4. Discussion
In the current study, we demonstrated for the first time that supplementation with vitamin D 24 h before an ultramarathon run significantly influences serum levels of arginine, DMA, citrulline, and ornithine. Notably, arginine levels did not change in athletes not supplemented with vitamin D. Serum arginine levels reflect arginine formation from citrulline and ornithine and its consumption during exercise. Previously, a decrease in serum arginine levels was observed after a triathlon; however, neither Cit nor Ort levels were measured in that study [In the current study, we observed that vitamin D supplementation was associated with exercise-induced changes in serum amino acids and their metabolites. We were not able to determine the mechanism through which vitamin D affects arginine levels during such an exhausting run. However, the significantly increased serum Phe + Tyr levels after the run in the placebo-supplemented athletes indicate that exercise stimulates proteolysis [Protein degradation products include methylated forms of arginine, such as SDMA and ADMA. Under physiological conditions, some proteins that contain Arg are methylated by protein arginine methyltransferases. This can lead to the formation of proteins with two methyl groups at one nitrogen atom of the guanidino group (ADMA) or Arg with one methyl group on each terminal guanidine nitrogen atom (SDMA). Both free ADMA and SDMA are released following proteolysis. Approximately 300 μmol of ADMA is formed in healthy adults, with 20% excreted by the kidneys and the remainder of the metabolised excreted by DDAH [Vitamin D supplementation also significantly affected ultramarathon-induced changes in the serum levels of amino acids. The levels of such amino acids as serine, alanine, leucine, isoleucine, valine, methionine, and others decreased after the run only in the athletes in the supplemented group. These amino acids were metabolised during the run. Hence, the observed decrease in their levels in the vitamin-D-supplemented athletes and lack of change or increase in the placebo group could be explained by the vitamin-D-induced inhibition of proteolysis. A significant decrease in essential and non-essential amino acid levels after a half-ironman triathlon was reported previously [Vitamin D modulates changes in kynurenine metabolism induced by endurance exercise [Collectively, the current study revealed distinct serum levels of some amino acids, including tyrosine and phenylalanine, after an ultramarathon race, which is noteworthy in light of the applied supplementation. The mechanisms underlying these effects may be multifaceted. The results suggest that an intake of vitamin D could suppress the protein degradation increase that occurs during sustained exercise. Furthermore, the Arg/ADMA ratio was lower in the vitamin-D-supplemented athletes than in the placebo group, despite the observed increase in DMA levels, indicating rapid ADMA metabolism. It is possible that the serum Arg/ADMA ratio could be different from that in tissues, as suggested above [