Decreased insulin sensitivity is associated with diabetes mellitus, ischemic heart disease, and hypertension, both independently and in association as what is called the metabolic syndrome. Although the negative effects of obesity, sedentary lifestyles, and high-fat diets on insulin sensitivity are well established, the influence of type and quantity of dietary carbohydrate is more controversial. This study aimed to assess the acute (24 h) effects of a high-sucrose compared with a high-starch diet on insulin sensitivity and to identify changes in blood metabolites that might lead to altered insulin sensitivity. Eight healthy adults consumed high-sucrose or high-starch diets (50% of dietary energy) in a randomized, crossover trial. Insulin sensitivity was assessed by a short insulin tolerance test the following morning. No differences were detected in insulin sensitivity, either for glucose metabolism [Kitt(glucose) (the rate constant for the decline in blood glucose concentrations) for sucrose diet = 3.86%/min, for starch diet = 3.72%/min; pooled SEM = 0.23] or for lipid metabolism [Kitt(NEFA) (the rate constant for the decline in blood fatty acid concentrations) for sucrose diet = 12.9%/min, for starch diet = 11.4%/min; pooled SEM = 1.18]. Profiles for blood glucose and serum insulin concentrations revealed higher peaks and lower troughs with the high-sucrose diet whereas area under the curve for glucose was higher with the high-starch diet (6780 +/- 245 mmol x L/min) than with the high-sucrose diet (6290 +/- 283 mmol x L/min) (P < 0.001). Plasma fatty acid concentrations showed a late postprandial rise with the sucrose-rich diet relative to the starch-rich diet, which was mirrored with a fractionally later peak in triacylglycerol concentrations.