A method to achieve control of dietary macronutrient composition in ad libitum diets consumed by free-living subjects

Objective: To validate a shop system in controlling macronutrient composition during ad libitum dietary intervention. Design: Six months randomized intervention trial. Setting: A shop at the department from which all foods were collected free of charge and registered by a purpose-designed computer system. Subjects: Sixty-five free-living obese subjects (25 kg/m(2) < BMI < 34 kg/m(2)) recruited through advertisement and from a waiting list at the Department. Total drop-out rate was 8%. Interventions: Ad libitum low-fat diets (30 energy-% (E%) fat): (1) High-protein (25 E% protein, HP) or (2) Low-protein, (12 E% protein, LP) or habitual diet (controls, C). Main outcome measures: Compliance was assessed by 24 h urinary nitrogen excretion (24 h UN). Results: After one month of dietary intervention 24 h UN increased significantly in the HP group and decreased significantly in the LP group (Group difference 95% CI):6.8 g (5.0-8.7 g), P < 0.0001). This group difference remained throughout the trial. There was good agreement between protein intake as estimated by the shop computer and as estimated from 24 h UN in both first (r = 0.86) and second half of the intervention (r = 0.80). Conclusion: The high dietary compliance demonstrates the potential of this method to control macronutrient composition in ad libitum dietary intervention studies in free-living subjects.