Metamucil Gas Production Clinical Study

Comparative Effects of Two Bulk Fibers, Psyllium and Methylcellulose, on Colonic Gas Production

By: Michael Levitt, M.D., The Minneapolis Veterans Affairs Medical Center, Minneapolis, MN

A high fiber intake is widely used for the treatment of constipation. While fiber intake can be increased via ingestion of fiber-rich foods, some individuals prefer to supplement their diets with commercially available preparations, which contain high concentrations of fiber. Most fiber products, such as psyllium, contain non-absorbed, fermentable material that potentially could serve as a substrate for reactions that release H2 and CO2 in the colon. Thus, a putative drawback to a high fiber intake is the widely held belief that fiber markedly increases the volume of intestinal gas. This potential for gas production has been forcibly transmitted to the public via advertisements for non-fermentable fiber preparations, such as methylcellulose, which allegedly do not cause excess gas. However, the quantity of fermentable material in psyllium (about 2.5g/dose) seemed too small to serve as a major source of gas production. Since the perception of gaseous symptoms is extremely susceptible to suggestion, we carried out blinded studies (a,b) to compare the gas-producing propensities of methylcellulose and psyllium.

Initially, we measured the breath H2 response of 10 healthy volunteers to ingestion of standard doses of methylcellulose and psyllium and 10g of lactulose (a non-absorbed sugar). As shown in Fig. 1, neither fiber preparation resulted in appreciable H2 production in contrast to the relatively enormous increase with lactulose.

chart breath

We next compared the gaseous symptoms of 25 healthy volunteers ingesting psyllium, methylcellulose, placebo, or lactulose in a randomized, double-blind crossover protocol. Neither psyllium nor methylcellulose increased flatus frequency or subjective impression of rectal gas over that observed with placebo, whereas lactulose caused significant increases in both symptoms. Lastly, 108 subjects who believed they had suffered from excessive gas during previous treatment with a high-fiber regimen were studied. These subjects were entered into a prospective, randomized, blinded protocol in which they received either methylcellulose, psyllium, placebo, or lactulose (about 25 subjects per treatment) for a one-week period. As shown in Fig. 2, neither methylcellulose nor psyllium was associated with a significant increase in flatus frequency, subjective impression of gas, or bloating.

Study chart
Flat frequency

Fig. 2. Flatus frequency perceived severity and bloating reported by subjects during one week periods on methylcellulose, psyllium, or lactulose. Light orange bars indicate symptoms during the baseline period and dark orange bars during the treatment period. Asterisks (*) indicate a significant increase in gaseous symptoms over that observed during the baseline period, and there was no difference between symptoms on methylcellulose versus psyllium.


When studied in blinded fashion, there is no evidence to suggest that either psyllium (a fermentable fiber) or methylcellulose (a non-fermentable fiber) causes a detectable increase in intestinal gas. Decisions concerning the use of a fiber preparation need not be based upon manufacturers' claims concerning the relative gas-producing properties of these preparations.

aLevitt MD, Furne J, Olsson S.
"The relation of passage of gas and abdominal bloating to colonic gas production."
Ann Intern Med.1996; 124:422:424.

bZumarraga L., Levitt MD, Suarrez F.
"Absence of gaseous symptoms during ingestion of commercial fiber preparations."
Aliment Pharmacol Ther. 1997;11:1067-1072

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