Dietary L-Malic Acid Enhances Growth Performance and Oxidative Myofiber Composition in Nursery Piglets

L-malic acid (L-MA) is a naturally occurring organic acid abundant in fruits and traditional plant-based foods. L-MA, chemically identical to the natural form, has potential for optimizing energy metabolism and strengthening the antioxidant system in animal production. Objective: This research evaluated the impacts of dietary L-MA on nursery piglets’ growth performance, carcass traits, and muscle development. Forty castrated male crossbred piglets (BW: 8.06 ± 0.43 kg) were randomly assigned into five groups and provided a basal diet containing 0%, 0.25%, 0.5%, 1%, or 2% L-MA for a 56-day feeding trial. Dietary 0.5–2% L-MA enhanced final BW, ADG, and ADFI. All L-MA (0.25–2%) groups showed increased carcass weight, muscle length, and loin eye area, as well as higher pH value and color, and reduced drip loss and cooking loss of muscle tissues. An increased muscular abundance of oxidative fibers was observed in response to L-MA, accompanied by upregulated MyHCI and MyHCIIa and downregulated glycolytic-fiber-related gene expressions. Additionally, L-MA enhanced muscular SOD, GSH-Px, and CAT activities and upregulated SOD, GSH-Px, CAT, and NRF2 expressions, accompanied by a reduced MDA content. Furthermore, L-MA elevated plasma and muscular SDH and MDH activities and suppressed LDH activity. Moreover, L-MA augmented transcription of the myogenic markers (Myf6 and MyoD) and mitochondrial-related genes (COX4, Cytc, TFAM, NRF1, PGC-1α, and Sirt1), increased mitochondrial abundance, as evidenced by enhanced TOM20 intensity and upregulated expressions of COX4, Cytc, TFAM, NRF1, PGC-1α, and Sirt1. These changes were accompanied by transcriptional alterations in the AMPK/SIRT1/PGC-1α/NRF1 signaling axis. Significant enhancement effects were observed at L-MA supplementation levels of 0.5% and 2%. In summary, dietary L-MA improves ADG, ADFI, and final BW of nursery piglets, as well as muscle characteristics relevant to subsequent pork quality by enhancing antioxidant capacity, stimulating oxidative metabolism, and promoting slow muscle fiber formation. The observed effects may be partially associated with the AMPK/PGC-1α signaling pathway at the transcriptional level, as well as an increase in mitochondrial abundance. While these findings suggest a potential enhancement of mitochondrial biogenesis, definitive proof would require functional assays. Collectively, these findings underpin the potential of L-MA as a functional nutrient, a natural-identical source for improving performance and muscle quality characteristics in pig production systems. In addition, this study also contributes to the development of high-quality pork.