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Gut Reactions



Gas and bloating

A variety of gastrointestinal complaints such as belching, bloating or flatulence are commonly attributed by people to excess of gas. This can occur due to excessive air swallowing, increased intraluminal production of gas due to malabsorbed nutrients, decreased gas absorption due to obstruction or expansion of intraluminal gas due to changes in atmospheric pressure. It is estimated that the volume of gas in the intestinal tract is approximately 200 ml in both fasting and postprandial states and it is composed mainly by nitrogen (N2), oxygen(O2), carbon dioxide (CO2), hydrogen(H2) and methane (CH4). The composition of gas varies in a given compartment of the gastrointestinal tract. For example, stomach gas contains high concentrations of N2 and O2 like the atmosphere, while flatus contains less O2 and more CH4. The odor in the flatus is attributed to minor components of it: sulfur compounds, short chain fatty acids, skatoles, indoles, volatile amines and ammonia. Medical conditions that can cause excess of intestinal gas include: intestinal obstruction, motility disorders, irritable bowel syndrome, malabsorption, infections, psychological causes (aerophagia), and dietary causes.

Sources of intestinal gas

Swallowing of air

Several milliliters of air, mainly N2, are incorporated in each normal swallow. This phenomenon, aerophagia, could be a manifestation of anxiety and could be potentiated by chewing gum, and smoking, most of swallowed air appears to be eructated. The most common symptoms reported by people with aerophagia are belching, bloating, abdominal pain and abdominal distension. Most people with aerophagia tend to expel air that have reached the esophagus rather than air that have reached the stomach.

Intraluminal production

Three of the five principle gases, CO2, H2, CH4, are produced with the bowel lumen. CO2 is derived from digestion of fat and protein in the upper gastrointestinal tract, from bacterial fermentation of intraluminal substrates or liberated from interaction of acid and bicarbonate. H2 production occurs predominantly in the colon, except under conditions of small intestinal bacterial overgrowth, and is produced and consumed by fecal bacteria. Methane (CH4)​​ is exclusively a product of bacterial metabolism. The tendency to produce CH4 in the colon may be related to genetic or environmental factors, and is increased in individuals with colorectal cancer, extensive ulcerative colitis and colonic polyps.

Carbohydrate malabsorption

​​Unabsorbed carbohydrates provided a substrate for rapid bacterial fermentation. Most common carbohydrates malabsorbed include lactose, fructose and indigestible oligosaccharides abundant in legumes.

Diffusion from blood

The partial pressure of intestinal gases determines the direction of diffusion of the gas between the intestinal lumen and blood stream.

Gas Disorders


Belching or eructation is defined as the retrograde passage of esophageal or gastric gas out of the mouth. It may be voluntary or involuntary. Involuntary belching typically follows a meal and is caused by the release of swallowed air after gastric distension. Belching may be facilitated by foods that relax the lower esophageal sphincter such as chocolate, fats and mints. Chronic, repetitive belching is a disorder caused by habitual air swallowing in which air may transit only to the esophagus before being vented. Diagnosis evaluation should be considered in individuals with chronic eructation who have additional symptoms suggesting underlying pathology. Treatment of chronic eructation consists of breaking the vicious cycle of aerophagia by understanding the process for which it happens. Specific recommendations that could alleviate symptoms include discontinuation of habits such as chewing gum, smoking, drinking carbonated beverages, and gulping food and liquids. Specific treatment of anxiety should be considered if this is a prominent symptom. If behavioral modifications fail there is medical therapy is available to decrease excessive belching and aerophagia.


​​The volume of gas passed per rectum varies from about 500 to 1500 ml per day. The frequency of flatus released varies between 10 to 20 times per day in healthy subjects. Although excessive passage of flatus or its foul odor may be a source of embarrassment to people, it is rarely associated with serious illness. Offensive odor, when present, may be due to sulfur-containing compounds, such as methanethiol, dimethyl sulfide, and hydrogen sulfide. A number of factors may account for bothersome flatus:

  • Alteration of intestinal motility or bacteria
  • Dietary factors, such as an increased intake of lactose, fructose, sorbitol, indigestible starches in fruits, vegetables and legumes; and carbonated beverages. Products such as pork may release trace concentrations of malodorous gases upon digestion.
  • Psychological factors that may create heightened sensitivity to normal passage of flatus

Several measures can be undertaken to reduce annoying flatus. Dietary measures including avoidance of foods that may contribute to the problem is an obvious initial step. Also limiting dietary ingestion of known gas producing foods such as cabbage, Brussel sprouts, wheat and potatoes could be helpful. A good diet modification strategy is the low FODMAP diet. This is a diet low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols. It has been demonstrated to reduce abdominal distension, bloating and flatulence. Over the counter products like Simethicone, which causes gas bubbles to break and coalesce is widely used for treating gaseous complaints. The proposed value of activated charcoal has been supported by some studies and refuted by others.

Functional abdominal distension and bloating

Bloating refers to a person's sensation of abdominal fullness, whereas distension connotes visible measurable increased in abdominal girth. The feeling of abdominal bloating is often attributed to excessive gas; however, the relationship between the amount of intestinal gas and symptoms is not straightforward. Some investigations have suggested that patients with functional bloating may have impaired transit of intestinal gas or heightened sensitivity to gaseous distension or exaggerated motor response to normal amount of gas. Treatment of functional abdominal distension and bloating is similar to the approach used in treating excessive flatus mentioned above.

Refractory symptoms, of course, should prompt a visit to a gastroenterologist!

Dr. Luis E. Mejia is a board-certified gastroenterologist member of the American College of Gastroenterology with more than 10 years of experience in gastrointestinal endoscopy and general GI practice with special interest in functional GI disorders. He obtained his medical doctor degree at the Pontificia Universidad Catolica Madre y Maestra (PUCMM) Santiago, Dominican Republic in 1995. He completed his Internal medicine and Gastroenterology training at Wayne State University/Detroit Medical Center in Detroit, Michigan. He has been in practice in Orlando, FL for nearly ten years. He is on staff at Orlando Gastroenterology, PA, Florida Hospital and Orlando Health. Visit

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