The glucose responses (GR) and insulin requirements (IRs) were measured by a glucose-controlled insulin infusion system for 5 h after 12 patients with insulin-dependent diabetes mellitus consumed each of three meals: a 1890-kJ standard meal, the standard meal with 840 kJ added protein, and the standard meal with 840 kJ added fat. The GR to the protein-added meal was greater (P = 0.005) than to either the standard or fat-added meals, because of an increase in the late (last 150 min) GR. The late IR was greater for the protein-added meal (P < 0.005). The IR was not changed after the fat-added meal. Therefore, the addition of protein (but not fat) energy to a meal increases both the postprandial GR and late IR. This finding suggests that diabetic patients who inject premeal insulin may need to increase their insulin dose when protein is added to a meal.