Effect of the intestinal flora on the urinary organic acid profile of rats ingesting a chemically simplified diet

Abstract

In order to estimate the influence of gut bacteria on animal metabolism, the excretion of organic acids, as monitored by gas-liquid chromatography, was compared in the urines of conventional and germ-free rats. Rats were maintained on a chemically simplified diet in order to minimize the effect of strictly exogenous compounds on the organic acid profile. Initial analysis of the excretion rates of 68 compounds, found reproducibly in the profile, indicated significant day-to-day and rat-to-rat variation, suggesting that haphazard comparison of experimental groups of animals might yield spurious differences with no biological significance. When repeated measures of the profile were analysed by a random effects analysis of variance model, no compound was found exclusively in the urine of either conventional or germ-free rats. Nevertheless, tricarballylate was significantly higher and both tartronate and vanillate significantly lower in conventional rat urine. The flora was implicated in these differences because caecal contents of conventional rats were found to convert such tricarboxylic acids as cis- and trans-aconitate to tricarballylate and to cause the dissimilation of both vanillate and tartronate, the latter compound being of dietary origin.