Ask the Expert: Dietary Fiber

The Physicians Committee

Ask the Expert: Dietary Fiber

Q: What is the difference between soluble and insoluble fiber?

A: Both soluble and insoluble fiber are valuable in preventing disease. Soluble fiber makes up about one-quarter of the fiber in food. It dissolves in water, slows digestion by slowing down the time it takes for the stomach to empty, and helps the body absorb nutrients from food. Oats, beans and other legumes, and some fruits and vegetables are all good sources of soluble fiber. Psyllium, a grain found in some cereals and in certain bulk fiber supplements, is also a good source of soluble fiber. Soluble fiber can help lower blood cholesterol, particularly if you have elevated cholesterol levels, and may help control blood sugar levels in people with diabetes. Both types of fiber are very important for controlling hormone levels in the body, particularly testosterone and estrogen. These hormones play a significant role in the prevention and survival of certain types of cancer. Fiber also helps to excrete toxins from the body and can help with weight control.

Insoluble fiber makes up about three-quarters of the fiber in food. It does not dissolve in water, and “holds” water which helps to create bulk and moisture to the stool. The water-holding quality of insoluble fiber creates a feeling of fullness in the stomach and helps foods pass through the stomach and intestines. It’s made up from the structural material of the cell walls of plants. It consists of cellulose, hemicellulose and lignin. Insoluble fiber passes through the gastrointestinal tract mostly undigested (the human body does not have the enzymes to break down insoluble fiber). Additionally, unlike soluble fiber, insoluble fibers are not metabolized by intestinal bacteria. The skins of many fruits and vegetables, seeds, nuts, wheat, and whole grains (also whole grain breads, cereals and pasta) are good sources of insoluble fiber. Though all plant cells contain both soluble and insoluble fibers in varying amounts, some foods are more abundant in one type of fiber. Some foods especially rich in the insoluble type of fiber are: grapes, prunes, apple skins, pear skins, berries, celery, beets, carrots, Brussels sprouts, turnips, cabbage, cauliflower, broccoli, rhubarb, red chard, asparagus, corn, pop corn, kidney beans, potato skins, and bran.

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Q: Why is fiber good for cancer prevention and survival?

A: Fiber is not digested by the human body, and so it sits in the digestive system waiting to be released.  Its mere presence extracts liquid from the body (a process called osmosis) that contains toxins and wastes, and the body is then able to expel these substances as feces.  Fiber in and of itself is not anti-carcinogenic as scientists once thought, but it is an important aid that the body needs to clean house, so to speak, and get rid of wastes and potentially carcinogenic substances.  These excess substances that the body may need to get rid of include hormones and cholesterol, both of which can contribute to disease. Fiber is only found in plant foods. 

Monroe KR et al. Dietary fiber intake and endogenous serum hormone levels in naturally postmenopausal Mexican American women: the multiethnic cohort study. Nutr Cancer. 2007;58(2):127-135.

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