Background: In recent years, the ketogenic diet (KD) has gained popularity beyond clinical settings, reaching the sports domain. Evidence suggests the KD may enhance aerobic endurance through increased lipid oxidation and reduced reliance on muscle glycogen. Conversely, high-intensity and anaerobic performance can be compromised due to limited glucose availability. Individual responses may vary depending on training type and adaptation duration. This systematic review critically evaluates the effects of the KD on athletic performance, examining metabolic adaptations and performance outcomes across endurance and anaerobic disciplines.

Methods: A systematic search of PubMed and ClinicalTrials.gov was conducted for studies published from 2015 to 2025. Randomized controlled trials (RCTs) and non-randomized trials (No-RCTs) including trained athletes or physically active individuals were selected. Inclusion criteria required quantitative performance and metabolic outcome measures. Clinical populations, sedentary individuals, and subjects with pre-existing metabolic disorders were excluded. Fifteen studies meeting these criteria were analyzed.

Results: The KD improved aerobic endurance in endurance-trained athletes via increased fat oxidation and reduced glycogen dependency. Anaerobic and high-intensity performance was often impaired, particularly during initial adaptation (<4 weeks), due to limited glucose availability. Adaptation periods exceeding four weeks mitigated some negative effects on anaerobic capacity. Inter-individual variability was evident, influenced by training type and body composition.

Conclusions: The KD may serve as an effective nutritional strategy for endurance athletes, provided sufficient adaptation time is allowed. In anaerobic and power sports, the KD should be applied cautiously, as reduced glycogen may compromise rapid energy production. Performance outcomes depend on adaptation duration, discipline, and individual characteristics. Further research is warranted to clarify the effects of genetics, adaptation length, and sport-specific energy demands.