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Short-Chain Fatty Acid Production and Fiber Degradation by Human Colonic Bacteria: Effects of Substrate and Cell Wall Fractionation Procedures^1,2,

Department of Animal Sciences and Division of Nutritional Sciences, University of Illinois, Urbana, IL 61801

Three dietary fiber sources (corn fiber, oat bran, wheat bran) were analyzed for chemical composition and potential fermentation by human colonic bacteria in vitro. Total dietary fiber (TDF) concentration of substrates was 64.3, 11.1 and 50.4 g/100 g dry matter for corn fiber, oat bran and wheat bran, respectively. Original material (ORIG), TDF fractions and simulated (SIM) cell wall fractions (produced by combining cellulose, hemicelluloses and pectic substances in proportions they represented in the cell wall) from each substrate were fermented in vitro for 6, 12, 18, 24 or 48 h using inoculum prepared from freshly voided feces from each of three human volunteers. Substrate dry matter remaining after 48 h of fermentation was 87.8, 39.8 and 73.5% for TDF fractions of corn fiber, oat bran and wheat bran, respectively. Disappearance of ORIG fractions was considerably greater than that of TDF due to fermentation of nonfibrous material. Disruption of cell wall structure during isolation of polysaccharide fractions allowed for dramatically increased fermentability of SIM relative to TDF. Averaged across all treatments, production of the short-chain fatty acids, acetate, propionate and butyrate, occurred in the molar ratio 63:21:16; however, profiles of short-chain fatty acids produced were influenced by both treatment and inoculum source. Extent of substrate fermentation varied among inoculum donors, implying that colonic microbial activities differ among individuals. Potential colonic fermentability of fiber sources was influenced by substrate, method of fiber preparation and inoculum source.

KEY WORDS: • dietary fiber • colonic fermentation • short-chain fatty acids • humans

¹ Based on work supported under a National Science Foundation Graduate Fellowship.

² Presented in part at the annual meeting of the Federation of American Societies for Experimental Biology, April 1–4, 1990, Washington, DC [Bourquin, L. D., Titgemeyer, E. C., Garleb, K. A. & Fahey, G. C., Jr. (1990) Effects of fiber source and isolation procedure on short chain fatty acid production and fiber degradation by human fecal bacteria in vitro. FASEB J. 4: A530 (abs.)].

³ National Science Foundation Graduate Fellow.

⁴ National Institutes of Health Postdoctoral Fellow.

⁵ Current address: Ross Laboratories, Columbus, OH 43215.

⁶ To whom correspondence and reprint requests should be addressed.

Manuscript received 21 October 1991. Revision accepted 19 March 1992.