Experimental Investigations Into the Role of Dietary Protein on the Control of Energy Intake and Body Mass in Humans

Evidence is needed to inform dietary strategies to promote a healthy weight and reduce cardiovascular risk. Dietary protein (P) may influence both energy intake (EI) and energy expenditure (EE). To determine the relationships between appetite control, body mass regulation and protein from different animal and vegetable sources, healthy lean subjects were studied in tightly controlled experiments. In a 5-way crossover study, 28 subjects aged 19-68 years consumed 2.5 MJ high-protein preload meals (33% energy from protein (P), 34% fat (F), 33% carbohydrate (C)) rich in meat, dairy, nuts and legumes, soy, or control (15%P, 34%F, 51%C), followed by test meals 180 min later. Appetite control mechanisms before, during and for 5 hours after eating, were assessed from analysis of serial blood samples and other physiological measurements. Mean EI was lower after soy (4.1±0.4 MJ) than control (4.6±0.4 MJ), meat (4.7±0.4 MJ), dairy (4.6±0.4 MJ) or nuts/legumes (5.0±0.5 MJ) preloads. Gastric emptying was slower and hunger decreased after soy, and after nuts/legumes, satiation increased and hunger reduced. Blood glucose and insulin excursions were significantly lower after the nuts/legumes and soy meals. Similar parameters, plus urinary nitrogen, EI and EE were measured in 10 adults during 3 five-day residential experiments. On days 1-3, they ate ad libitum diets providing 10%, 15%, or 25% protein, 30% fat and the remainder as carbohydrate. EI was fixed at maintenance requirements on day 4 and EE was measured by calorimeter. EI was lower during the 25%P (8.1±1.0 MJ/d) than the 15%P diet (8.7±1.0 MJ/d) with higher fullness scores, flatter glucose curves and higher pancreatic polypeptide concentration. EE was similar but fat oxidation was higher on the 25%P diet. Higher dietary protein promotes energy balance, through decreased energy intake and increased fat oxidation, improves metabolic risk factors, and may be useful to prevent weight gain and related metabolic diseases