BACKGROUND: The protein leverage hypothesis requires specific evidence that protein intake is regulated more strongly than energy intake. OBJECTIVE: The objective was to determine ad libitum energy intake, body weight changes, appetite profile, and nitrogen balance in response to 3 diets with different protein-to-carbohydrate + fat ratios over 12 consecutive days, with beef as a source of protein. DESIGN: A 3-arm, 12-d randomized crossover study was performed in 30 men and 28 women [mean +/- SD age: 33 +/- 16 y; body mass index (in kg/m2): 24.4 +/- 4.0] with the use of diets containing 5%, 15%, and 30% of energy (En%) from protein, predominantly from beef. RESULTS: Energy intake was significantly lower in the 30En%-protein condition (8.73 +/- 1.93 MJ/d) than in the 5En%-protein (9.48 +/- 1.67 MJ/d) and 15En%-protein (9.30 +/- 1.62 MJ/d) conditions (P = 0.001), stemming largely from lower energy intake during meals (P = 0.001). Hunger (P = 0.001) and desire to eat (P = 0.001) ratings were higher and fullness ratings were lower (P = 0.001) in the 5En%-protein condition than in the 15En%-protein and 30En%-protein conditions. Nitrogen excretion was lower in the 5En%-protein condition (4.7 +/- 1.5 g/24 h; P = 0.001) and was higher in the 30En%-protein condition (15.3 +/- 8.7 g/24 h; P = 0.001) compared with the 15En%-protein condition (10.0 +/- 5.2 g/24 h). Nitrogen balance was maintained in the 5En%-protein condition and was positive in the 15En%- and 30En%-protein conditions (P = 0.001). CONCLUSIONS: Complete protein leverage did not occur because subjects did not consume to a common protein amount at the expense of energy balance. Individuals did underconsume relative to energy requirements from high-protein diets. The lack of support for protein leverage effects on a low-protein diet may stem from the fact that protein intake was sufficient to maintain nitrogen balance over the 12-d trial. This trial was registered at clinicaltrials.gov as NCT01646749.
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