Higher Protein Intakes Predict Leaner Body Composition in Weight-Loss Participants—Findings from the International Weight Control Registry

Background A high protein diet combined with exercise is often recommended to promote fat loss and preserve muscle mass during weight loss. This secondary analysis evaluated the associations between dietary protein intake, physical activity, and body weight and composition among individuals engaged in purposeful weight loss attempts. Methods This study included participants (n = 203) in a three-week ancillary study of the International Weight Control Registry, enrolled between August 2022 and January 2023. Average protein intake (g/kg body weight/d) was estimated from three 24-hour, multi-pass dietary recalls. Body weight, percent body fat (% BF), percent muscle mass (% muscle), and physical activity were monitored using study-provided consumer-grade devices. Multiple linear regression models were used to evaluate associations between protein intake and daily steps and their interaction with body mass index (BMI), % BF, and % muscle. Results Total protein intake negatively predicted BMI (β [95% CI]=-0.51 [-0.62, -0.39], p < 0.001), % BF (β [95% CI]=-0.37 [-0.49, -0.26], p < 0.001), and positively predicted % muscle (β [95% CI] = 0.26 [0.20, 0.33], p < 0.001). Similarly, average daily steps negatively predicted BMI (β [95% CI]=-0.29 [-0.40, -0.17], p < 0.001), % BF (β [95% CI]=-0.23 [-0.35, -0.12], p < 0.001), and positively predicted % muscle (β [95% CI] = 0.14 [0.08, 0.21], p < 0.001). Total protein intake was significantly associated with BMI across all physical activity levels, with the strongest associations observed at 5,000 steps/day and weakening as the average daily steps increased. Conclusions Findings broadly support the significance of high protein intake in achieving a lower body weight and a more favorable body composition (i.e., lower % BF and higher % muscle) for individuals previously engaged in weight loss. Additionally, people who are less physically active may require higher protein intake to maintain a lower body weight.