Background: It is known that soluble fiber in the diet can lower blood lipid levels. It is less certain, however, that eating foods with soluble fiber will further lower blood lipids when the intake of saturated fat and cholesterol has already been reduced to very low levels. Furthermore, the mechanism of the lipid-lowering effect of fiber has not been elucidated.

Methods: To address these questions, we studied 43 volunteers with hyperlipidemia in a crossover study involving two four-month dietary periods. The two metabolic diets contained foods high in either soluble or insoluble fiber and were separated by a two-month National Cholesterol Education Program Step 2 diet. The metabolic diets were low in saturated fat (< 4 percent of total calories) and cholesterol (< 25 mg per 1000 kcal), high in carbohydrate (> or = 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 lipoprotein (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), respectively. In contrast, the ratio of total to HDL cholesterol was not significantly different during the two dietary periods. The loss of fecal bile acids was 83 +/- 14 percent greater during the soluble-fiber period than during the insoluble-fiber period (P < 0.001) and was related to the 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, respectively). 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: Very 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.