Beneficial Effects of High Dietary Fiber
abstract & commentary
Synopsis: A high-fiber diet resulted in improved glycemic control and a better lipid profile.
Source: Chandalia M, et al. N Engl J Med 2000;342:1392-1398.
Earlier this year, the dietary guidelines of the American Diabetes Association (ADA) were revised, recommending that saturated fats be replaced with carbohydrates or—alternatively—with cis monounsaturated fat. The ADA also recommended that the diet include at least 20-35 g/d of dietary fiber, because it has been established that dietary fiber has a cholesterol lowering effect. The expert panel considered the effects of dietary fiber on glycemic control to be minimal and considered it difficult to design a diet that contained a significant quantity of dietary fiber unless foods or supplements fortified with fiber were added. Chandalia and colleagues challenged this latter assumption, proposing that not only could they design a high-fiber diet in the absence of a supplement, but also that the diet would have a favorable effect on glycemic control.
Chandalia et al recruited 13 patients (12 men, 1 woman) with type 2 diabetes. The patients ate one meal at the center during the week; the remainder of meals were packaged to be eaten at home. For six weeks, the patients ate either a typical ADA diet or the high-fiber study diet. At the end of six weeks, diets were switched. Plasma glucose levels were closely monitored. Glycosylated hemoglobin was measured at baseline and at the end of each six-week interval. Serum lipids were also monitored. Fecal sterol balance was also studied in order to gain insight into the mechanism by which dietary fiber lowers serum cholesterol. Plasma glucose and insulin levels were measured on the last day of each dietary period, and 24-hour urine glucose excretion was quantitated on days 38 through 42.
The ADA diet provided 24 g/d of fiber (8 g soluble, 16 g insoluble). The study diet provided 50 g/d of fiber (25 g soluble, 25 g insoluble). To achieve this level of fiber, the following kinds of unfortified foods were used: cantaloupe, grapefruit, orange, raisins, lima beans, okra, sweet potato, winter squash, granola, oat bran, peas, zucchini, whole wheat bread, papaya, fruit cocktail, and fresh peaches. Dietary compliance was rated excellent. The patients’ weights did not change throughout the study.
Compared to the ADA diet, the high-fiber diet resulted in an 8.9% reduction in mean plasma glucose concentration, a 6.7% reduction in fasting plasma total cholesterol concentration, a 6.3% lowering of LDL cholesterol, and a 10.2% lowering of triglycerides. The high- fiber diet also reduced hyperinsulinemia, but glycosylated hemoglobin was only slightly lower, which Chandalia et al attribute to the short time period of the study. The sterol excretion study suggests that fiber exerts its cholesterol lowering effect via elimination of bile acids. The improved glycemic control may be a function of reduced or delayed absorption of carbohydrates.
Comment by Michael K. Rees, MD, mph
Chandalia et al note that the average fiber intake of people (including diabetics) living in western countries is 17 g/d. They demonstrate that it is relatively easy to construct a 50 g/d fiber diet without the use of supplements, and that the diet is palatable and well accepted. They recommend that dietary guidelines for patients with diabetes should emphasize an overall increase in dietary fiber through the consumption of unfortified foods, rather than use of fiber supplements—once again proving that eating your fruits and veggies is good for you!
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