When following a high protein diet, substituting fat for carbohydrate intake may offer a metabolic advantage for normal weight individuals. Eating a high protein, high fat meal meal significantly increased metabolic rate. But the effect was not duplicated in overweight or underweight individuals.
The study also found that overweight individuals have a higher metabolic rate than normal or underweight individuals (so much for the ‘slow metabolism’ theory). In fact, the underweight individuals had the slowest metabolic rate. After adusting for fat free mass (lean body mass), no significant difference was found in metabolic rate.