Metabolic markers linked to growth performance in Ross 708 broilers fed varying dietary crude protein levels

A study was conducted to investigate the effects of dietary crude protein (CP) levels on plasma metabolome profiles and growth performance in 480 male 0-day-old Ross 708 broilers (n = 8). Broilers were fed three-phase corn-soybean meal-based diets from d 0 to 42, consisting of starter (d 0 to 14; 21.0, 22.5, and 24.0% CP), grower (d 15 to 28; 19.0, 20.5, and 22.0% CP), and finisher diets (d 29 to 42; 17.5, 19.0, and 20.5% CP for Low, Med, and High CP groups, respectively). Plasma was collected on d 42 for untargeted metabolomic profiling. On d 42, the Low CP group showed lower body weight compared to the Med CP group (P = 0.014), without changes in feed intake or feed conversion ratio. Increasing dietary CP quadratically changed body weight (P = 0.009) and overall feed intake (P = 0.035). Compared with the Med CP group, the Low CP group had significantly lower plasma levels of linolenic acid, phosphate, alanine, pseudouridine, and glycine, whereas the High CP group had higher phenylethylamine, γ-tocopherol, and aminomalonic acid but lower phosphate (P < 0.05). Pathway analysis showed that the Low CP diet altered alanine, aspartate, and glutamate metabolism (FDR < 0.001), associated with higher TCA cycle intermediates such as fumarate and succinate. The Low CP diet also altered beta-alanine metabolism and phenylalanine, tyrosine, and tryptophan metabolism (FDR < 0.05). The High CP diet altered galactose and branched-chain amino acid metabolism (FDR < 0.05). Pearson correlation analysis showed that body weight was positively correlated with plasma phosphate (r = 0.607, P = 0.002), alanine (r = 0.437, P = 0.049), and linolenic acid (r = 0.405, P < 0.05). In conclusion, a low protein diet is associated with coordinated alterations in amino acid and tricarboxylic acid cycle metabolism, whereas a high protein diet imposes an internal metabolic burden associated with surplus amino acid catabolism without growth benefits, with plasma phosphate acting as a strong growth marker.