BACKGROUND: Little is known about the comparative effect of weight-loss diets on metabolic profiles during dieting. OBJECTIVE: The purpose of this study was to compare the effect of a low-carbohydrate diet (< or =20 g/d) with a high-carbohydrate diet (55% of total energy intake) on fasting and hourly metabolic variables during active weight loss. DESIGN: Healthy, obese adults (n = 32; 22 women, 10 men) were randomly assigned to receive either a carbohydrate-restricted diet [High Fat; mean +/- SD body mass index (BMI; in kg/m(2)): 35.8 +/- 2.9] or a calorie-restricted, low-fat diet (High Carb; BMI: 36.7 +/- 4.6) for 6 wk. A 24-h in-patient feeding study was performed at baseline and after 6 wk. Glucose, insulin, free fatty acids (FFAs), and triglycerides were measured hourly during meals, at regimented times. Remnant lipoprotein cholesterol was measured every 4 h. RESULTS: Patients lost a similar amount of weight in both groups (P = 0.57). There was an absence of any diet treatment effect between groups on fasting triglycerides or on remnant lipoprotein cholesterol, which was the main outcome. Fasting insulin decreased (P = 0.03), and both fasting (P = 0.040) and 24-h FFAs (P < 0.0001) increased within the High Fat group. Twenty-four-hour insulin decreased (P < 0.05 for both groups). Fasting LDL cholesterol decreased in the High Carb group only (P = 0.003). In both groups, the differences in fasting and 24-h FFAs at 6 wk were significantly correlated with the change in LDL cholesterol (fasting FFA: r = 0.41, P = 0.02; 24-h FFA: r = 0.52, P = 0.002). CONCLUSIONS: Weight loss was similar between diets, but only the high-fat diet increased LDL-cholesterol concentrations. This effect was related to the lack of suppression of both fasting and 24-h FFAs.