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 mucosa-associated 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.

To Cite This Article: Jan RU, Yu Z, Ahmad SZ, Zhong X, Zhang X, Wang M and Khan RU, 2026. Effects of rumen-undegradable protein and rumen-protected amino acids on small-intestinal microbiota in sheep with special reference to Chinese Hu breed. Pak Vet J, 46(7): 1636-1648. http://dx.doi.org/10.29261/pakvetj/2026.159