The continuing increase in the incidence of type 2 diabetes mellitus (DM2) and obesity in children and adolescents is attributable to excessive caloric intake. Abnormal lipid metabolism in the postprandial state leads to long exposure of the vasculature to hyperlipidemia. Most children and adolescents with DM2 are obese, and many have fasting hypertriglyceridemia. Clustering of hyperlipidemia, DM2 and obesity increases the risk for cardiovascular disease. We therefore studied lipids, insulin, C-peptide, and glucose in response to an oral fat load simulating the fat content of a high-fat, fast-food meal in 12 type 2 diabetic obese, 15 non-diabetic obese, and 12 non-diabetic non-obese (control) adolescents (aged 10-19 yr; 87% African-Americans). All three groups were age-, sex-, and sexual maturation-matched. Mean body mass indices were similar in the diabetes and obese groups (32.7 +/- 1.1 vs 35.8 +/- 1.6 kg/m2). All patients with DM2 had fasting C-peptide > 0.2 nmol/l (0.7 ng/ml) and negative diabetes-associated autoantibodies. Serum total cholesterol, triglyceride, high- and low-density lipoprotein cholesterol, insulin, C-peptide, and plasma glucose levels were measured at 0, 2, 4, and 6 h after the fat load. The area under the curve (AUC) was calculated by trapezoidal estimation. Triglyceride AUC was significantly greater in the diabetes group than in the other two groups (15.7 +/- 2.9 vs 9.2 +/- 0.7 and 7.5 +/- 0.7 mmol x h/l [1389 +/- 258 vs 819 +/- 60 and 663 +/- 62 mg x h/dl]; p < 0.02 and <0.004, respectively), as were insulin, C-peptide, and glucose AUCs. Incremental triglyceride response (delta triglyceride = peak - fasting) in the diabetes group was significantly higher than that in the control group (2.1 +/- 0.7 vs 0.8 +/- 0.1 mmol/l 189.7 +/- 58.4 vs 71.2 +/- 11.1 mg/dl]; p < 0.04). Insulin resistance was estimated using the homeostasis model assessment (HOMA), which was greater in the diabetes group than in the obese and control groups (14.4 +/- 2.8 vs 5.2 +/- 0.8 and 3.2 +/- 0.4; p < 0.001 and < 0.0001, respectively). The diabetes group was divided into subgroups of high and normal fasting triglycerides on the basis of triglyceride levels above and below the 95th percentile. The delta triglyceride in the subgroup with high fasting triglycerides was substantially greater than in the subgroup with normal fasting triglycerides (3.4 +/- 1.1 vs 0.8 +/- 0.2 mmol/l [305.2 +/- 96.8 vs 74.2 +/- 18.0 mg/dl]; p < 0.001). Total cholesterol and triglyceride AUCs were much greater in the high vs normal fasting triglycerides subgroup (33.0 +/- 2.9 vs 24.2 +/- 1.9 and 23.6 +/- 3.5 vs 7.8 +/- 0.6 mmol x h/l [1274 +/- 113 vs 934 +/- 72 and 2085 +/- 309 vs 692 +/- 49 mg x h/dl]; p < 0.02 and <0.0001, respectively), as were insulin and C-peptide AUCs. HOMA was greater in the high vs normal fasting triglycerides subgroup (20.8 +/- 4.0 vs 8.0 +/- 1.6; p < 0.0001). In addition to elevated plasma glucose levels, there were no significant differences in either insulin or lipid parameters among the diabetes subgroup with normal fasting triglycerides, the obese group, and controls. Our data suggest that postprandial hyperlipidemia in response to a fat loading test is present in adolescents with DM2 who already have fasting hypertriglyceridemia. The degree of insulin resistance as an underlying abnormality--not DM per se--determines the degree of postprandial lipemia.