Effects of Rumen-Undegradable Protein and Rumen-Protected Amino Acids on Small-Intestinal Microbiota in Sheep with Special Reference to Chinese Hu Breed

In ruminants, dietary crude protein does not directly determine the amino acids available to the small intestine because rumen-degradable protein (RDP) supports microbial synthesis, whereas rumen-undegradable protein (RUP) bypasses fermentation.Together, they define the metabolizable protein (MP) supply.The amino acid profile is delivered to the jejunum and ileum.Adjusting the RUP:RDP balance by using rumen-protected lysine (RP-Lys) and rumen-protected methionine (RP-Met) can improve growth and nitrogen-use efficiency.However, the response depends more on post-ruminal release and bioavailability on protection claims alone.Increasing evidence indicates that segment-specific luminal substrates and epithelial signaling are linked to microbial ecology.These effects include changes in mucosaassociated communities, metabolite networks, and barrier/immune markers.However, many studies rely on rumen or fecal measurements and cannot reliably infer mechanisms in the small intestine.This review synthesizes segment-level evidence relevant to Hu sheep.It also highlights how bioavailability and sampling compartment (digesta vs. mucosa) affect interpretation.In addition, it proposes a diet-substrate-epithelium-microbiota framework to guide future trials.Key gaps include limited causal designs, inconsistent reporting of protected amino acid release/availability, and insufficient integration of nitrogen partitioning with intestinal endpoints.Addressing these gaps will strengthen gut-resilient nutritional strategies in Hu sheep production systems.