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.

About half the population excretes methane in the breath. To see if methane producing status influenced serum acetate, we studied six methane producers and six nonproducers on three separate days. For 36 h they ate a polysaccharide-free diet alone, or with 20 g of unabsorbed sugar lactulose, or 20 g of fermentable fiber, guar, in random order. The mean fasting serum acetate concentration on the three test days in producers was higher than in nonproducers, 84 +/- 5 vs. 69 +/- 5 mumol/L (P < 0.05). Compared to the control diet, both lactulose and guar raised serum acetate concentration significantly in both groups of subjects. However, there was a significant interaction between methane producing status and diet. After lactulose consumption, postprandial serum acetate was similar in both groups of subjects, but guar consumption had a significantly greater effect in producers than nonproducers (98 +/- 8 vs. 73 +/- 5 mumol/L; P < 0.05). We conclude that methane producing status may influence serum acetate concentrations in humans, depending upon the type of fermentable carbohydrate in the diet.