The effects of a 4-week protein overfeeding protocol on body composition and influence of FTO genotype in exercise-trained men and women

Background High-protein diets have consistently been associated with favorable changes in body composition in both sedentary and exercise-trained populations. Despite abundant evidence supporting protein intakes above the recommended dietary allowance (RDA) for active individuals, the effects of deliberate protein overfeeding, particularly in those already consuming adequate protein, remain incompletely characterized. The primary purpose of this study was to examine the effects of a 4-week protein overfeeding protocol on body composition in exercise-trained men and women. The secondary purpose was to determine whether body composition adaptations differed by FTO rs9939609 genotype.Methods Fifty-six exercise-trained men and women (age: 22.40 ± 5.78 years; body mass: 75.47 ± 14.98 kg; BMI: 24.52 ± 3.50 kg/m2; resistance training: 5.12 ± 3.35 h/week) were enrolled in a 4-week dietary intervention. Participants were instructed to increase their daily protein intake by approximately 20% above baseline while maintaining habitual training. Body composition was assessed pre- and post-intervention using a multi-frequency bioelectrical impedance device (InBody® 270). Dietary intake was self-monitored via MyFitnessPal®. Thirty-five participants completed both pre- and post-testing assessments and constituted the primary analysis. Linear regression models adjusted for baseline values indicated that FTO genotype was not a significant predictor of body composition changes (p > 0.05).Results Protein intake trended toward a significant increase over the 4-week period (pre: 146 ± 66 g/day vs. post: 164 ± 79 g/day; p = 0.053), representing an approximate 12% increase. Total energy intake, carbohydrate, and fat intake did not significantly change. Lean body mass increased significantly (pre: 59.95 ± 14.14 kg vs. post: 60.62 ± 14.42 kg; p = 0.005; Cohen's d = 0.31) and total body water increased significantly (pre: 43.85 ± 10.35 L vs. post: 44.34 ± 10.55 L; p = 0.005). Fat mass (p = 0.424), body fat percentage (p = 0.149), and total body mass (p = 0.09) did not significantly change.Conclusions A 4-week modest protein overfeeding protocol in exercise-trained men and women was associated with a significant increase in lean body mass and total body water without concomitant increases in fat mass. These adaptations were not modified by FTO rs9939609 genotype. These findings support the contention that higher protein intakes do not promote fat accretion and may facilitate improvements in body composition in trained individuals.