Much attention has been traditionally paid to the role of amino acids in protein synthesis and their requirements in animal nutrition. Recently, there is growing interest in regulatory roles of amino acids in nutrient metabolism and immune responses. They are called functional amino acids which are necessary not only to animal growth and maintenance requirement, but also to the synthesis of bioactive molecules. Remarkably, L-arginine (Arg) can enhance conception rates, embryogenesis, blood flow, antioxidant activity, protein synthesis, immune cell proliferation, and intestinal development, affect whole body composition, thereby improve fertility, fetal and postnatal growth, health, and meat quality (Kong et al. Chin J Anim Nuti, 2009).

The current swine industry strives to wean piglets at an early age to prevent sow- originated infectious diseases and maximize sows’ production potential. This practice presents a tremendous challenge to neonatal pigs, whose immune system is not well developed in the first four weeks of life. Nutritional means, including dietary supplementation with glutamine, zinc, or herbs, have been used to improve the immunity of early-weaned piglets. However, due to low feed intake in the first week post-weaning, these approaches cannot provide piglets with sufficient Arg, an amino acid crucial for optimal immune responses. One of studies from group of Livestock Ecology and Healthy Production showed that dietary supplementation with 0.4-0.8% Arg for two weeks enhances both cellular and humoral immunity in piglets by modulating the production of leukocytes, cytokines and antibodies. These results indicate that increasing Arg provision is beneficial for optimal immune responses in young pigs and also have important implications for designing the next generation of improved formula for human infants (Tan et al. Amino acids, 2009a). Another study from our group tested the hypothesis that Arg may stimulate cell proliferation and prevent lipopolysaccharide (LPS)-induced death of intestinal cells. The results demonstrated a protective effect of Arg against LPS-induced enterocyte damage through mechanisms involving mammalian target of rapamycin (mTOR) and Toll-like receptor 4 (TLR4) signaling pathways, as well as intracellular protein turnover (Tan et al. Amino acids,2010).

Arg is a nutritionally essential amino acid for young pigs, particularly under stressful conditions. Increasing evidence shows that inadequate exogenous and endogenous provision of Arg to 7-21-day-old pigs fed milk protein (which is deficient in Arg) is a major factor limiting their maximum growth. Additionally, Arg deficiency can result in hyperammonemia, impaired vasodilation of blood vessels, as well as intestinal and immunological dysfunction. Using the young pig as an excellent animal model for studying infant nutrition discovered that a low level of mitochondrial N-acetylglutamate (NAG) limits in vivo intestinal Arg synthesis in sow-reared piglets. Therefore, oral administration of N- carbamylglutamate (NCG), which is a safe and metabolically stable analog of NAG, increases the endogenous synthesis of Arg, plasma Arg concentration, muscle protein synthesis, and whole-body growth in milk-fed piglets. Although these findings provide the proof of principle for an important role of Arg in neonatal nutrition and growth, little is known about the effects of supplementing Arg or NCG to a plant protein-based diet for weanling pigs on intestinal health and growth. Wu et al. (Amino Acids, 2010) found that dietary supplementation with 0.6% Arg or 0.08% NCG enhances intestinal heat stock protein 70 gene expression, intestinal growth and integrity, and the availability of dietary nutrients for whole-body weight gain in post-weaning pigs. Thus, Arg or NCG is a functional ingredient in the weaning diet to improve nutrition, health, and growth performance of these neonates.

Obesity in humans is a major public health crisis worldwide. In addition, livestock species exhibit excessive subcutaneous fat at market weight. However, there are currently few means of reducing adiposity in mammals. Another study from our group was conducted with a swine model to test the hypothesis that dietary Arg supplementation may increase muscle gain and decrease fat deposition. The results support our hypothesis that dietary Arg supplementation beneficially promotes muscle gain and reduces body fat accretion in growing-finishing pigs. The findings have a positive impact on development of novel therapeutics to treat human obesity and enhance swine lean-tissue growth (Tan et al. Amino Acids,2009b). In addition, Arg plays an important role regulating nutrient metabolism, but the underlying mechanisms are largely unknown. These novel findings suggest that dietary Arg supplementation alters the catabolism of fat and amino acids in the whole body, enhances protein synthesis in skeletal muscle, and modulates intestinal microbial metabolism in growing pigs (He et al. Amino Acids,2009).

Therefore, large-scale availability of feed-grade functional amino acids holds great promise for enhancing the efficiency of nutrient utilization in pigs.