Protein oxidation in non-exercising healthy adults under varying dietary conditions: Physiological determinants, effects on fuel partitioning, and implications for body weight regulation

Background: Protein oxidation (PROTOX) typically accounts for the smallest fraction of daily energy expenditure (24hEE) in humans compared to carbohydrate and lipid oxidation. However, inter-individual differences in PROTOX may explain differences in fuel partitioning and body weight change. We aimed to elucidate the physiological determinants of PROTOX under controlled 24-h dietary conditions, including eucaloric feeding, fasting, and overfeeding diets with variable protein content. Methods: Eighty-six weight-stable healthy volunteers with normal glucose regulation (67 M/19F; age: 37 ± 10 years; BMI: 26.7 ± 4.5 kg/m2, body fat by DXA: 29.0 ± 9.8 %) underwent 24hEE measurements by whole-room calorimetry during energy balance (20 % protein, 50 % carbohydrate), different overfeeding diets (200 % of the daily eucaloric requirement), including three normal-protein (20 %) diets (balanced: 50 % carbohydrate; high-carbohydrate: 75 % carbohydrate; high-fat: 60 % fat), low-protein (3 %) and high-protein (30 %), and 24-h fasting in a randomized crossover design. Urine samples were collected during each 24-h dietary intervention for quantification of PROTOX and catecholamine excretion rates by nitrogen excretion and high-performance liquid chromatography, respectively. Results: PROTOX during energy balance (mean ± SD: 372 ± 78 kcal/day) was positively associated with protein intake (r = 0.39, p < 0.001), fat free mass (r = 0.35, p < 0.001), but not with fat mass (p = 0.24). Higher PROTOX was associated with higher 24-h urinary norepinephrine (partial r = 0.27, p = 0.01), but not epinephrine (p = 0.48), excretion rates. During normal-protein diets, higher PROTOX was associated with lower lipid oxidation, but not carbohydrate oxidation. Inter-individual variability in PROTOX did not predict changes in weight or body composition over two years. Conclusion: Dietary protein content, lean body mass, and sympathetic nervous system activity are key determinants of PROTOX. Although PROTOX did not predict free-living weight gain, increased PROTOX is associated with decreased lipid oxidation, underscoring its role in fuel partitioning and whole-body energy and substrate balance.