Carbohydrate-restricted diets high in either monounsaturated fat or protein are equally effective at promoting fat loss and improving blood lipids.
BACKGROUND
When substituted for carbohydrate in an energy-reduced diet, dietary protein enhances fat loss in women. It is unknown whether the effect is due to increased protein or reduced carbohydrate.


OBJECTIVE
We compared the effects of 2 isocaloric diets that differed in protein and fat content on weight loss, lipids, appetite regulation, and energy expenditure after test meals.


DESIGN
This was a parallel, randomized study in which subjects received either a low-fat, high-protein (LF-HP) diet (29 +/- 1% fat, 34 +/- 0.8% protein) or a high-fat, standard-protein (HF-SP) diet (45 +/- 0.6% fat, 18 +/- 0.3% protein) during 12 wk of energy restriction (6 +/- 0.1 MJ/d) and 4 wk of energy balance (7.4 +/- 0.3 MJ/d). Fifty-seven overweight and obese [mean body mass index (in kg/m(2)): 33.8 +/- 0.9] volunteers with insulin concentrations >12 mU/L completed the study.


RESULTS
Weight loss (LF-HP group, 9.7 +/- 1.1 kg; HF-SP group, 10.2 +/- 1.4 kg; P = 0.78) and fat loss were not significantly different between diet groups even though the subjects desired less to eat after the LF-HP meal (P = 0.02). The decrease in resting energy expenditure was not significantly different between diet groups (LF-HP, -342 +/- 185 kJ/d; HF-SP, -349 +/- 220 kJ/d). The decrease in the thermic effect of feeding with weight loss was smaller in the LF-HP group than in the HF-SP group (-0.3 +/- 1.0% compared with -3.6 +/- 0.7%; P = 0.014). Glucose and insulin responses to test meals improved after weight loss (P < 0.001) with no significant diet effect. Bone turnover, inflammation, and calcium excretion did not change significantly.


CONCLUSION
The magnitude of weight loss and the improvements in insulin resistance and cardiovascular disease risk factors did not differ significantly between the 2 diets, and neither diet had any detrimental effects on bone turnover or renal function.