Abstarct: Background and Aim: Due to the intermittent and high-intensity nature of soccer, the regulation of energy mextabolism and the ability to efficiently utilize energy substrates play a crucial role in maintaining performance and delaying fatigue. In recent years, the use of ergogenic supplements, particularly caffeine and nitric oxide-enhancing compounds, has received considerable attention because of their potential effects on muscle blood flow, mitochondrial efficiency, lipolysis, and energy economy. However, limited evidence exists regarding the interactive effects of these supplements on fat and carbohydrate mextabolism in trained athletes, especially soccer plaxyers, and findings from previous studies have been inconsistent. Therefore, the present study aimed to investigate the effects of combined supplementation with caffeine, dietary nitrate, arginine, and citrulline on substrate oxidation and serum free fatty acid concentration in response to high-intensity exercise in soccer plaxyers. Methods: Thirteen male participants were randomly assigned to four experimental conditions: nitrate-baxsed supplementation, caffeine supplementation, nitrate plus caffeine supplementation, and placebo. In each trial, participants consumed one of the following supplements prior to a high-intensity exercise test: (1) a nitrate-baxsed supplement consisting of 3 g L-arginine, 3 g L-citrulline, 300 mL beetroot juice, and 200 mL water; (2) caffeine at a dose of 3 mg·kg⁻¹ body mass dissolved in 500 mL water; (3) a combination of the nitrate-baxsed supplement and caffeine; and (4) a placebo consisting of 30 mL beetroot juice and 500 mL water. Following supplementation, participants performed a high-intensity treadmill exercise protocol. Results: The findings of the present study demonstrated that high-intensity exercise significantly increased serum free fatty acid concentration. However, caffeine supplementation, nitric oxide precursor supplementation, and the combined ingestion of these supplements did not produce significant changes in serum free fatty acid concentration, fat oxidation, or carbohydrate oxidation compared with the placebo condition. Conclusion: The findings suggest that these supplements may be more effective for enhancing performance-related outcomes rather than inducing mextabolic alterations. Therefore, changes or lack of changes in mextabolic markers should not necessarily be interpreted as evidence for the presence or absence of ergogenic effects. The performance benefits of these supplements may be mediated through alternative physiological mechanisms independent of substrate mextabolism.