26 g of dietary fiber sources—hard red spring wheat bran (HRS), soft white wheat (SWW), corn bran (CB), soy hulls (SH), freeze-dried apple powder (AP), and freeze-dried carrot powder (CP)—was fed to 15 men as part of a mixed diet. Oral glucose tolerance and one hour postprandial serum glucose were measured after one month of each fiber source and were compared with findings measured after one month of the diet without added fiber. The subjects' energy intakes and expenditures were nearly constant throughout the study. The fiber sources contained 50.8% (HRS), 44.1% (SWW), 92.1% (CB), 86.7% (SH), 25.6% (AP), and 31.0% (CP) fiber. Oral glucose tolerance improved significantly when subjects ate CB, SH, AP, and CP. Corrected insulin response at the glucose peak (CIRp) and peripheral insulin sensitivity (GTp) improved when subjects were fed CB, SH, AP, and CP. Fasting plasma glucagon and plasma glucagon responses to oral glucose were significantly lower after feeding AP or CP. Hard spring wheat bran was associated with higher fasting plasma glucagon and a similar plasma glucagon response to oral glucose compared with those of controls. Postprandial serum glucose concentrations were significantly lower when subjects were fed fiber-supplemented (CB and SH) diets than when they were fed the same diet without fiber. Effects of the dietary fiber sources on carbohydrate metabolism seemed dependent on the composition of the source. Improved oral glucose tolerance was associated with improved insulin release and peripheral insulin sensitivity and decreased plasma glucagon concentrations.