Dietary Fiber and Lipid Metabolism 1

Abstract

Conclusions

The hypothesis that dietary fiber alters cholesterol levels by adsorbing bile acids, which would increase their excretion and alter cholesterol absorption, appears to be too simplistic based on the accumulated data. Some sources of dietary fiber (e.g., oat bran) reduce serum and/or liver cholesterol levels and simultaneously increase steroid excretion. However, these changes do not seem to be of a magnitude sufficient to account for the change in serum cholesterol. In addition, other sources which cause more substantial increases in steroid excretion cause no change in cholesterol accumulation (e.g., corn bran or cellulose) and, in humans, a reduction in serum cholesterol by beans is accompanied by a reduction in steroid excretion. A change in bile acid pool sizes in response to oat bran and the effects this may have on sterol balance may be an additional mechanism for the observed effects. Other situations (i.e., beans) do not fit either of the above mechanisms and will require further investigation.

At this point in time we can make several useful comments concerning the role of dietary fiber in lipid metabolism: (a) We cannot generalize about the effects of dietary fiber on lipid metabolism. We may be able to group sources by composition, to some extent, but the experience with oat bran and beans that, in spite of similar effects, mechanisms may be quite different suggests this must be done with caution. (b) Mechanisms appear to center more on effects on regulatory enzymes of cholesterol and bile acid synthesis than on adsorption and physical effects. We know very little about the effects changes in pool sizes of some of the minor bile acids may have in overall sterol balance, considering the key role this may play in explaining the observed effects of some dietary fiber sources, (c) In spite of these shortcomings, we have had experience with several sources of dietary fiber which have been shown to be effective in reducing hypercholesterolemia in men without substantial increases in fecal steroid concentrations. These experimental results should be incorporated into a more complete diet therapy for hypercholesterolemia employing all diet components.