Low rumen degradable starch promotes the growth performance of goats by increasing protein synthesis in skeletal muscle via the AMPK-mTOR pathway

Since starch digestion in the small intestine provides more energy than digestion in the rumen of ruminants, reducing dietary rumen degradable starch (RDS) content is beneficial for improving energy utilization of starch in ruminants. The present study tested whether the reduction of rumen degradable starch by restricting dietary corn processing for growing goats could improve growth performance, and further investigated the possible underlying mechanism. In this study, twenty-four 12-wk-old goats were selected and randomly allocated to receive either a high RDS diet (HRDS, crushed corn-based concentrate, the mean of particle sizes of corn grain = 1.64 mm, n = 12) or a low RDS diet (LRDS, non-processed corn-based concentrate, the mean of particle sizes of corn grain >8 mm, n = 12). Growth performance, carcass traits, plasma biochemical indices, gene expression of glucose and amino acid transporters, and protein expression of the AMPK-mTOR pathway were measured. Compared to the HRDS, LRDS tended to increase the average daily gain (ADG, P = 0.054) and decreased the feed-to-gain ratio (F/G, P < 0.05). Furthermore, LRDS increased the net lean tissue rate (P < 0.01), protein content (P < 0.05) and total free amino acids (P < 0.05) in the biceps femoris (BF) muscle of goats. LRDS increased the glucose concentration (P < 0.01), but reduced total amino acid concentration (P < 0.05) and tended to reduce blood urea nitrogen (BUN) concentration (P = 0.062) in plasma of goats. The mRNA expression of insulin receptors (INSR), glucose transporter 4 (GLUT4), L-type amino acid transporter 1 (LAT1) and 4F2 heavy chain (4F2hc) in BF muscle, and sodium-glucose cotransporters 1 (SGLT1) and glucose transporter 2 (GLUT2) in the small intestine were significantly increased (P < 0.05) in LRDS goats. LRDS also led to marked activation of p70-S6 kinase (S6K) (P < 0.05), but lower activation of AMP-activated protein kinase (AMPK) (P < 0.05) and eukaryotic initiation factor 2α (P < 0.01). Our findings suggested that reducing the content of dietary RDS enhanced postruminal starch digestion and increased plasma glucose, thereby improving amino acid utilization and promoting protein synthesis in the skeletal muscle of goats via the AMPK-mTOR pathway. These changes may contribute to improvement in growth performance and carcass traits in LRDS goats.