The Effects of <i>L</i>-citrulline Supplementation on the Athletic Performance, Physiological and Biochemical Parameters, Antioxidant Capacity, and Blood Amino Acid and Polyamine Levels in Speed-Racing <i>Yili</i> Horses

The objective of this study was to evaluate the effects of pre-exercise <i>L</i>-citrulline supplementation on the athletic performance of <i>Yili</i> speed-racing horses during a high-intensity exercise. On the 20th day of the experiment, blood samples were collected at 3 h and 6 h post-supplementation to measure the amino acid and polyamine concentrations. On the 38th day of the experiment, the horses participated in a 2000 m speed race, and three distinct blood samples were gathered for assessing blood gases, hematological parameters, the plasma biochemistry, antioxidant parameters, and NO concentrations. The results indicate that the <i>L</i>-citrulline group showed a significant increase in the plasma citrulline and arginine concentrations. Conversely, the concentrations of alanine, serine, and threonine were significantly decreased. The glycine concentration decreased significantly, while there was a trend towards an increase in the glutamine concentration. Additionally, the levels of putrescine and spermidine in the plasma of the <i>L</i>-citrulline group were significantly increased. In terms of exercise performance, <i>L</i>-citrulline can improve the exercise performance of sport horses, significantly reduce the immediate post-race lactate levels in <i>Yili</i> horses, and accelerate the recovery of blood gas levels after an exercise. Furthermore, in the <i>L</i>-citrulline group of <i>Yili</i> horses, The levels of the total protein of plasma, superoxide dismutase, catalase, and lactate dehydrogenase were significantly increased both 2 h before and 2 h after the race. The total antioxidant capacity showed a highly significant increase, while the malondialdehyde content significantly decreased. In the immediate post-race period, the creatinine content in the <i>L</i>-citrulline group significantly increased. In conclusion, this study demonstrates that <i>L</i>-citrulline supplementation can influence the circulating concentrations of <i>L</i>-citrulline and arginine in <i>Yili</i> horses, enhance the antioxidant capacity, reduce lactate levels, and improve physiological and biochemical blood parameters, thereby having a beneficial effect on the exercise performance of athletic horses.