Focus on Dietary Fiber
Dietary fiber is the indigestible residue of plants that is contained in foods. It can be water soluble or water insoluble.
Most studies of water-insoluble fibers, such as cellulose, have shown they have little effect on cholesterol levels. Water-soluble fiber, on the other hand, has been the focus of recent scientific studies on the cholesterol-lowering role of dietary fiber, but the new oat bran study has raised questions about its role.
Water-soluble fiber is abundant in oat bran and legumes such as peas and beans. Water-soluble fiber also includes gums and fruit pectin, which is used in making jelly.
About half the fiber in oat bran is water soluble. By contrast, only about 10% of the fiber in wheat bran and rice bran is water soluble.
Psyllium, the active ingredient in several bulk-forming laxatives, is another water-soluble fiber that has attracted attention as a potential cholesterol-lowering agent. The fiber is derived from a grain seed, which is grown in India and bears the same name.
Even though it now appears much less likely that oat bran can directly lower cholesterol, there are several theories about how soluble fibers such as psyllium might work. One theory is that soluble fiber binds cholesterol and bile acids--cholesterol breakdown products that stimulate the absorption of dietary fats--and prevents them from entering the bloodstream.
Another is that intestinal bacteria ferment soluble fibers into substances, called short-chain fatty acids, that can be absorbed into the body and may block the production of cholesterol.
But the new oat bran study raises another possibility: dietary fiber may often have only indirect effects on cholesterol levels, for example when fiber-rich foods are eaten instead of foods rich in cholesterol and saturated fats.
On the other hand, as the Council of Scientific Affairs of the American Medical Assn., pointed out last July, “dietary fiber is not a single substance, but an extremely complex mixture of substances.” Therefore, the new findings do not necessarily contradict research showing that other water-soluble fibers, such as psyllium, have direct cholesterol-lowering effects when taken in large amounts.
