The Relation of Passage of Gas and Abdominal Bloating to Colonic Gas Production

Methods

The study group consisted of 25 employees at a Veterans Affairs medical center (13 women and 12 men; 21 to 59 years of age) who had no recent history of bowel distress or ingestion of drugs that influence bowel function. For 1-week testing periods separated by 1-week washout periods, the participants' usual diets were supplemented at breakfast and dinner with either 20 g of a commercial flavoring; 5 g of the nonabsorbable disaccharide lactulose mixed with 20 g of commercial flavoring; 6.0 g of psyllium (sugar-free, orange-flavored Metamucil [Procter & Gamble, Cincinnati, Ohio]) containing 3.4 g of fiber; or 10 g of sugar-free Citrucel (SmithKline Beecham, Pittsburgh, Pennsylvania) containing 2 g of the nonfermentable fiber methylcellulose. We determined the order in which the supplements were administered by using a random numbers table. All supplements were taken in 1 cup of water and were orange and sweet, but they differed in taste and consistency.

Participants recorded each flatus passage, their impressions of flatus volume, and their sensations of abdominal bloating (feeling of distended intestine). The latter two symptoms were recorded at noon and bedtime on a scale of 0 equals none, 1 equals slight, 2 equals mild, 3 equals moderate, 4 equals strong, or 5 equals severe.

We determined the influence of the fiber preparations on breath hydrogen excretion in five participants. After an overnight fast, Citrucel (10 g) or Metamucil (6 g) was ingested in 1 cup of water and was followed 2 hours later by a similar second dose. Alveolar breath samples, collected before fiber ingestion and each hour for 12 hours, were analyzed for hydrogen by gas chromatography [1]. During this period, noncaloric fluids and one quarter pound of hamburger, a food that does not result in hydrogen production [2], were allowed.

Data are expressed as mean ±SD. We determined the significance of differences between means using the Wilcoxon signed-rank test. A P value of less than 0.05 was considered significant. The Human Studies Subcommittee of the Veterans Affairs Medical Center, Minneapolis, Minnesota, approved the study, and participants gave written informed consent. Procter & Gamble Co. played no role in gathering, analyzing, or interpreting data.

Results

During the control period (commercial flavoring), participants passed gas an average of 10 ± 5 times per day (Figure 1). This frequency increased to 19 ± 12 times per day (P < 0.01) with lactulose, whereas no significant change was seen with psyllium (12 ± 7 times per day) or methylcellulose (11 ± 5 times per day).

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Figure 1. The mean and 1 SD for all 25 participants during the 1-week control period and weeks during which the diet was supplemented with lactulose, psyllium, or methylcellulose is shown. Average daily frequency of flatus passage.

The subjective impression of rectal gas volume agreed with the emission count; the values for methylcellulose (6.4 ± 5.0) and psyllium (6.7 ± 5.4) did not differ significantly from the base value of 6.2 ± 4.0. A significantly increased score (9.6 ± 5.0; P < 0.01) was reported with lactulose.

Increased abdominal bloating scores relative to the control period (0.29 ± 0.45) were reported during ingestion of lactulose (1.2 ± 1.5), psyllium (0.77 ± 0.85), and methylcellulose (0.77 ± 1.0) (Figure 2).

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Figure 2. The mean and 1 SD for all 25 participants during the 1-week control period and the weeks during which the diet was supplemented with lactulose, psyllium, and methylcelluose is shown. Average daily score (sum of noon and bedtime scores) for abdominal bloating.

Ingestion of psyllium or methycellulose did not increase breath hydrogen concentration in any participant. Rather, the hydrogen concentration steadily declined from a mean of about 8 parts per million (ppm) to roughly 2 ppm at 12 hours.

Discussion

We studied the relation between abdominal bloating and intestinal gas in 25 healthy volunteers whose diets were supplemented with lactulose or psyllium for 1-week periods. Both of these compounds supply substrate for gas-forming reactions by colonic bacteria. An absorbable commercial flavoring or methylcellulose, a reportedly nonfermentable fiber with water-binding properties similar to those of psyllium, served as non-gas-producing controls.

During the control period, participants passed gas 10 ± 5.0 times per day, a value similar to previous reports of 13.6 times per day for U.S. men [3] and 8 times per day for British research participants [4]. Lactulose ingestion resulted in significant increases in flatus frequency (mean, 19 times per day), subjective impression of gas volume, and sensations of bloating. In contrast, psyllium ingestion did not significantly increase flatus frequency or the subjective impression of gas volume. This apparent lack of gas production was supported by the declining breath hydrogen concentrations observed after psyllium ingestion, in contrast to the increases routinely observed after lactulose ingestion [5, 6]. Either psyllium provides little substrate for gas-producing reactions, or the hydrogen released during fermentation is rapidly consumed [7].

Despite the absence of gas production with psyllium, participants reported increased bloating, which they interpreted as increased gas. Participants reported similar increases in bloating with the nonfermentable fiber methylcellulose. Lasser and colleagues [8] previously observed this dissociation between bloating and intestinal gas in a study show ing that patients with severe bloating had normal bowel gas volumes.

The bloating associated with psyllium and methylcellulose presumably reflects the increased fecal mass that results from the water-holding capacity of fiber. Because “gaseousness” associated with fiber therapy is assumed to result from bacterial fermentation of fiber, nonfermentable fibers are marketed as less “gaseous” than their fermentable counterparts. However, if distention associated with fiber reflects the beneficial property of these materials (that is, increased fecal bulk), a low “gas” fiber (one not associated with “gaseous” symptoms) may be inherently impossible.

It should be stressed that we evaluated subjective responses to dietary supplements that differed in taste and appearance. In addition, potential variables such as diet, air swallowing, and the colonic flora could not be controlled. With these limitations in mind, the results of our study suggest that the initial step in evaluating patients with “too much gas” should be to determine whether the symptom refers to excessive rectal gas or to bloating. The occurrence of more than 20 flatus passages per day indicates excessive rectal gas, which in turn usually means that excessive fermentable substrate is reaching the colonic flora. This substrate could be inherently indigestible carbohydrates (such as the oligosaccharides in legumes [9]) or malabsorbed compounds (such as lactose malabsorbed by lactase-deficient participants).

Roentgenographic and endoscopic tests are of little value in patients with excessive rectal gas; the only potentially useful studies are absorption tests. Treatment includes limiting the quantity of fermentable material that reaches the colon. In contrast, persons who report abdominal bloating are probably not producing excessive gas. Although partial bowel obstruction can cause such symptoms, these symptoms (when longstanding) are normally associated with other symptoms that are typical of the irritable bowel syndrome (such as alternating diarrhea and constipation or symptom relief with defecation). Studies to visualize the gastrointestinal tract are rarely useful in such patients. Therapy should consist of treating of the underlying irritable bowel syndrome.