Transfer of blood urea nitrogen to cecal microbial nitrogen is increased by mannitol feeding in growing rabbits fed timothy hay diet

Abstract

The presence of the fermentable sugar D-mannitol in the diet improves nitrogen (N) utilization in rabbits. To clarify the mechanism by which D-mannitol improves N utilization, we studied the effect of D-mannitol on the fate of blood urea N in growing rabbits. Growing rabbits received a control diet or a diet containing D-mannitol, which were formulated by adding 80 g/kg glucose or D-mannitol to timothy hay. After 9 days of feeding of the experimental diets, 15 N-urea was administrated intravenously under anesthesia 1 h before slaughter. The blood urea level (concentration of both urea N (43.6% of the control group (CG), P , 0.05) and 15 N (95% of the CG, P , 0.05) in blood serum) was reduced in the mannitol group. The concentration and amount of N, and 15 N atom % excess in the contents of the cecum and colon were higher (P , 0.05) in the rabbits fed the mannitol diet than in rabbits fed the control diet, especially in the cecum. The consumption of mannitol caused bacterial proliferation in the cecum characterized by marked short-chain fatty acid production (165% of the CG, P , 0.05), decreased cecal ammonia N (73% of the CG, P , 0.05) and elevated cecal bacterial N (150% of the CG, P , 0.05). On the other hand, addition of D-mannitol to the diet decreased N (80% of the CG, P , 0.05) and 15 N (77% of the CG, P , 0.05) excretion in the urine. These results indicate that D-mannitol increases the transfer of blood urea N to the large intestine, where it is used for bacterial N synthesis.Keywords: fermentable sugar, D-mannitol, nitrogen, growing rabbits ImplicationsCecotrophy allows rabbits to reingest microbial products from the cecum as cecotrophs. Cecotrophy has considerable significance for nitrogen (N) metabolism in rabbits. A previous study indicated that as a fermentable sugar, D-mannitol increases N utilization through cecotrophy by deceasing urinary N excretion in rabbits. In the present study, we clarified the possible mechanism by which D-mannitol improves N utilization in growing rabbits. The results indicate that D-mannitol can be used as a cecal microbial stimulator to promote the utilization rate of dietary N in rabbits fed a diet of timothy hay. Moreover, it might aid in reducing the N load passed on to the environment. IntroductionIt is well documented that fermentable carbohydrates can increase urea flux from the blood to the digestive tract and alter fermentation in the large intestine of rats (Ré mé sy and Demigné, 1989;Younes et al., 1995b;Henningsson et al., 2002). In rabbits, a considerable portion of endogenous urea is transferred to the alimentary tract (Regoeczi et al., 1965), and the exchange between blood urea nitrogen (N) and cecal ammonia N is well established Parker, 1985a and1985b). However, the exact fate of blood urea in rabbits fed a diet containing fermentable sugar remains unclear.Urea, as an end product of ammonia and protein metabolism, is synthesized in the liver of mammals and passed into the blood stream (Stewart and Smith, 2005). Most of the blood urea is eliminated ...

Transfer of blood urea nitrogen to cecal microbial nitrogen is increased by fructo‐oligosaccharide feeding in guinea pigs

“…Although an indigestible sugar has been shown to increase the concentration of bacterial N in the large intestine of rabbits (Xiao et al . ), in the present study the concentration of bacterial N in the large intestine of guinea pigs was not altered by consuming an indigestible sugar. The fluid digesta containing intestinal bacteria is selectively retained in the cecum for a much longer time than the particle digesta by the colonic wash‐back separation mechanism in the rabbit (Cork et al .…”

“…In rabbits, an indigestible sugar such as mannitol has been shown to increase transfer of blood urea N into the large intestine for bacterial N synthesis, and to contribute to the increase of N use (Xiao et al . ). In the present study in guinea pigs, FOS decreased the amount of total 15 N in urine and increased the 15 N atom % excess, and the amount of 15 N, in the total, protein and bacterial N in the cecum.…”

“…In previous studies, addition of indigestible sugars such as FOS or mannitol to the diet decreased urinary N excretion and improved N utilization of both guinea pigs and rabbits (Xiao et al . , ; Kawasaki et al . ).…”

“…The concentration of N, and the atom % excess of 15 N within total, protein, bacterial, ammonia and urea N, of samples (liver, gut contents and blood serum) were analyzed according to the method described by Xiao et al . (). The concentration of ammonia N in the serum was analyzed according to the Fujii‐Okuda method (Okuda & Fujii ), using a kit (Ammonia; Wako Pure Chemical Industries, Ltd, Osaka, Japan).…”

“…These effects have a close relationship with enhanced flux of blood urea N toward the large intestine (Rémésy & Demigné 1989). Furthermore, in rabbits, indigestible sugars such as mannitol increase N utilization and transfer of blood urea N into the large intestine, where it is used for bacterial N synthesis and contributes to the improvement of N use (Xiao et al 2011(Xiao et al , 2012. In guinea pigs, the addition of FOS to the diet also improves N use (Kawasaki et al 2013).…”