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Effect of Lactose on ⁵-Steroid-reducing Activity of Intestinal Bacteria in Gnotobiotic Rats¹

The Rega Institute, University of Leuven, 10 Minderbroedersstraat, B-3000 Leuven, Belgium

Seventy percent of cholesterol was converted into coprostanol in the large intestine of gnotobiotic rats associated with Eubacterium 21,408 plus Clostridium Cl-8 when the animals were fed a purified 56% starch diet. The extensive biohydrogenation of cholesterol, however, did not promote the fecal output of C-27 sterols in animals fed the basal diet without added cholesterol and did not inhibit the rise of serum and liver cholesterol in cholesterol-fed animals. When 75% of dietary starch was replaced by lactose, the pH of the cecal contents decreased by 0.8 unit to pH 6.1, the numbers of Eubacterium 21,408 decreased 100-fold and coprostanol formation was suppressed for more than 70% early after starting lactose feeding. In vitro, lactose did not inhibit pure cultures of Eubacterium 21,408, but suppressed this bacterium in mixed cultures with Clostridium Cl-8, E.coli or Str.faecalis. The inhibitory effect of lactose could be vitiated by keeping the pH of the medium above pH 6.5 by addition of CaCO₂ or KOH. These investigations suggest that the inhibitory effect of dietary lactose on the ⁵-steroid-reducing activities of the intestinal microflora is due to suppression of biohydrogenating microorganisms by the acids produced in the cecum and the colon by microbial fermentation of unabsorbed lactose.

KEY WORDS: • biohydrogenation • cholesterol • coprostanol • lactose • microflora

¹ Supported by a grant from the ‘Onderling Overlegde Acties’ of the ‘Fonds voor Collectief Fundamenteel Onderzoek’.

Manuscript received 30 October 1973.