Guar and psyllium are fermented by human fecal bacteria in vitro. To see if they were fermented in vivo, eight subjects were studied over 3 separate days, in random order, while eating a polysaccharide-free diet. Twenty grams guar or psyllium, taken at breakfast, had no effect on breath hydrogen levels over 14 h. Mean breath methane and serum acetate concentrations after guar, 37 +/- 1 ppm and 93 +/- 6 mumol/L, respectively, were significantly greater than after control, 20 +/- 2 ppm (P less than 0.05) and 62 +/- 4 mumol/L (P less than 0.01), and psyllium, 20 +/- 2 ppm (P less than 0.05) and 78 +/- 6 mumol/L (P less than 0.05). Serum acetate increased after guar (area under curve 193 +/- 56 mumol.h/L; P less than 0.02) but decreased after psyllium and control. We conclude that guar is fermented in the human colon, producing rises in breath methane and serum acetate but not hydrogen. Although psyllium had no effect on hydrogen, methane, or acetate, this does not prove that it is not fermented.