Low net energy diet compromises pork tenderness by promoting larger muscle fibers and diminishing intramuscular fat through fiber type reprogramming

To investigate the effects of low net energy (NE) diets on the meat quality of Tunchang pigs, a total of 96 Tunchang pigs at 55 days of age and with an initial body weight of 26.42 ± 1.04 kg were randomly assigned to four treatment groups with NE levels of 9.82 (control group, CG), 9.57 (EY1), 9.32 (EY2), and 9.07 MJ/kg (EY3). Each treatment consisted of 6 replicates with 4 pigs per replicate. The trial lasted for 63 days. The results showed that dietary NE levels had no significant effects on average daily gain or feed conversion ratio ( P > 0.05), whereas the average daily feed intake was significantly higher in the EY3 group than other groups ( P = 0.018). Compared with CG group, pigs in the EY3 group exhibited higher shear force ( P = 0.008) and lower intramuscular fat (IMF) content ( P = 0.046) in the longissimus dorsi muscle. In addition, low NE diets increased muscle fiber diameter, cross-sectional area, and relative mRNA expression of MYHC IIb ( P < 0.05), while reducing the relative mRNA expression of MYHC I and MYHC IIa ( P < 0.05). Proteomic analysis indicated that low-NE diets modulated key cytoskeletal proteins in muscle cells, including myosin-binding protein C slow type, myosin regulatory light chain 3, troponin C slow type, myosin heavy chain 2, and cysteine and glycine-rich protein 3, thereby promoting a shift from oxidative to glycolytic muscle fibers. Metabolomic results further confirmed this fiber-type transition, showing an accumulation of glycolytic metabolites, depletion of tricarboxylic acid cycle intermediates, and reduced biosynthesis of unsaturated fatty acid metabolites. Overall, reducing dietary NE to 9.32 MJ/kg had no adverse effects on growth performance or meat quality of Tunchang pigs; however, a further reduction to 9.07 MJ/kg induced glycolytic fiber remodeling, increased muscle fiber diameter and cross-sectional area, and decreased IMF deposition, ultimately impairing meat tenderness.