EFFECT ON BLOOD-LIPIDS OF VERY HIGH INTAKES OF FIBER IN DIETS LOW IN SATURATED FAT AND CHOLESTEROL

Background. It is known that soluble fiber in the diet can lower blood lipid levels. It is less certain, however, that eating foods with sol uble fiber will further lower blood lipids when the intake of saturate d fat and cholesterol has already been reduced to very low levels. Fur thermore, the mechanism of the lipid-lowering effect of fiber has not been elucidated. Methods. To address these questions, we studied 43 vo lunteers with hyperlipidemia in a crossover study involving two four-m onth dietary periods. The two metabolic diets contained foods high in either soluble or insoluble fiber and were separated by a two-month Na tional Cholesterol Education Program Step 2 diet. The metabolic diets were low in saturated fat (<4 percent of total calories) and cholester ol (<25 mg per 1 000 kcal), high in carbohydrate (greater-than-or-equa l-to 60 percent of total calories), and very high in fiber (>24 g per 1000 kcal). Results. Blood lipids fell to their lowest levels by week 4 of both study diets. When the soluble-fiber period was compared with the insoluble-fiber period, the subjects' total, low-density lipoprot ein (LDL), and high-density lipoprotein (HDL) cholesterol levels were found to be lower by a mean (+/-SE) of 4.9+/-0.9 percent (P<0.001), 4. 8+/-1.3 percent (P<0.001), and 3.4+/-1.3 percent (P = 0.014), respecti vely. In contrast, the ratio of total to HDL cholesterol was not signi ficantly different during the two dietary periods. The loss of fecal b ile acids was 83+/-14 percent greater during the soluble-fiber period than during the insoluble-fiber period (P<0.001) and was related to th e differences in total and LDL cholesterol and apolipoprotein B levels (r = 0.42, P = 0.005; r = 0.49, P<0.001; and r = 0.33, P = 0.035, res pectively). The difference in serum cholesterol levels between the two dietary periods was greater among the men (7.5+/-1.2 percent, P<0.001 ) than among the women (3.4+/-1.2 percent, P = 0.008). Conclusions. Ve ry high intakes of foods rich in soluble fiber lower blood cholesterol levels even when the main dietary modifiers of blood lipids - namely, saturated fat and cholesterol - are greatly reduced.