Comparative Effects of Low Glycemic Index, High Glycemic Index, and Ketogenic Diets on Body Composition, Metabolic Parameters, and Aerobic Capacity in Endurance Athletes: A 12- Week Randomized Trial

Background The optimal dietary strategy to enhance endurance training adaptations remains contentious. This study compared the effects of low-glycemic index (Low-GI), high-glycemic index (High-GI), and ketogenic (Low CHO High Fat - LCHF) diets on body composition, metabolic health, and VO₂ max in endurance athletes. Methods Fifty-seven male endurance athletes (age: 27.5 ± 4.9 years) were randomly assigned to a Low-GI (n = 19), High-GI (n = 19), or LCHF (n = 19) diet for 12 weeks. All participants followed a standardized endurance running program. Body composition, VO₂ max (via direct gas analysis during a Bruce Protocol test), and fasting blood parameters were assessed pre- and post-intervention. Results Significant main effects of time were observed for all groups, showing improvements in body composition and VO₂ max (p < 0.001). A significant time × group interaction was found for macronutrient intake, rate of perceived exertion (RPE), and weekly mileage (p < 0.001). The High-GI group showed no change in RPE and a decrease in mileage. The Low-GI and LCHF groups reported lower RPE and increased mileage. The LCHF group exhibited a significant increase in LDL cholesterol. Conclusion A structured training program improved fitness and body composition regardless of diet. However, diet type significantly influenced training behavior. The High-GI diet was associated with worse subjective outcomes, while Low-GI and LCHF diets were better tolerated. The LCHF diet's impact on LDL cholesterol warrants caution.