Short-Term Nutritional Supplementation Accelerates Creatine Kinase Normalization in Adolescent Soccer Players: A Prospective Study with Regression Analysis

Background: Creatine kinase (CK) serves as a biomarker of muscle stress and damage and is often elevated in athletes after intense training. Persistent CK elevations may indicate subclinical muscle dysfunction or incomplete recovery. Methods: This prospective, randomized study included 100 male adolescent soccer players (aged 13–15 years) with elevated CK levels (>1000 U/L). Participants were allocated into two equal groups: an intervention group receiving nutritional supplementation and a control group without supplementation. Both groups abstained from training for 7 days. CK was measured at baseline, day 3, and day 7. Non-parametric tests were used to compare groups. Additionally, multiple linear regression and Lasso regression models were applied to identify predictors of CK percentage reduction over 7 days. Results: The supplementation group exhibited significantly greater CK reduction at both day 3 (median decrease: 55% vs. 28%) and day 7 (84% vs. 50.8%) compared with controls (p < 0.001 for both). Regression analyses indicated that nutritional supplementation had the highest predictive weight for CK decrease. The Lasso model achieved strong performance (R2 = 0.714; MAE = 0.068), showing that approximately 71% of the variability in CK reduction could be explained by the intervention. Conclusions: Short-term nutritional supplementation with antioxidants and amino acids may significantly accelerate CK return toward athlete-appropriate CK values during rest in adolescent soccer players with elevated CK. Regression modeling confirmed supplementation as the main determinant of CK reduction, supporting the inclusion of targeted nutritional strategies in recovery protocols for youth athletes.