Effects of dietary fiber and carbohydrate on glucose and lipoprotein metabolism in diabetic patients

G Riccardi and AA Rivellese
Institute of Internal Medicine and Metabolic Diseases, Second Medical School, University of Naples, Italy.

Dietary recommendations for the treatment of diabetic patients issued by national and international diabetes associations consistently emphasize the need to increase carbohydrate consumption. However, these recommendations have been questioned on the basis of growing evidence that, in both insulin-dependent and non-insulin-dependent diabetic patients, a high-carbohydrate diet does not offer any advantage in terms of blood glucose and plasma lipid concentrations compared with a high-fat (mainly unsaturated) diet. It has been shown repeatedly that a high-carbohydrate diet increases plasma insulin and triglyceride levels and can deteriorate blood glucose control in the postprandial period. However, much of the controversy between advocates and detractors of dietary carbohydrate can be settled by taking into account dietary fiber. Several studies have shown that the adverse metabolic effects of high-carbohydrate diets are neutralized when fiber and carbohydrate are increased simultaneously in the diet for diabetic patients. In particular, these studies demonstrated that a high-carbohydrate/high-fiber diet significantly improves blood glucose control and reduces plasma cholesterol levels in diabetic patients compared with a low-carbohydrate/low-fiber diet. In addition, a high-carbohydrate/high-fiber diet does not increase plasma insulin and triglyceride concentrations, despite the higher consumption of carbohydrates. Unfortunately, dietary fiber represents a heterogenous category, and there is still much to understand as to which foods should be preferred to maximize the metabolic effects of fiber. There are indications that only water-soluble fiber is active on plasma glucose and lipoprotein metabolism in humans. Therefore, in practice, the consumption of legumes, vegetables, and fruits--rich in water-soluble fiber--should be particularly encouraged. The mechanisms by which dietary fiber exerts its hypoglycemic and hypolipidemic activities are unknown. However, the ability of dietary fiber to retard food digestion and nutrient absorption certainly has an important influence on lipid and carbohydrate metabolism. The beneficial effects of high-fiber foods are also exerted by some foods not particularly rich in fiber. The fiber content and physical form of the food can influence the accessibility of nutrients by digestive enzymes, thus delaying digestion and absorption. The identification of these foods with a low-glycemic response would help enlarge the list of foods particularly suitable for diabetic patients. In conclusion, a diet low in cholesterol and saturated fat should be recommended to all diabetic patients to prevent cardiovascular disease. A balanced increase in consumption of fiber-rich foods and unsaturated fat is the most rational way to replace foods rich in saturated fat and cholesterol in the diabetic diet.