Carbohydrate metabolism was assessed in 20 glucose-intolerant subjects before and after 12 wk on a high-carbohydrate diet (HC) or the diet combined with exercise training (HC-EX). The diet provided 60% of energy as carbohydrate and 20% as fat. Neither treatment altered fasting glucose or insulin concentrations or their response to a meal. During a glucose clamp (216 pmol insulin/L) glucose disposal increased from 13.2 +/- 0.83 to 14.6 +/- 0.83 mumol.kg fat-free mass-1.min-1 (P < 0.05) in both groups. During more pronounced hyperinsulinemia (654 pmol/L) glucose disposal did not change significantly (49.9 +/- 3.8 to 50.7 +/- 3.8 mumol.kg fat-free mass-1.min-1). Muscle glycogen increased in the HC-EX group (78.5 +/- 8.1 to 161.1 +/- 15.7 mmol glucose/kg muscle), with no changes in the HC group. These results do not support the recommendation to increase the dietary carbohydrate content for improving postprandial glucose metabolism or insulin action in glucose-intolerant adults unless combined with exercise training, which promotes muscle glycogen storage.