, is different.
  • The part least able to cope is the small intestine. That's because its diameter is small (hence, the name - small intestine) as compared to the large intestine, or to the stomach, and because the wall of the small intestine is more 'brittle' (less compliant), as compared to the stomach or large intestine.

Normally, whatever gas gets made or introduced along the GI tract passes along and out of the body quickly - and so either there is never enough to cause uncomfortable expansion of any one place along the tract - or, if there is enough to cause pain, that kind of discomfort is really transitory.

(An old fashioned name for the kind of crampy pain that comes with recurrent bouts of "too much" gas, is colic. Although the sort of crying jags that small babies get (infantile colic) got its name from parent's interpretation of the baby's distress as coming from gas, it's doubtful that gas has anything to do with that colic, at all.)

Many adults suffer from what's called irritable bowel syndrome, and a good number of these attribute their symptoms to "gas". There are adults who have a problem with excessive farting or particularly strong smelling release of intestinal gas, and a few have a compelling need to burp because of stomach distension. For some, these issues impact everyday life every single day of their lives, and even impair the ability to socialize, to work or shop in public. In other words, there are a number of people who are chronicaly disabled - at least to some extent, in their own opinions, by gas. Physicians call these disorder "functional" GI disorders, because there is nothing that tests abnormally about the GI system.

• However, when it comes to Medicine, there is little to no formal training of physicians in "gas".
• That's a shame, really, because a lot of people suffer from "gas", or think that they do.
• For many with so-called functional bowel disorders (like irritable bowel syndrome), research studies that place extra gas in the bowel duplicate their symptoms- so, they may well be exactly right about that.

This article reviews the current medical textbooks, and recently published journal articles, about gas in the human gastrointestinal tract.

Where does gas come from?

Actual gas in the gut comes from the diet and from the digestive process. Here's how:

Some air is included with every swallow- whether we are eating or drinking, or not. That has to be, because there is always air in the mouth.

Depending on just how a person swallows, there can be more or less air taken in; but there is always at least a tiny bit. Air is mostly nitrogen and oxygen, and a bit of carbon dioxide and other gasses, and since the gas in the stomach is air - it's those gasses that are the ones in the stomach. A little bit of the oxygen and the carbon dioxide gets absorbed by the lining of the gut, but most passes - either up and out in a burp- or into the intestine from the stomach.

• Now, sometimes people gulp air either on purpose or as an unconscious habit. The kind of subjective feeling of 'gas' is that results is most frequently relieved by repetitve burping.

Once food leaves the stomach, more gas is produced as a by-product of normal digestion. The first big burst of gas made (rather than swallowed) is mostly carbon dioxide- here's why:

• The stomach did its part in digestion by pounding and kneading, and disolving food in acid - a strong acid, hydrochloric acid (HCl). Those acidic contents are what drip from the stomach through the pyloric valve into the start of the small intestine.
• There it mixes with enzymes made by the pancreas, and is now called chyme. Pancreatic juice is not just loaded with enzymes, it's also very alkaline- the opposite of acid when it comes to pH.
• As the heavily alkaline juice neutralizes the acid stuff from the stomach, carbon dioxide is released. (You can duplicate this reaction by adding baking soda to vinegar.) In the first part of the small intestine, about 200 ml ( ounces) of gas exists in healthy people, more or less. That's a scant cup - for the cooks among us.

Although GI doctors and physiologists have studied how chyme moves along the rest of digestive tract, there has not been that kind of attention paid to the movement of gas. That's a pertinent difference, because it's clear that gas moves separately from the rest. For one thing, digested food in that chyme passes along pretty much the same whether a person is standing, sitting, or lying down. Not true for gas: when laying down in a horizontal position gas stays put, by and large. When upright, gas passes.

• Some gas is swallowed air, some of that is air in food, some air is with food . Other kinds of gasses also get drunk, or eaten (like carbon dioxide in fizzy drinks).
• Besides what's taken in, there is production of gas in the gut, beyond the stomach.
  • In the small intestine, it's about cupfulls - and it's mostly carbon dioxide. CO2 is given off from the chemical reaction of alkaline digestive juices in the small intestine neutralizing the acidic contents entering it from the stomach.
  • In the large intestine, it's bacterial metabolism of fermentable substrates releases CO2, H2, and CH4, as well as a variety of trace gases

Where does the feeling of gas come from? What causes 'gas'?

• What's also known is the reaction of people to artificial gas placed in known places along the gut, and in known quantities.

1. "Intestinal gas dynamics have recently been investigated using a gas challenge test: exogenous gas is continuously infused into the intestine while measuring gas evacuation, girth, and abdominal symptoms. With this technique a series of mechanistic studies have been carried out both in healthy subjects and in patients with gas symptoms."

How much gas gets made and swallowed, normally?

How much gas does it take to cause gas pains?

It's not a question of how much- it's a question of movement. As long as the gas is evacuated at the same rate it gets made, then large amounts can get made without discomfort.

• "Healthy subjects evacuate as much gas as infused without discomfort. 2
• Indeed, jejunal gas infusion at a rate up to 30 ml/min (that is, 1.8 l/h) is well tolerated by the majority of the healthy population. However, intraluminal gas displacement is not a passive process. Further studies rather indicated that the gut actively propulses gas because transit of gas down the gastrointestinal tract is more effective in the erect position, opposing flotation forces, than when supine. 3 Transit of gas, like that of solids and liquids, is modulated by a series of reflex mechanisms: intraluminal nutrients, particularly lipids, delay gas transit, 4 whereas mechanical stimulation of the gut, for instance mild rectal distension, has a strong prokinetic effect. 5

Gas transit is normally very effective but if a certain amount of gas is retained within the gut, subjects may develop abdominal distension and symptoms. Different experimental models of gas retention were used to show that while abdominal distension is related to the volume of gas within the gut, perception of abdominal symptoms depends both on intestinal motor activity (gas is better tolerated when the gut is relaxed)7 and on the intraluminal distribution of gas (gas is better tolerated within the colon than within the long, but poorly compliant, small intestine).

Using the same methodology it was shown that patients complaining of abdominal bloating, either with irritable bowel syndrome or functional bloating, have impaired gas transit and develop intestinal gas retention, abdominal distension, and/or abdominal symptoms in response to gas loads (12 ml/min jejunal gas infusion for 2–3 hours) that are well tolerated by healthy subjects. 4, 9, 10 Interestingly, symptoms induced by the gas challenge test in patients by and large replicate their customary complaints. Scintigraphic studies using gas labelled with radioactive xenon produced striking data indicating that the small bowel is responsible for impaired gas transit in these patients, in contrast with the common idea of gas being retained in the colon. 11 The ileocaecal region is an area with sphincteric function likely implicated in this dysfunction. However, very elaborate studies with gas infusion at various levels of the gut showed that gas retention is due to impaired propulsion in more proximal parts of the small bowel because while jejunal gas loads were retained, clearance of gas directly infused into the distal ileum or caecum was normal.

“The small bowel is responsible for impaired gas transit in these patients, in contrast with the common idea of gas being retained in the colon”

“The amount of gas also depends on the composition of the colonic flora, which is very stable in each subject but exhibits high interindividual variations, so that some subjects are prone to excessive gas production and evacuation”
Since modification of colonic flora is not yet an effective option, treatment is focused on dietary instructions, helping the patient identify the high flatulogenic offending foodstuffs. 13

Aerophagia
Some patients complain of excessive eructation, as if their gastric production of gas is unlimited. Really, these patients inadvertently swallow air that accumulates in the stomach and is then released by belching, with patient satisfaction. 1 Frequently, the process is triggered by a basal dyspeptic-type symptom of epigastric fullness that patients misinterpret as excessive gas in the stomach, and during repetitive and ineffective attempts of belching, air is introduced into the stomach with increasing discomfort. The patient’s misconception is reinforced by the partial relief experienced when eructation finally occurs. In most of the cases a clear explanation resolves the problem but in some patients psychological abnormalities may be involved, requiring special management.

Impaired anal evacuation
Self restraint anal gas evacuation in healthy subjects produces gas retention, 7 and this mechanism may also operate in patients with impaired anal evacuation due to functional outlet obstruction. Furthermore, faecal retention would prolong the fermentation process, increasing gas production. In contrast with patients with excessive flatus, these patients complain of difficult evacuation and abdominal gas retention, and the problem can be resolved by biofeedback retraining. 14

Abdominal gas symptoms in irritable bowel syndrome and related syndromes
Gas related symptoms are the most frequent and troublesome complaints in patients with functional intestinal disorders, particularly irritable bowel syndrome and functional bloating, but the situation here is far less clear than in the conditions described above. Experimental studies in these patients have demonstrated a series of abnormalities in intestinal handling of gas loads but how do these abnormalities relate to symptoms? The clinical relevance of intestinal gas in this context can be established by addressing a series of questions.

Do these patients produce more intestinal gas?
Gas production was initially measured by Levitt’s group using a washout technique, and was found to be normal in patients. 15 Hydrogen, which accounts for a large proportion of colonic gas production, is partly absorbed into the blood and excreted by breath. 1 A more recent and largely quoted study measured gas excretion (breath plus anal) by indirect calorimetry in irritable bowel syndrome patients on a standard diet and showed that hydrogen excreted was increased but the total gas volume excreted (hydrogen plus methane) was not different than in healthy controls. 16 Indirect evaluation of hydrogen production by breath tests has shown either normal production 17 or increased production, attributed to various causes, such as hyperactive gas producing colonic flora, 16 small bowel bacterial overgrowth, 18 or small bowel malabsorption. 19 The level of evidence supporting these interpretations is questionable. Nevertheless, it seems that the total volume of gas produced in these patients is not much larger than in healthy subjects.

Do these patients have more gas within the gut?
Three independent studies showed that the gas surface in plain abdominal radiographs was 28%–118% larger in irritable bowel syndrome patients than in controls. 20–22 As the normal volume of intestinal gas is approximately 200 ml, 9, 10, 15 this difference would hardly justify the symptoms. Furthermore, other studies using computed tomography 23 or the washout technique 9, 10, 15 could not detect differences between patients complaining of bloating and healthy controls.

Hence impaired gas transit in these patients does not result in global gas retention. Conceivably, the abnormalities detected by the gas transit studies affect intraluminal gas distribution and result in segmental gas pooling and focal distension, without net increments in total gas volume.

Is abdominal distension a fact?
Abdominal distension is the most common gas symptom in patients with functional intestinal disorders. However, this patient claim is difficult to verify. Frequently, these patients report that distension develops during the day and resolves after overnight rest, and this variability may be the key to substantiate the subjective sensation. Several studies measuring girth changes with a tape measure, 23, 24 computed tomography, 23 and, more recently, inductance plethysmography, 25 have shown that, indeed, the subjective sensation is associated with objective abdominal distension.

“Patients with bloating do have objective abdominal distension but it may not necessarily be due to a true increment in intra-abdominal volume”
The abdominal wall normally adapts to its content. It has been recently shown that an intra-abdominal volume load, produced by colonic gas infusion, induces in healthy subjects an increment in tonic activity of the abdominal muscles that can be measured by electromyography, 26 and this response is probably mediated via viscerosomatic reflexes. 27 This adaptation of the abdominal wall to intra-abdominal volume loads is impaired in patients complaining of bloating who fail to contract their abdominal muscles, and this abnormal response is associated with exaggerated abdominal distension and bloating. 26 Hence patients with bloating do have objective abdominal distension but it may not necessarily be due to a true increment in intra-abdominal volume, but to abdominal wall dystony with abdominal redistribution and protrusion of the anterior wall.

The ultimate question: is it really gas that matters?
Gas transit studies have consistently shown that patients complaining of intestinal gas symptoms have impaired handling of intestinal contents, related to abnormal gut reflexes, which may result in segmental pooling and focal gut distension (fig 1). Additional evidence indicates that these patients also have a sensory dysfunction with increased perception of intraluminal stimuli. 28, 29 As described above, recent data further suggest that viscerosomatic reflexes controlling abdominal wall tone may also be affected, so that segmental pooling within the gut may lead to abdominal wall dystony and distension. However, this does not imply that gas is necessarily the offending element, but rather other intraluminal components could trigger the abnormal responses and thus be responsible for the abdominal symptoms that patients misinterpret and attribute to intestinal gas. The main contribution of gas studies has been demonstration of subtle dysfunctions of intestinal motility that were missed, or at least not consistently observed, with conventional methodologies. Hopefully, extension of these studies may help both in the classification of patients complaining of gas symptoms based on pathophysiological mechanisms, and in identification of objective markers to test mechanistically oriented treatment options.

These patients pass large volumes of sometimes odoriferous gas per anus. Gas evacuation depends on the volume of gas produced by colonic bacteria during fermentation of unabsorbed food residues arriving into the colon. 1 Hence the volume of gas depends on diet, and most subjects may experience flatulence after eating foods rich in fermentable residues, such as beans. The amount of gas also depends on the composition of the colonic flora, which is very stable in each subject but exhibits high interindividual variations, so that some subjects are prone to excessive gas production and evacuation. 1 These subjects do not complain of abdominal symptoms unless they have associated irritable bowel syndrome, because healthy subjects propulse and evacuate large intraluminal gas loads without symptoms.

But how much flatus is normal? And what does normal flatus smell like?

"The rate of gas excretion per anus by healthy subjects ingesting a normal diet ranges from 476 to 1491 mL/day (mean, 705 mL/day), with an average frequency of the passages of gas per rectum of 10 times per day and an upper limit of normal of 20 times per day."

• Azpiroz, F : Intestinal gas dynamics: mechanisms and clinical relevance
• Dainese R, Serra J, Azpiroz F, et al. Influence of body posture on intestinal transit of gas. Gut 2003;52:971–4.
• Salvioli B, Serra J, Azpiroz F, et al. Origin of gas retention in patients with bloating. Gastroenterology 2005;128:574–9.
• King TS, Elia M, Hunter JO. Abnormal colonic fermentation in irritable bowel syndrome. Lancet 1998;352:1187–9.
• Chapter 10 - Intestinal Gas in Feldman: Sleisenger & Fordtran's Gastrointestinal and Liver Disease