The metabolic consequences of two hypocaloric diets were assessed in 10 obese men. The study, performed on a metabolic ward, compared the response of these men to two cholesterol-free liquid formula diets of differing composition (10 kcal/kg per day, 70% carbohydrate, 20% protein, 10% fat versus 70% fat, 20% protein, 10% carbohydrate) but identical in calories. These were administered for 14 days in a random order and each diet was preceded by a 7-day control weight maintenance diet (30 kcal/kg per day, 40% carbohydrate, 20% protein, 40% fat). The low calorie diets were well tolerated by the men and effected similar losses of nonaqueous body weight. Fasting glucose and insulin decreased significantly in these men after they ingested either weight loss diet for 14 days, but the change in each parameter was greater for high fat as compared to high carbohydrate (15% versus 7% and 67% versus 35%, respectively, P less than 0.01). In contrast, fasting glucagon concentration decreased in these subjects to a greater extent in response to the high carbohydrate diet (35% versus 16%, P less than 0.01). This adaptive response thus resulted in a 50% fall in insulin:glucagon molar ratio for high fat and no change for high carbohydrate weight loss. Despite these hormonal alterations no change in glucose tolerance was observed. Fasting serum triglyceride and cholesterol levels declined in these subjects to a greater extent following the high fat compared to the high carbohydrate regimen (45% versus 28%, P less than 0.01 and 8% versus 3%, not significant, respectively). These changes reflected decrements in very low density lipoproteins alone. Despite similar increments in free fatty acid levels, (350% versus 270%, not significant) serum ketone body (beta-hydroxybutyrate and acetoacetate) concentrations increased 7-fold on the high fat diet compared to the high carbohydrate diet, P less than 0.001. The hyperketonemia of these men in response to the high fat, low calorie diet suggested the occurrence of a shift in hepatic free fatty acid metabolism toward ketogenesis rather than triglyceride synthesis. The associated decrease in the insulin: glucagon molar ratio raised the question of a possible role for these hormones in the adaptation.