Excretion of benzoic acid derivatives in urine of sheep given intraruminal infusions of 3-phenylpropionic and cyclohexanecarboxylic acids.

The quantitative relationship between the urinary excretion of benzoic acid (BA) and the uptake of 3-phenylpropionic (PPA) and cyclohexanecarboxylic (CHCA) acids was assessed. PPA and CHCA are produced in the rumen by microbial fermentation of lignocellulosic feeds and metabolized, after absorption, to BA which is excreted in the urine mainly as its glycine conjugate hippuric acid (HA). Four sheep nourished by intragastric infusions of all nutrients were given continuous ruminal infusions of PPA (8, 16 or 24 mmol/d) either alone or with CHCA (8 or 16 mmol/d) in a factorial experiment. The treatments were allocated to ten consecutive 6 d periods, with a control being repeated at periods 1, 5 and 10. PPA and CHCA ruminal absorption rates, estimated using the liquid-phase marker Cr-EDTA, were 0.78 (SD 0.29)/h and 0.88 (SD 0.28)/h respectively. For the control, HA excretion was only 0.22 (SD 0.33) mmol/d and free BA was absent. For the other treatments, both HA and free BA were present and HA accounted for 0.85 (SD 0.05) of total BA: The urinary excretion of total BA showed a significant linear correlation (r = 0.997, P < 0.001) with the amounts of PPA and CHCA infused. The urinary recovery of infused PPA and CHCA as total BA was 0.79 (SE 0.01). Faecal excretion of BA and its precursors was negligible. Results of this study show that urinary total BA is a potential estimator of the absorption of PPA + CHCA produced in the rumen.