Long-term leucine supplementation increases body weight in goats by controlling appetite and muscle protein synthesis under protein-restricted conditions

An inadequate amino acid (AA) supply in animals under protein-restricted conditions can slow skeletal muscle growth. Protein translation can be activated by short-term leucine (Leu) stimulation; however, whether muscle mass increases under long-term Leu supplementation and how the gut and muscle respond to Leu supplementation are largely unknown. In this study, we investigated if muscle mass increases with long-term Leu supplementation under protein-restricted conditions. We identified changes in the link between the gut and muscles under different amino acid supply conditions, using goats as the study object. A total of 27 Xiangdong black male goats with average initial body weight (BW) of 10.88 ± 1.22 kg were randomly divided into three dietary treatments: a normal protein diet (NP, 14.24% crude protein [CP]); a low protein diet (LP, 8.27% CP with supplemental 1.66% rumen-protected lysine [RPLys] and 0.09% rumen-protected methionine [RPMet]); and LP diet with rumen-protected Leu (RPLeu) (LP + RPLeu, 8.75% CP with supplemental 1.66% RPLys, 0.09% RPMet and 1.46% RPLeu). The animal trial lasted for 110 d, consisting of 20 d of adaptation and a 90 d of experimental period. The results showed that long-term protein restriction increased gut tryptophan hydroxylase 1 (TPH1) activity (<i>P</i> < 0.001), tryptophan (Trp) catabolism (<i>P</i> < 0.001), and 5-hydroxytryptamine (5-HT) synthesis (<i>P</i> < 0.001), which all subsequently reduced goat appetite. Long-term Leu supplementation inhibited 5-HT synthesis (<i>P</i> < 0.001), decreased Trp catabolism in the gut, and increased appetite in goats. Long-term protein restriction enhanced jejunal and ileal branched-chain amino acid transferase (BCAT) (<i>P</i> < 0.001) and branched-chain α-Keto acid dehydrogenase (BCKD) (<i>P</i> = 0.048) activities, which increased branched-chain amino acid (BCAA) catabolism. Immunofluorescence results showed that protein restriction decreased the intestinal mucosal expression of solute carrier family 1 member 5 (<i>SLC1A5</i>) (<i>P</i> = 0.032) and solute carrier family 7 member 5 (<i>SLC7A5</i>) (<i>P</i> < 0.001), reduced BCAA transport from the mucosa to the blood, lowered BCAA levels in the blood (<i>P</i> < 0.001). Western blot results showed that protein restriction inhibited mammalian target of rapamycin (mTOR) pathway activation in goat muscles. Leu supplementation increased BCAA translocation from the intestine to the blood and promoted activation of the muscle mTOR pathway and protein synthesis. In conclusion, our results suggest that Leu supplementation in low-protein diets improves appetite and alleviates the inhibition of muscle protein synthesis in goats.