Melatonin Enhances Muscle Development and Suppresses Fat Deposition in Cashmere Goats by Implicating Gut Microbiota and Ameliorating Systemic Antioxidant Status

Goat meat is widely valued as a healthy option due to its lean nature, yet strategies to further optimize its intrinsic nutritional composition remain a key objective. This study examined the influence of melatonin on muscle development and visceral fat deposition in cashmere goats, focusing on its role in augmenting systemic antioxidant capacity and modifying gut microbiota. Thirty goat kids were randomly assigned to a control or a melatonin-treated (2 mg/kg body weight) group. Melatonin implantation induced a metabolic shift characterized by reduced visceral fat deposition (perirenal, omental, and mesenteric fat; <i>p</i> < 0.05) without impacting intramuscular fat. Concurrently, it promoted muscle accretion, as demonstrated by an increase in crude protein content and hypertrophy of muscle fibers in the Longissimus thoracis et lumborum, Gluteus medius, and Biceps femoris muscles (<i>p</i> < 0.05). These effects were underpinned by an enhanced systemic antioxidant capacity (elevated CAT, GSH-Px, T-AOC, and reduced MDA; <i>p</i> < 0.05), changes in gut microbiota, and a concomitant improvement in gastrointestinal morphology, evidenced by increased rumen papilla length and intestinal villus height. Melatonin enriched beneficial genera (e.g., <i>Succiniclasticum</i>, <i>Butyrivibrio</i>, <i>Akkermansia</i>), which were significantly correlated with reduced adiposity and improved protein deposition. These improvements resulted from the concerted actions of an enhanced systemic antioxidant defense system and a beneficially modulated gut microbial community. This trial observed no effect on intramuscular fat deposition, suggesting that improving intramuscular fat may require a systematic fattening regimen. This study provides a scientific foundation for employing melatonin as a nutritional strategy in goat production to improve meat quality.