Randomized trial on protein vs carbohydrate in ad libitum fat reduced diet for the treatment of obesity

OBJECTIVE To study the effect on weight loss in obese subjects by replacement of carbohydrate by protein in ad libitum consumed fat-reduced diets. DESIGN Randomized dietary intervention study over six months comparing two ad libitum fat reduced diets (30% of total energy) strictly controlled in composition: High-carbohydrate (HC, protein 12% of total energy) or high-protein (HP, protein 25% of total energy). SETTING AND PARTICIPANTS Subjects were 65 healthy, overweight and obese subjects (50 women, 15 men, aged 18-55 y) randomly assigned to HC (n = 25), HP (n = 25) or a control group (C, n = 15). All food was provided by self-selection in a shop at the department, and compliance to the diet composition was evaluated by urinary nitrogen excretion. MAIN OUTCOME MEASURE Change in body weight, body composition and blood lipids. RESULTS More than 90% completed the trial. Weight loss after six months was 5.1 kg in the HC group and 8.9 kg in the HP group (difference 3.7 kg, 95% confidence interval (CI)(1.3-6.2 kg) P \textless 0.001), and fat loss was 4.3 kg and 7.6 kg, respectively (difference 3.3 kg (1.1-5.5 kg) P \textless 0.0001), whereas no changes occurred in the control group. More subjects lost \textgreater 10 kg in the HP group (35%) than in the HC group (9%). The HP diet only decreased fasting plasma triglycerides and free fatty acids significantly. CONCLUSIONS Replacement of some dietary carbohydrate by protein in an ad libitum fat-reduced diet, improves weight loss and increases the proportion of subjects achieving a clinically relevant weight loss. More freedom to choose between protein-rich and complex carbohydrate-rich foods may allow obese subjects to choose more lean meat and dairy products, and hence improve adherence to low-fat diets in weight reduction programs.