Excessive Flatulence – Causes of Lower Gut (Bowel, Colon) Gas

What is Flatus?

Flatus is the gas that is passed out of the gut through the anus. It is commonly referred to as a ‘fart’. Expelling gas from the gut is a normal physiological process, whether through the mouth as a belch (‘burp’), or through the anus as flatus. Approximately 600 to 700 milliliters of gas is passed out in a day with less than 200 ml remaining in the gut. Gas in the lower gut, which includes the latter part of the small intestine, colon and rectum, is passed out of the system as flatus.

Flatus as a Diagnosis

The act of passing out flatus is known as flatulence. It is accepted as a part of everyday life unless it becomes excessive, unusually loud, extremely offensive odor or accompanied by other gastrointestinal symptoms like fecal incontinence, diarrhea, constipation or pain.

Flatulence can vary from being almost silent to loud with different descriptions to describe the sound. The odor may also vary, from flatus that has almost no odor, to an unpleasantly aromatic or severely offensive smelling expulsion. A person may even report variations in temperature when passing flatus. However none of these factors are clinically significant.

Natural Bowel Gas Production in Humans

Flatus is a combination of several gases including methane, hydrogen and carbon dioxide with smaller quantities of hydrogen sulfide and nitrogen. The composition is a result of a number of processes that contribute to the presence of gas in the lower gut.

  1. Air swallowing (aerophagia) occurs in every person to varying degrees and excessive air swallowing is commonly associated with eating or drinking too fast, a blocked nose, mouth breathing, cigarette smoking and gum chewing. Most of the air is passed out through the mouth as a belch but residual amounts pass further down into the gut.
  2. Foods that are high in indigestible material like fiber and with high quantities of sulfur compounds are more likely to contribute to gas formation in the bowel. Carbonated beverages are another contributing factor although much of this gas is passed out through belching.
  3. Chemical digestion is the break down of foods by the action of digestive enzymes. These chemical reactions inadvertently lead to gas production (byproduct) in most instances.
  4. Intestinal bacteria contribute significantly to the presence of bowel gas. These naturally occurring microorganisms consume residual nutrients and waste products in the small and large intestine releasing gas as a byproduct.
  5. Blood gas diffusion contributes to a small amount of gas in the lumen of the gastrointestinal tract. Nitrogen diffuses out of the blood stream and into the gut while hydrogen may enter the blood stream from the gut.

Most of the gas that enters or is produced in the upper gut is evacuated through belching, however, small amounts do pass into the lower gut to be passed out as flatus. Normally, a person passes flatus anywhere between 15 to 20 times in a day. The volume, odor and even sound of the flatus that is passed may vary from person to person and even during the course of the day.

Anywhere between 1,000 milliliters to 2,000 milliliters of gas is produced in the gut per day. This is primarily a result of chemical digestion and bacterial fermentation. Most of this gas is evacuated from the bowels but about 200 milliliters remain within the gut.

Bacteria in the bowel produce most of the gas in the lower gut. The large intestine has the largest population of these bacteria, which ingest remnants of food and waste matter in the colon. This is known as microbial fermentation. It is a vital process since these colonic bacteria produce vitamin K and enzymatically digest any remaining carbohydrates in the bowel. These nutrients are then absorbed by the the colon wall.

The main gases that accumulate in the bowel due to bacterial fermentation includes methane, carbon dioxide and hydrogen. Small amounts of nitrogen pass from the bloodstream into the lumen of the gut. This is eventually expelled from the rectum during a bowel movement or during the course of a day as flatus.

Causes of Excessive Flatulence

Apart from the normal process of gas production explaineded above, there are other contributing factors to the gas in the gut that are not pathological (due to disease). Some of these may also result in excessive flatulence but in most cases, the gas is passed out in belching. This includes :

Other non-pathological factors that may contribute to both excessive belching and excessive flatulence includes :

  • ‘Gassy’ foods (refer to Gassy Stomach)
  • High fiber diets
  • Medication

Impaired Digestion

Digestive problems (maldigestion) may arise for a number of reasons. Deficiency and inactivity of certain digestive enzymes prevents nutrients from being absorbed from the gut. This residual nutrition allows intestinal bacteria a larger nutrient supply for consumption thereby increasing the gas production in the gut.


A number of gastrointestinal infections can disrupt the normal physiological processes within the gut. Via various mechanisms, these infections may lead to excessive gas in the gut. Even after the infection resolves, a disturbance in the population of the normal intestinal flora (bowel bacteria) may continue to contribute to excessive belching and flatulence.

Refer to Stomach Infections for other causes.

Delayed Transit

If the transit of food and waste materials through the gut is delayed, either as a result of a disorder with gut motility or an obstruction (or pseudo-obstruction), the normal intestinal bacteria will have a longer period of time to consume residual nutrients thereby producing excessive gas.


Irritable bowel syndrome (IBS) is a functional bowel disorder the cause of which is not clearly understood. Excessive flatulence is often reported in IBS, both in constipation-predominant and diarrhea-predominant IBS. The exact cause of this excessive gas is also not clearly understood but may be due to impaired transit of intestinal gas (Reference 1).


  1. Impaired transit and tolerance if intestinal gas in IBS. PubMed.gov

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