Effect of Substrate Concentration on Short Chain Fatty Acid Production inIn vitroCultures of Human Faeces with Lactulose, a Rapidly Fermented Carbohydrate

Abstract

Objective : Fermentation of unabsorbable carbohydrate produces short chain fatty acids (SCFA) and rapidly lowers human colonic pH. SCFA produced in vivo will be absorbed. However, in vitro batch cultures are often used for assessing the fermentation characteristics of a carbohydrate. These models have several limitations. The accumulation of fermentation products due to increased substrate concentration may transform bacterial activity. In addition the enzymes involved in the fermentation process may become saturated producing misleading results. The objective of this study was to determine the effect of substrate concentration on SCFA production in vitro . Design : The effect of substrate concentration was estimated using incremental portions of lactulose in an in vitro fermentation model. Comparison of the total SCFA produced from increasing amounts of carbohydrate with those calculated by multiples of that produced from 2.5 mg/ml lactulose showed a progressive decrease in the production of SCFA with increasing lactulose. Results : The SCFA production from 10 mg/ml lactulose was significantly less than that from 7.5 mg/ml and when compared with amounts predicted from multiples of 2.5 mg/ml lactulose, was significantly reduced by 40–60%. Conclusion : Substrate concentration must be carefully selected to give a true index of SCFA production from rapidly fermentable carbohydrates. Keywords: fermentation, lactulose, short chain fatty acids, bacteria.