Evidence suggests that soluble fiber is more effective at lowering cholesterol, but both types of fiber are important for your health. One of the ways soluble fiber may lower blood cholesterol is through its ability to reduce the amount of bile reabsorbed in the intestines. It works like this: When fiber interferes with absorption of bile in the intestines, the bile is excreted in the feces. To make up for this loss of bile, the liver makes more bile salts. The body uses cholesterol to make bile salts. So in order to obtain the cholesterol necessary to make more bile salts, the liver increases its production of LDL receptors.
These receptors are responsible for pulling cholesterol out of LDL molecules in the bloodstream. Therefore, the more bile salts are made from the liver, the more LDL cholesterol is pulled from the blood. There is more to be learned about the relationship between soluble fiber and cholesterol, however. It is also possible that one of the short-chain fatty acids produced by the fermentation of soluble fiber in the large intestines may inhibit the amount of cholesterol produced by the liver.
Research has shown that increasing soluble fiber by 5 to 10 g a day reduces LDL cholesterol by about five percent. Oat bran and oatmeal, as well as psyllium and barley, are rich in beta-glucan, a soluble form of fiber, which has been shown to lower total cholesterol and LDL cholesterol.