Moderate Reduction in Dietary Protein Improves Muscle Composition and Modulates Gut Microbiota and Serum Metabolome Without Compromising Growth in Finishing Pigs

Reducing dietary crude protein (CP) while sustaining growth performance and minimizing nitrogen emissions is a critical challenge in swine production. Beyond growth efficiency, the influence of low-protein diets (LPDs) on meat quality traits, gut microbiota, and systemic metabolism in finishing pigs remains insufficiently understood. In this study, 180 healthy crossbred finishing pigs (Duroc × Liangguang Small Spotted; initial body weight 85.49 ± 4.90 kg) were assigned to three dietary regimens for 35 days (six replicate pens per treatment, ten pigs per pen, male/female = 1:1): Control (CON, 15.5% CP), Low-Protein 1 (LP1, 14.5% CP), and Low-Protein 2 (LP2, 13.5% CP). Growth performance and nutrient digestibility were not impaired by protein reduction. Notably, LP1 pigs exhibited thicker backfat (<i>p</i> < 0.05), while LP2 pigs showed decreased concentrations of specific fatty acids (C12:0-C22:1n9) and essential amino acids (aspartic acid, glutamic acid, lysine) compared with LP1 (<i>p</i> < 0.05), indicating that dietary protein levels affected muscle composition. Cecal microbiota analysis revealed distinct shifts, with <i>Prevotella</i> spp., <i>Faecalibacterium</i> spp., and <i>Plesiomonas</i> spp. enriched in CON, whereas LP1 promoted <i>Eubacteriaceae</i> spp., <i>Christensenellaceae</i> spp., and <i>Clostridia</i> spp. (<i>p</i> < 0.05). Serum metabolomics further distinguished groups: LP1 reduced bile secretion and cholesterol metabolism pathways (<i>p</i> < 0.05) and LP2 further suppressed cholesterol metabolism and primary bile acid biosynthesis (<i>p</i> < 0.05), with a trend toward reduced phenylalanine metabolism (<i>p</i> = 0.07). Collectively, these findings demonstrate that moderate dietary protein reduction, when balanced with essential amino acids, maintains growth, reduces nitrogen output, and beneficially alters muscle composition, gut microbiota, and host metabolic pathways, offering nutritional strategies to enhance pork quality and promote sustainable pig production.