High fiber diets have been recommended for the general population and for the nutritional management of patients with type 2 diabetes. Soluble dietary fiber retards digestion and absorption of the associated dietary carbohydrate, thus flattening the postprandial rise in plasma glucose and insulin concentrations. Some foods such as beta-glucan fiber containing oats and barley, and soluble fibers isolates such as pectin, guar, psyllium, or glucomannan have a high viscosity which gives them the greatest blood glucose lowering effect. Viscous fibers, as a result of their rheological properties, form gel with the food and human digesta and consequently reduce postprandial increases in plasma glucose concentrations in both normal and diabetic subjects in positive relation to their level of viscosity. Insoluble fibers such as cellulose and wheat bran have little effect.