The metabolic effect of high-protein low-carbohydrate (HP) diets on body composition and glucose homeostasis remains incompletely understood. This study assesses the respective roles of the increased protein:carbohydrate ratio (P:C) and the resulting moderate decrease in energy intake in the metabolic effects of HP diets. Rats had free access to normal (NP; 14%) or high (HP; 53%) total milk protein isoenergetic diets, or were fed the NP diet but restricted to the energy intake of HP rats (NPr), which was 89.1 +/- 9.3% that of NP rats. After 8 wk, body weight was lower in HP and NPr rats than in NP rats. In HP rats, the lower body weight was associated with a lower adipose tissue mass and a reduced proportion of large adipocytes. HP rats also had an improved oral glucose tolerance and insulin sensitivity, as assessed by the homeostatic model assessment index, compared with NPr and NP rats, and these effects were related solely to the increased P:C. These data suggest that the reduced energy intake of rats fed a high-protein, low-carbohydrate diet explains the lower fat deposition but an increased P:C per se improves glucose homeostasis.