Transfer of blood urea nitrogen to cecal microbes and nitrogen retention in mature rabbits are increased by dietary fructooligosaccharides.

To estimate the effect of fructooligosaccharides (FOS) on N utilization, seven mature rabbits were fed a diet containing 5% glucose or FOS for 8 days. During the last 5 days, total feces and urine were collected to measure N levels (Experiment 1). To examine N transfer from the blood to cecal microbes, eight rabbits were fed the same diets as in Experiment 1. After 9 days of feeding, 2 g of glucose or FOS was given orally. Two hours later 20 mg of (15) N-urea was administered via the ear vein, and 1 h later cecal and blood samples were collected (Experiment 2). Urinary N excretion was lowered by FOS feeding (P < 0.05). Total bacterial N and (15) N in the cecum was significantly higher in FOS-fed animals (P < 0.05). Urea N in the cecum was lower in FOS-fed rabbits (P < 0.05). Similarly, (15) N atom % excess of cecal urea N was also lower in FOS-fed rabbits than in glucose-fed rabbits. These results suggest that FOS in the diet increases the transfer of blood urea N to the cecum for bacterial synthesis, thereby increasing N utilization.