Soluble fiber attracts water and turns to gel during digestion. This process slows digestion and the rate of nutrient absorption from the stomach and intestine. Soluble fiber is found in oat bran, barley, nuts, seeds, dried beans and legumes, lentils, peas, and some fruits and vegetables. Insoluble fiber appears to speed up the passage of food through the stomach and intestine. It also adds bulk to the stool.
Insoluble fiber is found in wheat bran, vegetables, and whole grains.
A diet high in fiber has been thought to reduce the risk of cancers of the rectum and colon. Since fiber speeds up the passage of food through the digestive tract, it may help limit exposure to cancer-causing agents in food. Recent research, however, has questioned the fiber-cancer link. Eating more fruits and vegetables helps provide a good supply of fiber, vitamin A , vitamin C , beta carotene, other carotenoids, and other valuable substances called phytochemicals. Studies have indicated that a diet high in these nutrients and fiber can reduce the risk of developing several kinds of cancer, including stomach, colorectal, esophagus, larynx, and lung cancer.