Dietary protein and urinary nitrogen in relation to 6-year changes in fat mass and fat-free mass.

ABSTRACT

BACKGROUND: In contrast to the physiological expectation, observational studies show that greater protein intake is associated with subsequent body weight (BW) gain. An increase in fat-free mass (FFM) due to the anabolic effects of protein could explain this. OBJECTIVE: To examine associations between protein intake and subsequent changes in fat mass (FM) and FFM in longitudinal, observational data. DESIGN: A health examination, including measures of FM and FFM by bioelectrical impedance at baseline and follow-up 6 years later, was conducted. Diet history interviews (DHI) were performed, and 24-h urinary nitrogen collection at baseline was done. In total, 330 participants with DHI, of whom 227 had validated and complete 24-h urine collection data, were analyzed. Macronutrient energy substitution models were used. RESULTS: Mean estimated protein intake was 14.6 E% from DHI and 11.3 E% from urinary nitrogen. Estimated from DHI, FM increased 46 g per year, with every 1 E% protein substituted for fat (95% confidence interval (CI) = 13, 79; P = 0.006), and FFM increased 15 g per year (1, 30; P = 0.046). Results were similar in other substitution models. Estimated from urinary nitrogen, FM increased 53 g per year, with 1 E% protein substituted for other macronutrients (24, 81; P < 0.0005), and FFM increased 18 g per year (6, 31; P = 0.004). CONCLUSION: Within a habitual range, a greater protein intake was associated with BW gain, mostly in FM. This is in contrast to the expectations based on physiological and clinical trials, and calls for a better understanding of how habitual dietary protein influences long-term energy balance, versus how greater changes in dietary proteins may influence short-term energy balance.