Keeping things moving along in your digestive system isn't fiber's only claim to fame. More and more research indicates that a high-fiber diet may help prevent diabetes, heart disease, and cancer.

Fiber, found in fruits, vegetables, whole grains, and legumes, simply is the portion of plant foods that humans cannot digest. You know that fiber is part of a healthy diet, but you might not know how insoluble and soluble fiber work—plus how much your body needs. Here's the rough breakdown.

Insoluble fiber
What is it? Insoluble fiber is the coarse, chewy part of a plant that does not dissolve in water. It forms a plant's structure and can be found in the outside tissue—think fruit skins, stringy vegetables, and crunchy whole grains.

What does it do? Insoluble fiber passes through the body largely intact; it soaks up water like a sponge, adding bulk and softness to the stool. This not only prevents constipation, but also speeds the rate at which food goes through your system. That's good news for your colon because a shorter transit time helps prevent diverticulosis (bulging tissue pouches in the colon) and hemorrhoids. Insoluble fiber also moves harmful toxins and cancer-causing substances—in alcohol, pesticides, processed foods, preservatives, and additives—out of the colon more quickly (Journal of Nutrition, 1998, vol. 128, no. 4; American Family Physician, 1995, vol. 51, no. 2).

Diets rich in insoluble whole grains help people lose weight—and keep it off.

Some scientists believe that fiber reduces colon cancer risk, but research results are mixed (Journal of the American Medical Association, 2005, vol. 294, no. 22). Still, according to Karen Collins, MS, RD, a nutrition adviser to the American Institute for Cancer Research, "while we wait for further research on exactly how dietary fiber may protect against colorectal cancer, a high-fiber, plant-based diet ... remains one of the most positive steps we can take in reducing risk of cancer."

More good news: Diets that are rich in insoluble whole grains help people lose weight—and keep it off (American Journal of Clinical Nutrition, 2003, vol. 78, no. 5).

Which foods contain the most? Whole-wheat bread, wheat bran, rye, most other whole grains, cabbage, beets, carrots, brussels sprouts, turnips, cauliflower, and apple skin.

Soluble fiber
What is it? Soluble fiber—found inside plant cell walls—dissolves and thickens in water to form a sticky, gel-like substance. It gives oatmeal its gummy texture and cooked beans their mushy centers.

What does it do? As it passes through the digestive system, soluble fiber binds to dietary cholesterol, helping the body to eliminate it. This reduces blood cholesterol levels, which may help reduce heart disease risk (Journal of Cardiovascular Nursing, 2006, vol. 21, no. 1; Current Atherosclerosis Reports, 2003, vol. 5, no. 6).

Soluble fiber eases the symptoms of irritable bowel syndrome and other digestive problems by helping to regulate both constipation and diarrhea. It also slows the absorption of simple sugars; this helps control the rise in blood sugar levels after a meal, which is particularly helpful for people with diabetes (Molecular Nutrition and Food Research, 2005, vol. 49, no. 6; New England Journal of Medicine, 2000, vol. 342, no. 19).

Which foods contain the most? Oats (which have the highest amount of soluble fiber of any grain), oat bran, beans, peas, rice bran, citrus fruits, strawberries, and apple pulp.

Too much, too fast?
Adding a lot of fiber to your diet too quickly can cause intestinal gas, abdominal bloating, and cramping. Increase roughage gradually over a period of a few weeks. And remember to drink plenty of water, which helps fiber pass smoothly through the digestive system.

—H.K.J.

Putting it together
According to the National Academy of Sciences Institute of Medicine, adults need 21 to 38 grams of dietary fiber each day. Here's how to get enough.

  1. Always go for whole grains. Look for bread, cereal, and crackers that are 100 percent whole grain—meaning no refined flour. If the label doesn't say "100 percent," check the ingredient list for refined culprits like white flour (usually listed as bleached or unbleached enriched wheat flour), semolina or durum flour, and rice flour.
  2. Try other 100 percent whole grains, including brown rice, bulgur wheat, oatmeal, barley, buckwheat, cracked wheat, quinoa, spelt berries, and amaranth.
  3. Eat more legumes. They offer loads of soluble fiber, plus healthy plant protein. Add them to dips, soups, and salads. Substitute legume-based dishes (such as lentil soup, bean burritos, or rice and beans) for those made with meat.
  4. Load up on vegetables and fruit. If possible, eat the skins; they contain tons of insoluble fiber. Enjoy whole fruits instead of fruit juices, which have no fiber.

H. K. Jones is a registered dietitian and freelance writer based in Washington, D.C.