Serine-to-glycine ratios in low-protein diets regulate intramuscular fat by affecting lipid metabolism and myofiber type transition in the skeletal muscle of growing-finishing pigs

Serine and glycine are 2 of the first-affected nonessential amino acids in low crude protein(CP)diets for pigs.Therefore,we explored the effects of different dietary serine-to-glycine ratios on growth perfor-mance and lipid metabolism in growing-finishing pigs.A total of 160 crossbred healthy barrows,with a similar body weight of around 59.50 kg,were randomly allotted into 1 of 5 treatments(8 pens per treatment and 4 pigs per pen).The serine-to-glycine ratios of the 5 dietary treatments were as follows:diet A(NORMAL group),1.18∶1(16%CP);diet B(LOW group),1.2∶1(12%CP);diet C(S2G1 group),2∶1(12%CP);diet D(S1G2 group),1∶2(12%CP);and diet E(S1G1 group),1∶1(12%CP).We found that the pigs fed a low CP diet(12%CP),when maintaining serine-to-glycine ratio at 1∶2 and a total amount of 1.44%,had the same average daily gain as the pigs fed a normal CP diet(16%CP)(P>0.05),but they had increased intramuscular fat(P<0.05).Furthermore,they exhibited higher expression of genes involved in lipid oxidation(P<0.05),which was regulated by modulating methylation levels in the promoters of acyl-CoA oxidase 1(ACOX1)and acyl-CoA dehydrogenase medium chain(ACADM).When compared with the pigs fed a normal CP diet,these pigs had more oxidative myofibers(P<0.05),which were regulated by AMPK-PGC-1αand Calcineurin-MEF2/NFAT pathways in a coordinated manner.Our findings suggested that a dietary serine-to-glycine ratio of 1:2 is beneficial for improving meat quality in pigs fed a low CP diet.