Recently, two commercial enteral formulae for diabetic patients have been made available in Spain: a high-complex-carbohydrate, low-fat formulation (HCF) and a low-carbohydrate formulation (RCF). This study compares the effects of the two enteral nutritional formulae in patients with non-insulin-dependent diabetes mellitus (type 2 diabetes) treated with sulfonylurea or insulin. Fifty-two type 2 diabetes patients were randomly assigned to receive one of the two enteral formulae. Test enteral formula breakfast (250 cc) were consumed at approximately 0900 h after routine medications (insulin or oral agents) had been taken. Venous blood samples were obtained during fasting, before medication, and at 30 and 120 min after the start of the meal. The glycemic response of patients to the HCF was significantly greater than to RCF, but lower than in the sulfonyl type 2 diabetes treated groups. The incremental glucose response was within acceptable levels except in insulin treatment type 2 diabetes patients given HCF. Glucose, insulin, and C-peptide responses were higher in HCF than RCF groups. Two-factor analysis of variance on mean increments of blood glucose and C-peptide from basal levels to 30 min show the type of enteral nutrition as the main factor (P = 0.0010 and P = 0.0005, respectively). The RCF formula supplies 50.0% of energy as fat and 33.3% as carbohydrates, so it may be a ketogenic diet. It was found that both ketone bodies were higher after RCF than after HCF ingestion, but without statistical significance. We conclude that the partial replacement of complex digestible carbohydrates with monounsaturated fatty acids in the enteral formulae for supplementation of oral diet may improve glycemic control in patients with type 2 diabetes. The long-term effects of enteral diets high in monounsaturated fatty acids need further evaluation in patients with type 2 diabetes.