Enteric fermentation and intestinal nitrogen metabolism in the growing pig
Microbes in the gastrointestinal tract of pigs can have a substantial impact on the utilization of dietary amino acids (AA) for various body functions. This may be attributed to microbe-induced endogenous gut protein secretions, microbial degradation of AA and microbial protein synthesis prior to the terminal ileum (i.e. upper gut). Research objectives were to determine the effect of feeding additional fermentable fiber (12% pectin) on synthesis rate of gut proteins (i.e. whole intestinal, mucosal and mucin proteins), microbial degradation of AA, and use of pre-formed AA from dietary (DP) and endogenous proteins (EP) versus ammonia (NH3) for microbial protein synthesis in the upper gut of growing pigs. A preliminary study showed that a flooding dose of valine had no impact on key indicators of metabolic status (plasma AA, glucose, insulin) in growing pigs. This method is, therefore, suitable for measurement of protein synthesis in tissues with high protein turnover rates. A relatively simple size exclusion chromatography method was then developed to isolate mucins from intestinal mucosal samples. Feeding additional fermentable fiber had no effect on fractional (FSR, %/d) and absolute (ASR, g/d) synthesis rate of total protein in the whole jejunum or colon of pigs, but it increased the FSR and ASR of mucosal proteins in the colon as well as the FSR of mucin protein in the colon. In isotope tracer studies, it was shown that microbial AA in both digesta- and mucosa-associated microbes in the upper gut of pigs are synthesized mainly from pre-formed AA from DP and EP, and that a large proportion of NH3 in ileal digesta is derived via microbial fermentation of DP and EP. In this study, intake of additional fermentable fiber had only a minor influence on the relative contributions of NH3, DP and EP to microbial AA, or to NH 3 in ileal digesta. It is, however, important to consider that the rate of microbial protein synthesis and fermentation, and thus AA losses, may have increased when feeding fermentable fiber. These AA losses associated with enteric fermentation should be considered when estimating the bioavailable dietary AA supply in pigs.