To examine extra-alimentary effects of high-carbohydrate, high-fiber (HCF) diets, insulin-mediated glucose disposal employing the euglycemic clamp and hepatic glucose output (HGO) employing [6,6-2H2]glucose were measured in 12 healthy young and old individuals before and after 21-28 d of an HCF diet. Diet lowered fasting concentrations of glucose from 5.3 +/- 0.2 to 5.1 +/- 0.1 mmol/L (p less than 0.01) and insulin from 66.0 +/- 7.9 to 49.5 +/- 5.7 pmol/L (p less than 0.01). Fasting serum cholesterol decreased from 5.17 +/- 0.18 to 3.80 +/- 0.20 mmol/L (p less than 0.01) in young individuals and from 6.15 +/- 0.52 to 4.99 +/- 0.49 mmol/L (p less than 0.01) in elderly individuals. Fasting serum triglyceride concentrations, basal HGO, and insulin suppression of HGO were unchanged by the diet. Glucose disposal rates increased from 18.87 +/- 1.66 before 23.87 +/- 2.78 mumol.kg-1.min-1 after the diet (p less than 0.02). Therefore, HCF diets may improve carbohydrate economy by enhanced peripheral sensitivity to insulin.