The effect of wheat-bran fibre on the anaerobic metabolism of cholic acid by mixed faecal bacteria

Abstract

The mode of metabolism of the primary bile acids CA and CDCA by intestinal bacteria involves enzymic attack on the hydroxyl functions. This involves oxidereduction and 7adehydroxylation of the hydroxyl groups, the latter being the predominant reaction in the gut. 7a-Dehydroxylation yields the secondary bile acids DCA and LA from CA and CDCA respectively. The involvement of the secondary bile acids as possible co-carcinogens or promoters (Hill, 198 1) in the colon and the purported protective effect of dietary fibre in colorectal carcinogenesis (Reddy, 1980) has stimulated interest in the metabolic studies of bile acids both in oitro (Morotomi et al., 1979) and in oivo (Pomare et al., 1974). This study examines the effect of wheat-bran fibre on CA metabolism by mixed faecal flora in anaerobic batch culture. Mixed faecal flora from a healthy volunteer was cultured using Brain Heart Infusion broth with 0.02% sodium cholate as substrate and with varying additions of fibre. Cultures were incubated under 80% N,, 10% C02, 10% H2 and sampled at regular intervals for CA metabolites, bacterial analysis and culture pH. In order to quantify CA metabolites 5ml aliquots of culture were extracted with ethyl acetate, dried, methylated with ethereal diazomethane and finally quantified by g.1.c. as trimethyl silyl ether derivatives of the bile acid methyl esters. G.1.c. was conducted on a Pye 304 chromatograph fitted with a 3% OV-1 column, using a temperature gradient of 160-260°C at 4C/min. Bacterial metabolites of CA were identified by gas-chromatography-mass spectrometry on a Dupont 49 1 B series mass spectrometer coupled to a Varian aerograph 2780 gas chromatograph. Results showed that addition of wheat-bran fibre to the culture medium had a marked effect on CA metabolism. In fibre-free cultures the CA substrate was rapidly 7adehydroxylated to DCA with 100% conversion by 47h. During an intermediate period (12-30 h) there was production of keto derivatives of CA, i.e. 7a,l2a-dihydroxy-3-0xoSB-cholanoic acid and 3a, 12a-dihydroxy-7-0xo-5~-cholanoic acid, 3a,7a-dihydroxy-l2-oxo-5~-cholanoic acid, 3,7,12trioxo-SB-cholanoic acid and 3a,7j?, 12a-trihydroxy-SBcholanoic acid. Addition of 1% wheat-bran fibre retarded the metabolism of CA although the same metabolites were produced; 100% conversion of CA to DCA was not evident until 54h. Addition of 2.5% and 5% wheat-bran fibre completely inhibited 7a-dehydroxylation and 3a, 12a-dihydroxy7-oxo-5c this study substantiates these findings. The end products of fermentative carbohydrate breakdown are volatile fatty acids which lower culture pH, hence cultures with added fibre all had a consistently lower pH throughout the incubation