Background: Professional triathlon training is widely recognized as one of the most physiologically demanding forms of training, combining the multicomponent development of physical, biomechanical, and neuromuscular capacities across the three disciplines of swimming, cycling, and running. In this research, the authors measured the impact of two different training periods in elite male athletes on the serum concentrations of GLP-1, GIP, testosterone, and NF-κB and assessed their potential correlations. Methods: We compared a group of 37 triathletes, aged between 25 and 50, during preparatory and competitive periods, with a group of 20 healthy untrained males. The aim of the study was to assess the concentrations of testosterone, GLP-1, GIP, and NF-κB in both groups and in different exercise periods. We determined the markers using the ELISA method. Results: The results demonstrated an increase in GLP-1 concentration between two training groups (preparatory 44.73 ± 5.34 (pmol/L) vs. competitive 45.92 ± 6.13 (pmol/L); p < 0.001). Moreover, higher values of GIP serum concentration were observed in the training groups compared to the control group (33.21 ± 3.54 (ng/mL) vs. 38.83 ± 4.5 (ng/mL); p = 0.038). There were no statistically significant differences (p > 0.05) between the groups in terms of testosterone and NF-κB concentrations. There was also a statistically significant, strong positive correlation between NF-κB and GLP-1, GIP during each training period. In spite of the absence of definite changes in mean values of testosterone concentration between the groups, a minor increase in the median value in the group during the competitive period and a significantly lower median value in the control group were detected, suggesting a trend toward statistical significance. Conclusions: These findings may have clinical significance in athletes, as incretins and NF-κB could serve as biomarkers of metabolic adaptation to exercise and support individual training optimization. Further research is needed to investigate these relationships.
