A gluten-free diet does not alter performance outcomes in nonceliac athletes undergoing sprint interval training: a pilot trial.

Introduction: The popularity of gluten-free diets (GFD) among athletes has increased due to perceived benefits for performance, well-being, and body composition, despite limited evidence in non-celiac individuals. Methods: This parallel-group pilot study evaluated the effects of a 6-week GFD vs. a mixed diet (MD), both combined with sprint interval training (SIT), on metabolic and performance-related parameters in 15 male endurance athletes (GFD: n = 6; MD: n = 9). Outcomes included body composition, time trial performance (distance during a 60 min run on a 400-metre track), metabolic (respiratory exchange ratio (PER), substrate oxidation rates, maximal fat oxidation [MFO], and FatMax-intensity at which MFO occurs), and performance-related (ventilatory threshold [VT], respiratory compensation point [RCP], peak oxygen uptake [V̇O2peak], and time to exhaustion [TTE]) markers during a ramp incremental running test. Gastrointestinal quality of life (GIQLI) was also assessed. Results: The GFD group achieved significant reductions in weight and BMI compared to the MD group (Δweight: GFD: -2.70 [-3.20 to -1.73] kg vs. MD: -0.30 [-1.75 to 1.35] kg, p = 0.018, r = 0.569; ΔBMI: GFD: -0.75 [-1.00 to -0.50] kg·m-² vs. MD: -0.10 [-0.55 to 0.35] kg·m-², p = 0.026, r = 0.549). Both groups demonstrated improved time trial distance (Δ: GFD: 0.76 [0.56 to 1.57] km vs. MD: 0.60 [0.50 to 0.90] km, p = 0.313, r = 0.328) and TTE (Δ: GFD: 1.24 [0.61 to 1.80] min vs. MD: 0.70 [0.19 to 0.92] min, p = 0.088, r = 0.442), with V̇O2peak increases appearing more pronounced in the GFD group (ΔV̇O₂peak: GFD: 9.10 [1.80 to 12.38] ml·kg-1·min-1 vs. MD: 3.20 [-1.95 to 10.40] ml·kg-1·min-1, p = 0.388, r = 0.246). Group differences in metabolic changes were mixed (ΔMFO: GFD: 0.075 [-0.070 to 0.190] g·min-1 vs. MD: 0.200 [-0.145 to 0.310] g·min-1, p = 0.607, r = 0.152; ΔFatMax: GFD: -12.90 [-23.56 to 1.79] %V̇O₂peak vs. MD: 1.00 [-10.23 to 12.11] %V̇O₂peak, p = 0.181, r = 0.365), and GIQLI scores showed no significant changes. Conclusion: Although a GFD showed modest benefits for weight management and aerobic performance, its metabolic effects were variable, and challenges with nutritional deficiencies highlight the need for caution in non-celiac athletes.