Although dietary recommendations for diabetics stress the need for increased carbohydrate and dietary fiber, the effectiveness of dietary fiber in improving insulin sensitivity remains controversial. The aim of this study was to compare the effects of a soluble fiber (guar gum) and an insoluble fiber (wheat bran) on insulin sensitivity in streptozotocin-induced (STZ) diabetic rats. Consequently, the rats were divided into two groups and one half were rendered diabetic with streptozotocin. The STZ diabetic and nondiabetic rats were further randomized and fed a diet containing dietary fiber (7 g/100 g diet) from either guar gum or wheat bran. The hyperinsulinemic clamp technique, combined with infusion of the glucose analog, 2-deoxyglucose (2DG), was utilized to examine insulin sensitivity. Bran-fed STZ diabetic rats were significantly (P < 0.001) hyperglycemic, which was ameliorated by guar gum. Insulin-mediated glucose disposal was increased by the guar diet compared with the bran diet in both the STZ diabetic rats [17.7 +/- 2.2 vs. 11.8 +/- 2.4 mL/(kg x min), P < 0.05] and the nondiabetic rats [20.5 +/- 2.8 vs. 15.5 +/- 1.5 mL/(kg x min), P < 0.05]. The accumulation of 2DG in peripheral muscles reflected the changes in insulin sensitivity with a trend for increased 2DG uptake in the majority of analyzed tissues in rats fed the guar diet, both nondiabetic and STZ diabetic, compared with the bran-fed rats. Accompanying these alterations in insulin sensitivity, guar gum suppressed food intake in the hyperphagic diabetic rats by 20% (P < 0.001). The present results demonstrate the effectiveness of guar gum in improving insulin sensitivity in STZ diabetic rats and suggest that reduced food intake may be an important mechanism of action of guar in hyperphagic diabetic rats.