1. Introduction
Exercise affects serum levels of amino acids and their metabolites, with important metabolic consequences. Since vitamin D impacts skeletal muscle protein degradation, we hypothesised that it would also impact exercise-induced changes in serum amino acid levels and the serum levels of arginine metabolites, influencing the body’s ability to synthesise NO. Accordingly, we analysed the effect of a single high-dose vitamin D supplementation on the serum levels of various amino acids in ultramarathon runners. Thirty-five male amateur runners were assigned to the supplemented group, administered 150,000 IU vitamin D in vegetable oil 24 h before the run (According to an increasing number of studies, vitamin D sufficiency positively influences skeletal muscle strength and endurance performance [The active metabolite of vitamin D, 1,25(OH)Nevertheless, the effects of vitamin D on muscle and human performance still remain to be completely understood. In addition to the proven effect on muscle tissue, there are still many pathways that may be affected by vitamin D and its activity.It is well-established that endurance exercise capacity is determined by working muscle blood flow, which is determined by cardiac output and microvascular blood flow [Vitamin D has been reported to exert some antioxidative effects [