Case

A student aged 23 years was born and educated in India and attended an Australian university for postgraduate studies. Shortly after arrival he began to experience abdominal discomfort, bloating and occasional bouts of loose bowel habit. Routine blood tests (blood count and chemistry) were normal. An upper gastrointestinal endoscopy was performed. The examination was normal and the rapid urease test for Helicobacter pylori was negative. Small bowel biopsies showed a normal duodenal mucosa, but the lactase level was reduced to 2 U/g protein (normal 20–120). Other disaccharidase levels were normal. Symptoms were attributed to lactose intolerance secondary to the increased intake of lactose-containing foods since arrival in Australia. At a follow-up consultation he was improved, but still had some abdominal discomfort, bloating and increased flatus. The high dietary intake of lentils was discussed and his diet modified, with symptomatic benefit.

Introduction

Many patients present complaining of ‘excessive gas’, which can indicate excessive passage of wind through the mouth or anus, abdominal bloating or audible bowel sounds (borborygmi). In population studies about 20% of individuals report abdominal bloating. The causes of a sensation of excess gas include air swallowing or ingestion of poorly absorbed but fermentable foods such as baked beans, bran or brassica vegetables. Other causes include lactose intolerance (due to lactase deficiency; see ch 18) or malabsorption (e.g. due to coeliac disease, giardiasis or bacterial overgrowth). Functional gastrointestinal disease can also produce a sensation of excessive gas.

History

An accurate history is essential. Determine if the problem is excessive belching (usually due to air swallowing), abdominal bloating or distention, or excessive flatus. Check for evidence of irritable bowel syndrome (IBS), which is characterised by abdominal pain and discomfort with altered bowel habit, a sensation of incomplete rectal evacuation and bloating which is often present in the afternoon. Enquire about the presence of alarm symptoms such as vomiting, bleeding or weight loss and investigate these symptoms if they are present. An adequate dietary history is required. In adults, lactose intolerance is the usual status for non-Caucasians. In general, origin from Northern Europe usually indicates normal lactase levels.

Physical Examination

Observe the patient during the interview. You may observe gulping of air and repetitive belching in the patient with aerophagia. An abdominal examination is required to exclude abdominal masses or evidence of intestinal obstruction. In most patients with a complaint of bloating the examination is normal.

Investigations

Breath hydrogen testing for lactose intolerance

Gas production in the human gut is determined by two factors: first, the amount of fermentable substrate that evades digestion in the small bowel and reaches the colon (Fig 8.1); and, secondly, by the individual characteristics of the colonic flora. Lactose intolerance is due to a deficiency of the enzyme lactase in the brush-border membrane of the intestinal villous cell and is manifested by abdominal cramps, borborygmi, bloating, excessive flatus (bacterial fermentation) and diarrhoea (osmotic effect) following milk ingestion. Primary lactase deficiency is genetically determined and lactose intolerance occurs from late childhood in populations who are not of northern European ancestry.

Figure 8.1 Schematic diagram showing arrival in the colon of carbohydrate (CHO) that has escaped digestion in the small intestine.

The breath hydrogen (H₂) test following a 50 g lactose challenge is a non-invasive, sensitive and specific method to identify subjects with lactase deficiency (Fig 8.2). The subject is prepared by an overnight fast following a meal of meat and rice and avoidance of foods rich in starch and fibre (to reduce baseline H₂ levels): smoking is prohibited. A baseline breath test is obtained and then a 50g test load of lactose is administered. Breath samples are obtained at half-hourly intervals for 4 hours. An abnormal result is characterised by breath hydrogen exceeding 20 parts per million over the baseline. Lactose-intolerant patients may experience the usual symptoms listed above. The majority of patients with lactase deficiency have adjusted their diet and generally avoid symptoms associated with lactose-containing foods. Management is by the use of commercial milk products containing lactase, hard and mature cheeses (minimal residual lactose) and yoghurt (auto-digesting).

Figure 8.2 The lactose breath H₂ test.

Breath hydrogen testing for bacterial overgrowth of the small intestine

The syndrome of bacterial overgrowth of the small intestine is characterised by the presence of diarrhoea, malabsorption and weight loss. Potential causes include altered anatomy (e.g. a blind loop associated with a Billroth II gastrectomy) and altered motility (e.g. scleroderma or intestinal pseudo-obstruction). Confirmation of the diagnosis is by demonstration of increased microbial numbers (over 10⁵ organisms/mL) in the proximal small intestine. Alternative, non-invasive techniques use the metabolic action of the bacteria. These indirect diagnostic tests include measurement of the production of H₂ after exposure of a fermentable substrate (e.g. glucose or rice flour) to pathologically high levels of intestinal bacteria. A positive test is characterised by a high fasting breath H₂ level after dietary preparation as mentioned above and the finding of a rise in breath hydrogen of at least 12 ppm within 2 hours of a 50 g glucose challenge (Fig 8.3).

Figure 8.3 The 2-hour glucose breath H₂ test. In this patient with scleroderma and bacterial overgrowth, the basal breath H₂ is elevated and rises further after exposure to glucose.

Laboratory Tests

These tests are rarely required. If organic disease is suspected check the haemoglobin, iron, red cell folate and vitamin B₁₂ levels. Check the serum albumen level for evidence of malabsorption. The tissue transglutaminase (TTG IgA) antibody is a sensitive and specific test for coeliac disease.

Physiology of Gas

The limited available information about the content, composition and role of intestinal gases is due in part to the difficulties of obtaining data in humans. The information on intestinal gas is based on (1) the ‘wash out technique’ (of Levitt and colleagues) in which an inert gas (argon) is infused rapidly into the jejunum and gases are collected with a rectal tube for analysis; (2) analysis of colonic gas collected via a rectal tube; and (3) scattered observations have been obtained by sampling gases at various levels of the gastro-intestinal tract. Based on the ‘wash out technique’ the normal gastrointestinal tract contains about 100 mL of gas. The passage of gas per rectum ranges from 200 to 3,000 mL (mean 700 mL) per 24 hours. Based on observations in young healthy subjects, the normal frequency of passage of flatus is 13.6 ± 6 times a day. The volume of flatus increases dramatically after ingestion of poorly absorbed carbohydrates such as baked beans or brassica vegetables.

Composition

The composition of gases in the gut varies with the site of sampling. For example, nitrogen concentration in the human stomach approaches atmospheric, suggesting that it originates from swallowed air. The five gases nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), hydrogen (H₂) and methane (CH₄) comprise about 99% of intestinal gas (Table 8.1, Figure 8.4). Nitrogen is the predominant gas and O₂ is present in low concentrations. Carbon dioxide in the upper small bowel reflects the interaction of gastric acid and bicarbonate secretion from the pancreas, biliary tree and small intestine. Most of the small bowel CO₂ appears to be absorbed. Carbon dioxide in the lower small bowel results from the interaction of bicarbonate and organic acids. Breath CO₂ can be studied by the use of isotopic labelling of the carbon atoms. Three gases (CO₂, H₂ and CH₄) are produced in the gut by bacterial action on unabsorbed carbohydrate. Hydrogen production is usually limited to the colon and is dependent on the ingestion of fermentable substrates, which escape absorption in the small intestine (Fig 8.1). In general, about 90% of most staple carbohydrate is absorbed in the small bowel but 10% escapes absorption and passes to the colon. Baked beans contain unabsorbable oligosaccharides such as stachyose and raffinose and these produce large quantities of gas after interaction with colonic bacteria. About 15% of hydrogen production in the colon is absorbed into the circulation and expired from the lungs. About 30% of normal individuals produce significant amounts of CH₄. The production is constant and unrelated to food intake.

Table 8.1 Gastrointestinal gas

∗ Plus traces of odiferous gases that are socially significant.

Figure 8.4 Mechanisms influencing rate of accumulation of gas in the gastrointestinal tract. Air is swallowed (1) and a sizeable fraction is then eructated (2). The O₂ of gastric air diffuses into the blood draining the stomach (3). The reaction of H⁺ and HCO₃^– yields CO₂ (4), which rapidly diffuses into the blood (5) while N₂ (6) diffuses into the lumen down a gradient established by the CO₂ production. In the colon (7), bacteria produce CO₂, H₂ and CH₄, which diffuse into the blood perfusing the colon (8). Bacteria also consume O₂ and H₂ (10). N₂ diffuses into the colon (9) down a gradient established by bacterial production of CO₂, H₂ and CH₄. The net result of all these processes determines the composition and rate of passage of gas per rectum (11).

From Dr Michael Levitt, in Johnson LR, ed. Physiology of the gastrointestinal tract. 3rd edn. New York: Raven Press; 2004, with permission.

All of the five major gases are odourless. The odour of flatus is due to trace quantities of other gases produced by colonic bacteria. Ammonia is due to urea breakdown; indoles and skatoles are produced by protein breakdown; and hydrogen sulphide (H₂S) and methanethiol result from amino acids with sulfur content. There are few published data on these odiferous gases.

Symptoms Associated with Intestinal Gas

Swallowed air (O₂ + N₂) is the main source of gas in the upper gastrointestinal tract. Some gas is swallowed during regular meals and this is intensified by gulping when excited. At times patients with troublesome, frequent and noisy belching are encountered. There is usually no detectable underlying pathology and the situation probably represents an acquired habit. Although this area has not been well studied, the likely explanation is that air is sucked back into the oesophagus during each act of belching. The most useful therapeutic option is to concentrate on avoiding the next belch.

Aerophagia refers to the unconscious habit of swallowing air. The subject needs to avoid smoking, chewing gum and carbonated beverages.

The origin of odoriferous gases after garlic ingestion has been studied by Suarez and colleagues from Minneapolis. The halitosis initially originates from the mouth and subsequently from the gut (metabolism in the colon causing large concentration in alveolar air). Oral hygiene may reduce the halitosis from the mouth and manipulation of the diet may limit gas production from the gut flora.

A sensation of abdominal bloating, sometimes accompanied by an increase in girth (distention), is a common and frustrating symptom in patients with IBS. Abdominal x-rays generally show normal quantities of gas in the intestines. A wash-out technique by Lasser et al. has been applied to quantify intestinal gas volume and composition in patients with IBS using normal healthy subjects as controls. Collection of gas at the rectum demonstrated similar volumes and composition of gas in the two groups. The patients with IBS experienced increased symptoms in response to the infusion of gas and there was increased retrograde flow of gas as determined by aspiration via a gastric tube. This experiment supports the view that abnormal gut sensation occurs in some patients with IBS, and this may explain the sensation of bloating. Visible abdominal distension can occur in patients with IBS. This has been studied by Accarino and colleagues from the Barcelona group of researchers. They studied morphovolumetric differences between computerised scans obtained before and during a severe bloating episode. The symptom was due to the descent of the diaphragm and protrusion of the anterior abdominal wall.

The buoyancy of stool is determined by the content of gas within the stool and not the content of fat. In the laboratory, stools must contain gas to float. Floating stools sink when their gas volume is compressed by positive pressure. The floating stool should not be considered a sign of steatorrhoea.

Based on observations in healthy young students the frequency of passage of flatus is 14 per day. The frequency and volume of flatus varies widely, and depends on the composition of the colonic flora. Maximum gas production occurs after poorly digested food such as baked beans and brassica vegetables (cabbage, brussels sprouts and broccoli), which should be avoided by patients with excessive flatus. Least rectal gas follows ingestion of carbohydrates such as rice flour, which is fully absorbed in the small intestine. The addition of fibre in patients with IBS is an important therapeutic option but can temporarily aggravate abdominal discomfort and bloating for 2–3 weeks.

Hydrogen and methane may reach combustible concentrations in the colon (greater than 4%). It is for this reason that the use of electrocautery to remove colonic polyps is restricted to the prepared colon. Fermentable substrates such as lactulose are best avoided in bowel preparation before electrocautery.

The offensive odour in flatus is due to sulfur-containing compounds. These include hydrogen sulfide, methanethiol, dimethylsulfide, short-chain fatty acids, skatoles, indoles, volatile amines and ammonia. Various approaches have been evaluated to minimise problems with odoriferous rectal gases. The only product that absorbed virtually all of the sulfide gases was briefs constructed from an activated carbon fibre fabric. Charcoal-containing pads worn inside the underwear were less effective and their role is limited by incomplete exposure of the activated carbon to the gases.

Key Points