The Effects of Different Dietary and Glycogen-Loaded Two-a-Day Training Approaches on Muscle Damage and Inflammatory Markers for Enhancing Metabolic Efficiency

Background/Objective: Insufficient research has examined the muscle damage and inflammatory effects of training performed in a fasted state with reduced (low) glycogen reserves. Methods: This study aimed to investigate the acute effects of exercise sessions performed under fasting (FST) and liquid nutrient-supplemented postprandial (PPD) conditions, as well as with full and reduced body glycogen stores, on muscle damage markers creatine kinase (CK) and lactate dehydrogenase (LDH), and the inflammatory marker interleukin-6 (IL-6). Eleven male amateur football players (mean age 19.91 ± 1.64 years) were included in the study. Participants performed a single 60 min aerobic endurance exercise session at 70% VO2max under the liquid nutrient-supplemented PPD condition, and two 60 min sessions with a 60 min recovery interval under 10–12 h FST conditions. Data were analyzed using paired samples t-test, ANOVA, Wilcoxon, and Friedman tests, depending on parametric and nonparametric assumptions. Results: In pre-exercise (PRE-FE) measurements, no significant differences were observed between the FST and PPD conditions for GLU, CK, LDH, and IL-6 levels. In postexercise (POST-FE) measurements, GLU levels were significantly higher in the PPD condition than in the FST condition (p < 0.05), whereas LA, CK, LDH, and IL-6 levels did not differ significantly between the two conditions. In the FST condition, following the second exercise session performed with default reduced glycogen availability (POST-SE), CK levels significantly increased from 279.36 ± 16.7 U/L to 335.63 ± 20.3 U/L, LDH levels from 178.72 ± 38.1 U/L to 208.09 ± 50.7 U/L, and IL-6 levels from 2.60 ± 2.4 pg/mL to 8.69 ± 5.8 pg/mL (p < 0.05). In the PPD condition, no significant changes were observed in any of the parameters following a single exercise session (p > 0.05). Conclusions: The two-a-day training approach performed with default reduced glycogen availability may increase muscle damage and inflammatory responses. These findings are considered as an inference that low-glycogen training should not be implemented during the pre-competition period because of the risk of inducing muscle damage.