Effects And Mechanisms Of Dietary Starch Levels And Types On Growth And Meat Quality Of Finishing Pigs

Feeding strategy is an effective means to improve meat quality. Previously studies have shown that different dietary starch levels and starch types could regulate muscle energy storage status or muscle fibre type at slaughter, which further influences meat quality through affecting postmortem muscle energy metabolism. This study investigated the effects of dietary starch levels and starch types on growth performance, carcass traits, meat quality, postmortem muscle metabolism and muscle fibre type of finishing pigs. In addition,researches were designed to study the effect of phosphoinositide 3 kinases/AKT-hypoxia inducible factor-1? (PI3K/AKT-HIF-1?) pathway on postmortem glycolysis and the effects of microRNAs (miRNAs) on the transformation of myofibre type, to explore the possible mechanism.1. Effects and mechanisms of dietary starch levels on growth and meat quality of finishing pigsA total of 72 barrow (Duroc×Landrace × Yorkshire, DLY) pigs with similar initial body weight (65.0 ? 2.0 kg) were uniformly allotted into three iso-energetic and iso-nitrogenous experimental diet treatments, with three replicates per treatment and eight pigs per replicate.All diets were formulated to meet the nutrient requirements of NRC (2012) for finishing pigs. The traditional diet (diet A) of finishing pigs was used as the control diet, the starch level of which was 44.1%. The starch levels of diets B and C were formulated by decreasing approximately 15% or 30%, on the basis of diet A, were 37.6% and 30.9%respectively. In the case of iso-energetic diets, the levels of dietary fat and neutral detergent fiber (NDF) varied with the change of dietary starch level. Diet A contained 44.1% starch,12.6% NDF and 5.9% crude fat; diet B contained 37.6% starch, 15.4% NDF and 9.5%crude fat; diet C contained 30.9% starch, 17.8% NDF and. 14.3% crude fat The duration of the experiment was 28 days, on the day of slaughter, after a 12-h fast, two or three per pen with medium bodyweight (eight per treatment) were selected, weighed individually and slaughtered. Samples from M. longissimus lumborum were collected for subsequent analysis. The results were shown as follow.(1) Effects of dietary starch levels on the growth performance, carcass traits, meat quality and myofibre typs of finishing pigsCompared with diet A, pigs fed diet C had lower average daily feed intake (ADFI),average daily gain (ADG) and back fat depth (P < 0.05), whereas no significant differences in feed: gain ratio (F/G), live weight, carcass weight and carcass yield of finishing pigs were observed among the three treatments (P > 0.05). Diets B and C increased pH45min value (P < 0.05), and diet C decreased drip loss in M. longissimus lumborum (P < 0.05)compared with those fed diet A. There were no significant differences of pH24h value, meat color (L*,a*,b*),cooking losses, shear force and textural characteristics of M.longissimus lumborum in all of treatments (P > 0.05). In addtion,greater mRNA expressions of myosin heavy-chain (MyHC)-I and MyHC-IIa and lesser expressions of MyHC-?x and MyHC-IIb (P < 0.05) in M. longissimus lumborum were found in pigs fed diet C, than in pigs fed diet A. Compared with diet A, miR23a expression was decreased (P< 0.05) in pigs fed diet C and associated with increased level of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1?) (P < 0.05), whereas no significant difference in myocyte enhancer factor 2C (MEF2C) mRNA expression was observed among the three treatments (P > 0.05). Contrary to miR23a, pigs fed diet C had increased levels of miR499 and miR208b compared with those fed diet A (P < 0.05).Converse to the elevations of miR499 and miR208b, the mRNA expressions of SRY-box containing gene 6 (Sox6) and specificity protein 3 (Sp3) were decreased in pigs fed diet C compared with pigs fed diet A (P<0.05),whereas no significant difference in thyroid hormone receptor associated protein 1 (Thrapl) mRNA expression was observed among the three treatments (P > 0.05)(2) Effects of dietary starch levels on postmortem muscle energy metabolism of finishing pigsCompared with diet A, pigs fed diets B and C had greater ATP content in M. longissimus lumborum (P < 0.05). Compared with diet A, pigs fed diet C showed greater activity of creatine kinase (CK) (P < 0.05) and lower content of phosphocreatine (PCr) (P < 0.05) in M. longissimus lumborum. No differences in contents of ADP, AMP and creatine (Cr)among three treatments were observed (P > 0.05). Compared with diet A, meat from pigs fed diet C showed decreased content of lactate and lower glycolytic potential (GP) (P <0.05), whereas no significant difference in glycogen was observed among the three treatments (P > 0.05). Compared with diet A, the percentage bound activitiy of hexokinase(HK) in M. longissimus lumborum was decreased in pigs fed diet B (P<0.05), and the percentage bound activities of HK and pyruvate kinase (PK) in M. longissimus lumborum were decreased in pigs fed diet C (P < 0.05), whereas no significant difference in the percentage bound activities of lactate dehydrogenase (LDH) was observed among the three treatments (P > 0.05). In addition, compared with diet A, pigs fed diets B and C decreased the level of phosphorylated AKT (P < 0.05) and increased the level of hydroxy-HIF-la (P< 0.05). Compared with diet A, the bound proteins amounts of HK ? and PKM2 in M.longissimus lumborum were decreased in diets B and C (P < 0.05), and the bound protein expression of LDHA was decreased in diet C (P < 0.05). However, diet treatments did not significantly affect the amounts of soluble proteins of HK ?, PKM2 and LDHA (P > 0.05).2. Effects and mechanisms of dietary starch types on growth and meat quality of finishing pigsNinety Yorkshire × Landrace × Meishan barrows with an average body weight of 68.0±2.0 kg were randomly allotted to three dietary treatments. Each treatment consisted of five replicates (pen) with six pigs per pen. The experimental diets were formulated to meet the nutrient requirements for 75-100 kg pigs (NRC 2012). Purified waxy maize starch (WMS),nonwaxy maize starch (NMS) and peas starch (PS) were used as starch sources. The analyzed amylose / amylopectin ratio of diets were 0.07, 0.19 and 0.28 respectively. The duration of the experiment was 28 days, on the day of slaughter, after a 12 h fast, two per pen with medium bodyweight (ten per treatment) were selected, weighed individually and slaughtered. Samples from M. longissimus lumborum were collected for subsequent analysis. The results were shown as follow.(1) Effects of dietary starch types on the growth performance, meat quality, changes of postmortem muslce proteins and myofibre typs of finishing pigsCompared with WMS diet, PS diet increased ADG and loin eye area (P < 0.05), and decreased F/G and back fat depth (P < 0.05), whereas no significant differences in ADFI,live weight, carcass weight and carcass yield of finishing pigs were observed among the three treatments (P > 0.05). The chemical compositions of M. longissimus lumborum were not different among diet treatments (P > 0.05). Compared with WMS diet and NMS diet,PS diet had greater pH45min value (P<0.05) and lower drip loss (P<0.05) in M.longissimus lumborum. Compared with WMS diet, PS diet had lower cooking loss in M.longissimus lumborum(P < 0.05),while insignificant effects were presented on pH24h value,meat color (L*, a*, b*) and shear force (P > 0.05). Compared with WMS diet, PS diet increased T22 peak area ratio (P < 0.05) and decreased T23 peak area ratio (P < 0.05) in M.longissimus lumborum, while no effect was observed on T21 peak area ratio. In addition,pigs fed with PS diet had greater sarcoplasmic protein solubility in M. longissimus lumborum at 24 h postmortem (P < 0.05) than those fed with WMS diet, while no effects were observed on total protein solubility and myofibrillar protein solubility (P > 0.05). Diet treatments did not significantly affect the SDS-PAGE photograph of sarcoplasmic protein and myofibrillar protein of M. longissimus lumborum at 24 h postmortem (P > 0.05).Compared with WMS diet, PS diet increased the mRNA expressions of MyHC-I and MyHC-?a (P < 0.05) and decreased the mRNA expression of MyHC-IIb (P < 0.05) in M.longissimus lumborum, while no effect was observed on the mRNA expression of MyHC-?x. Compared with WMS diet, PS diet decreased the expression of miR23a (P <0.05) and increased the mRNA expression of PGC-la (P < 0.05) in M. longissimus lumborum, while no significant difference in MEF2C was observed among the three treatments (P > 0.05). Contrary to miR23a, pigs fed diet C had increased level of miR499(P < 0.05), associated with decreased level of Sox6 (P < 0.05) in M. longissimus lumborum.There were no significant differences in expressions of miR208b,Sp3 and Thrapl were observed among the three treatments (P > 0.05).(2) Effects of dietary starch types on postmortem muscle energy metabolism of finishing pigsCompared with WMS diet, PS diet increased the activity of CK and content of ATP (P <0.05) in M. longissimus lumborum, decreased the contents of PCr and AMP in M.longissimus lumborum (P < 0.05),while no effects occurred on Cr and ADP concentrations(P > 0.05). Compared with WMS diet, pigs fed with PS diet showed lower glycogen content and GP in M. longissimus lumborum (P<0.05), while no effect was observed on the content of lactate. In addition, pigs fed with PS diet led to a drop in the percentage bound activity of HK compared with those fed with WMS diet (P < 0.05), but it insignificantly affected the percentage bound activities of LDH and PK in M. longissimus lumborum(P>0.05). Compared with WMS diet,PS diet decreased the level of phosphorylated AKT (P < 0.05) and increased the level of hydroxy-HIF-l?(P<0.05).Furthermore, pigs fed PS diet had greater amount of bound HK II in M. longissimus lumborum (P < 0.05), whereas, diet treatments did not significantly affect the amounts of bound proteins of PKM2 and LDHA, as well as soluble proteins of HK II, PKM2 and LDHA (P>0.05) .As stated above, the conclusions are as follow:(1) With iso-energetic and iso-nitrogenous, the low starch diet (diet C) decreased the growth performance of finishing pigs, this might be due to the reduction of feed intake induced by poor palatability and early satiety of feeding. The low starch diet improved meat quality of M. longissimus lumborum, this mainly achieved through promoting the fast-to-slow myofibre type transformation and decreasing the muscle glycolysis rate early postmortem. The transformation of fast-to-slow myofibre type might be associated with the downregulation expression level of miR23a, which promote the slow-to-fast myofibre type transformation, and the upregulation expression levels of miR208b and miR499, which promote the fast-to-slow myofibre type transformation. In addition, the reduction of muscle glycolysis rate early postmortem might have two reasons. Firstly, the fully degradation of PCr could provide more ATP to delay muscle glycolysis occur early postmortem. On the other hand, the inhibited PI3K/AKT-HIF-1? pathway decreaed the bingding of glycolytic enzymes to subcellular, and the activities of bound glycolytic enzymes were greater several fold than soluble glycolytic enzymes.(2) With iso-energetic and iso-nitrogenous, the high amylose diet (PS diet) improved the growth performance and carcass traits of finishing pigs. This mainly due to the high amylose diet resulted in the prolonged but continuous release and absorption of glucose,which can favor lean deposition. The high amylose diet improved meat quality of M.longissimus lumborum, this mainly achieved through promoting the fast-to-slow myofibre type transformation, as well as influencing postmortem muscle energy metabolism and further affect muscle proteins degeneration and improved the ratio of immobilized water.The transformation of fast-to-slow myofibre type might be associated with the downregulation expression level of miR23a and the upregulation expression level of miR499, which promote the fast-to-slow myofibre type transformation. Moreover, the more ATP generated from PCr degradation delayed muscle glycolysis occuring early postmortem and the inhibited PI3K/AKT-HIF-1? pathway decreased the rate of muscle glycolysis.