Philip Roudebush,     Topeka, Kansas

Production of Intestinal Gas

The tendency to treat flatus as a humorous topic has obscured appreciation of the complex physiology that underlies the formation of intestinal gas. The quantitatively important gases in the intestinal tract are nitrogen (N₂), oxygen (O₂), hydrogen (H₂), carbon dioxide (CO₂), and methane (CH₄). These odorless gases make up more than 99% of the intestinal gas volume in people and pet animals (Web Table 51-1). The characteristic unpleasant odor of intestinal gas comes primarily from the presence of trace gases that contain volatile sulfur compounds such as hydrogen sulfide (H₂S), methanethiol (CH₃SH), and dimethylsulfide (CH₃SCH₃). The noxious odor of flatus in humans and dogs correlates most strongly with the concentration of hydrogen sulfide.

Gas in the gastrointestinal (GI) tract is normal and is derived from four primary sources (see Web Table 51-1): aerophagia (O₂ and N₂); interaction of gastric acid and alkaline food, saliva, or pancreatic bicarbonate (CO₂); diffusion from the blood (CO₂ and O₂); and bacterial metabolism/fermentation (CO₂, H₂, CH_4, and a variety of trace gases, including volatile sulfur compounds). Gases can be removed from the gut via passage out the esophagus or anus, diffusion into the blood, and consumption by bacteria. The net of these processes proximal to a given site in the GI tract determines the volume and composition of gas passing that site.

Swallowed air is thought to contribute the most to gas in the digestive tract. Air can enter the stomach in association with the swallowing of liquids or solids. Studies using ultrafast computed tomography in humans show that a mean of 17 ml of air accompanies the swallowing of 10 ml of water. Given the quantity of food and fluid ingested each day, large amounts of air normally may enter the stomach. If not belched, swallowed air passes through the GI tract with minimal absorption and often is passed per rectum. Air also can be swallowed in the absence of food or water ingestion via the propulsion of a bolus of air into the pharynx. This may be the cause of excessive flatus commonly seen in many brachycephalic breeds. Vigorous exercise and rapid and competitive eating situations may exacerbate aerophagia. Gas is moved along the gut independently of solids and liquids, and intestinal transit time is considerably shorter for gases. Air introduced into the stomach can result in flatus within 15 to 35 minutes. It has been estimated that gases can move 10 cm per second through the GI tract. The interaction between hydrochloric acid and alkaline food, saliva, or bicarbonate secreted by the pancreas produces carbon dioxide in the stomach and intestines. Studies in dogs showed that levels of carbon dioxide in the duodenum doubled after a meal. Belched gas largely is swallowed air plus variable quantities of carbon dioxide.

A large amount of gas is formed from bacterial fermentation in the colon. Substrates for bacterial gas production include dietary substances (fiber, poorly digestible protein, and carbohydrate), and endogenous sources (mucin, bile acids). Foods that contain large amounts of nonabsorbable oligosaccharides (e.g., raffinose, stachyose, verbascose) or other fermentable carbohydrates are likely to produce large amounts of intestinal gas. Dogs and cats lack the digestive enzymes needed to split oligosaccharides into absorbable monosaccharides. Therefore bacteria in the colon rapidly ferment these sugars, producing hydrogen and carbon dioxide. Soybeans, beans, peas, and other legumes contain large quantities of nonabsorbable oligosaccharides and often are associated with excessive flatus. Many fibers used in pet foods are fermented by colonic microflora and may contribute directly to flatus. Rapidly fermentable fibers in pet foods include pectins and most gums. Fiber-containing foods may contribute to flatus indirectly through reduced dry matter digestibility. Intestinal gas production also is increased by fresh or dried foods containing fructose, resistant starches, and fermentable fiber (e.g., apples, prunes, bananas).

Diseases that cause maldigestion or malabsorption often are associated with borborygmus, abdominal distention, and excessive flatus because large amounts of malassimilated substrates are available for bacterial fermentation. Flatus is also frequent in adult dogs and cats fed excessive amounts of lactose-containing foods.

Sulfur-containing gases are the major malodorous components of human and canine flatus. Dietary sources of sulfur (sulfates and sulfur-containing amino acids), and endogenous sulfur-containing compounds (e.g., mucin, taurocholate) are converted by sulfate-reducing bacteria to the odoriferous compounds hydrogen sulfide, methanethiol, and dimethylsulfide. Onions, nuts, spices, cruciferous vegetables (e.g., broccoli, cabbage, cauliflower, Brussels sprouts), and carrageenan contain high levels of sulfate and often increase production of malodorous gases. High-protein ingredients also may increase malodorous gas production.

Assessment of Patients with Flatulence

Pet owners often express concerns with clinical manifestations of flatulence and may describe an increase in frequency of belching, flatus, or borborygmus, objectionable odor of flatus, or abdominal distention. In one study 43% of dog owners reported flatus in their otherwise healthy pet dogs, and 13% of owners reported objectionable odor associated with the flatus episodes (Jones et al, 1998). Dogs housed indoors and less active dogs were more likely to have evidence of flatus. Temperament, frequency of feeding, specific diet, eating habits, age, gender, and history of previous GI disease were not found to be risk factors for flatulence in this study.

At times eliciting a history of dietary change or dietary indiscretion in association with flatulence may be possible. Specific foods, major food ingredients, treats, supplements, and opportunities for dietary indiscretion should be evaluated. A thorough assessment also should include verification of the feeding method currently used. Items to consider include feeding frequency, amount fed, how the food is offered, access to other food (e.g., a dog eating food for cats in the same household), relationship of feeding to exercise, and who feeds the animal.

There is widespread belief that some individuals are consistently more flatulent than others. This is probably true because studies in humans have shown great variability in the frequency of flatus. Such variation probably occurs in pet animals as well. Rectal gas excretion rates in humans range from 400 to 1500 ml/day (mean 705 ml/day). Humans eating their usual diet passed gas per rectum an average of eight to 10 times per day with an upper normal limit of 20 times per day. In general, frequency of flatus correlates with volume of intestinal gas; thus increases and decreases in episodes of flatus can be used to obtain a relative idea of changes in intestinal gas volume. Frequency of gas passed per rectum has not been evaluated in pets whose owners have complained of problems with flatus.

Belching, abdominal distention, and flatus may develop in conjunction with other GI signs, including weight loss, diarrhea, and steatorrhea. This type of history is suggestive of an underlying small intestinal disorder. Examples of chronic intestinal disorders often associated with flatulence include exocrine pancreatic insufficiency, inflammatory bowel disease, gluten-sensitive enteropathy, food sensitivity, antibiotic-responsive diarrhea, and lymphangiectasia. In one study 26% of cats with chronic diarrhea and/or vomiting had flatus, and 11% of these cats had abdominal distention (Guilford et al, 2001). Cats with clinical evidence of flatulence always should be evaluated closely for underlying chronic GI problems such as inflammatory bowel disease or food sensitivity.

Excessive aerophagia is a risk factor for flatulence and is seen with brachycephalic breeds, working and sporting canine breeds, and dogs with aggressive and competitive eating behaviors. Dietary indiscretion and ingestion of certain pet food ingredients may be risk factors for certain individuals. Excessive belching, rapid eating, and aerophagia also have been identified as risk factors for gastric dilation-volvulus (GDV) syndrome and should be considered important clinical findings in dogs at risk for GDV syndrome.

In many cases physical examination findings are unremarkable in dogs and cats with flatulence. Intestinal gas often can be detected during abdominal palpation, but it is difficult to assess the quantity of gas from palpation alone. Laboratory testing usually is not indicated. Animals may be in poor body condition if objectionable flatus occurs secondary to an underlying GI condition. Further evaluation is necessary if concomitant vomiting, diarrhea, or weight loss is present.