Dietary Isoacids Improve Early-Life Feed Efficiency, Reduce Abdominal Fat Deposition, and Modulate Intestinal and Metabolic Responses in Broiler Chickens

This study evaluated the effects of increasing dietary inclusion levels of branched-chain volatile fatty acids, also called isoacids (ISO), on the growth performance, carcass traits, intestinal health, and breast muscle metabolomic profile of broiler chickens. A total of 700 one-day-old male Cobb 500 chicks were assigned to five corn–soybean meal-based diets with 10 replicate pens (14 birds/pen) in a completely randomized design: A Positive Control (PC) diet formulated to meet nutritional requirements was supplemented with 0.0, 0.10, 0.20, 0.40, or 0.60% of ISO (supplied as a blend of isobutyrate and 2-methylbutyrate). Data were analyzed using General Linear Models with orthogonal polynomial contrasts, PERMANOVA for microbiome beta diversity and ANOVA with FDR correction for metabolomics. Growth performance was monitored across feeding phases. At day 42, carcass traits, breast meat quality, intestinal permeability, cecal short-chain fatty acids (SCFAs), and cecal and ileal microbiota were assessed, alongside liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) metabolomics of the Pectoralis major muscle. Dietary ISO significantly improved average daily weight gain (p < 0.0001) and feed conversion ratio (p < 0.0001) during the first 21 days (early-life), with an optimal inclusion level of 0.40%. While overall carcass and breast yields were unaffected, ISO supplementation significantly and quadratically reduced abdominal fat deposition (p < 0.0001), with an optimal inclusion range between 0.10% and 0.40%. ISO did not compromise intestinal permeability or alter microbial alpha diversity, but it significantly restructured the cecal microbiota (beta diversity; Bray–Curtis p = 0.0213) and linearly increased cecal isobutyric acid accumulation in the cecal content (p = 0.0443). Breast muscle metabolomics revealed a dose-dependent, nonlinear response; the 0.40% inclusion level maximized the relative abundance of galactolipids, whereas the highest dose (0.60% ISO) upregulated sphingomyelins, lysophospholipids, and acylcarnitines. In conclusion, a specific dietary ISO supplementation dose of 0.4% serves as an effective nutritional strategy to boost early feed efficiency and reduce abdominal adiposity. Doses exceeding this optimal level, such as 0.60%, offer no further productive benefits and may trigger cellular stress.