Twenty-two subjects, six of them obese, five latent diabetics, six overt diabetics, and five control subjects, were given a daily dietary supplement of 16 g guar gum and 10 g pectin for three successive days. Postprandial blood glucose and insulin responses to an initial standard meal and to a fiber-enriched meal given 72 h later were compared. A significant decrease of blood glucose levels was found in all subjects, with maximal response in the obese group. We conclude that supplementing the diet with certain fibers can be regarded as an important therapeutic measure in the treatment of diabetic and obese patients.