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Fermentation of Carbohydrate in Rat Ileal Excreta Is Enhanced with Cecal Inocula Compared with Fecal Inocula^1,2,3,

Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, WI 53706

The differential fermentative capacities of microflora from two regions of the large bowel and how fermentation was altered by prior exposure of the microflora to the substrate to be fermented were studied using an in vitro fermentation system. Bacterial inocula were prepared from cecal contents and feces from three groups of rats fed purified diets containing 100 g/kg dietary fiber from canned peas or psyllium seed husk, or a nonpurified diet containing 170 g/kg dietary fiber. The substrate for all fermentations was ileal excreta from colectomized rats fed a purified diet containing 100 g/kg dietary fiber from canned peas. Anaerobic fermentations were conducted for 0, 3, 6, 12, 24, 48 and 96 h. Sugars of the unfermented polysaccharides were measured by gas chromatography following acid hydrolysis; disappearance was the measure of fermentation. Independent of inoculum source, >90% of the starch and arabinose and 75% of the uronic acids, but <30% of non-starch glucose (the measure of cellulose), were fermented by 24 h. Cecal inocula fermented arabinose and uronic acids more quickly (P < 0.05) and fermented more (P < 0.05) non-starch glucose than fecal inocula. Inocula adapted to psyllium seed husk fermented starch faster (P < 0.05) and non-starch glucose, arabinose and uronic acids more slowly (P < 0.05) than inocula adapted to peas or nonpurified diet. Bacterial efficiency of carbohydrate utilization, the increase in muramic acid/mole carbohydrate fermented, was greatest (P < 0.05) with cecal inocula adapted to peas. We conclude that using cecal microflora as the inoculum source provides a more accurate index of fermentation during transit through the large bowel and that noncellulosic and storage polysaccharides of the plant cell wall are utilized before cellulose.

KEY WORDS: • dietary fiber • gas chromatography • rats • sugars

¹ Presented in part at Experimental Biology 94, April 24–28, 1994, Anaheim, CA [Monsma, D. J. & Marlett, J. A. (1994) In vitro fermentation of rat ileal excreta is affected by source of bacterial inoculum. FASEB J. 8: A152 (abs.)].

² Supported by National Institutes of Health grant DK21712 and by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, Madison, WI.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

Manuscript received 31 July 1995. Revision accepted 30 October 1995.