Disposition of D-[U-14C]tagatose in the rat

The purpose of this experiment was to determine the disposition of D-tagatose, under development as a low-calorie sweetener, in conventional and germ-free male rats. One group of conventional rats was fed a diet containing D-tagatose (100 g/kg) mixed with the nonpurified diet (900 g/kg) for 28 days. Then, [U-14C]-labeled D-tagatose was administered as a single dose (approximately 220-380 kBq) to 4 of these adapted rats, as well as to 15 conventional and germ-free rats with no prior exposure (i.e., unadapted) to D-tagatose. Eleven of the 19 dosed animals (4 adapted conventional, 3 unadapted conventional and 2 unadapted germ-free, all dosed orally, plus 2 unadapted conventional dosed intravenously) were placed in metabolism chambers and samples of CO2, urine, and feces taken at regular intervals. At termination, a complete material balance was obtained based on the recovery of 14C. Over the 6-h digestive period, D-tagatose was metabolized to release 39.9 and 13.9% of the oral dose as CO2 in the adapted conventional rats and in the unadapted germ-free rats, respectively. Total releases approximated 68 and 22%, respectively. The difference in CO2 evolution is ascribed to microbial fermentation of D-tagatose in the gut of the conventional rats. The role of adaptation was confirmed by finding 93% less D-tagatose in the feces of the adapted conventional rat than in the feces of the unadapted conventional rat. The intestinal absorption of D-tagatose in the rat is estimated to be 20%. The results demonstrate that D-tagatose is metabolized primarily by microorganisms in the gut of the rat, with an upper limit between 15 and 20% of oral dose metabolized by the host.