Fiber is indigestible carbohydrate found in the cell walls of plant foods. Although fiber isn't an essential nutrient, a high-fiber diet keeps your digestive system running smoothly, can help you lose weight and may lower your risk of heart disease. The two types of dietary fiber, soluble and insoluble, both contribute to health in different ways.

Soluble fiber (including pectins and gums) dissolves in fluids in the large intestine and forms a gel. A diet high in soluble fiber helps lower cholesterol levels because it binds intestinal bile acids that contain fat and cholesterol, which are removed in the stools instead of being reabsorbed into the bloodstream. Soluble fiber also reduces the symptoms of irritable bowel syndrome and other digestive problems by helping to regulate both constipation and diarrhea, and slows the absorption of simple sugars, which helps control the rise in blood sugar after eating. Good sources of soluble fiber include legumes (beans), oats, barley, fruit (apples, for example) and psyllium seed (used to make Metamucil and similar products).

Insoluble fiber (cellulose, hemicellulose and lignins) doesn't dissolve in intestinal fluids, but instead soaks up water like a sponge, adding bulk and preventing constipation by making it easier for the intestines to move waste matter — including potential carcinogens — along and out of your system more quickly. A diet high in insoluble fiber can help prevent diverticulosis (formation of pouches in the large intestine, or colon) and hemorrhoids. Good sources of insoluble fiber include whole grains, wheat bran and some fruits and vegetables.

Overall, fiber-rich foods can help you lose weight or maintain weight loss because they tend to be low in calories, take longer to chew and make you feel full faster. High-fiber foods are also naturally rich in vitamins and minerals. But if you want to add more fiber to your diet, do it gradually. Adding too much fiber at once can cause gas, bloating or diarrhea. Be sure to increase your water intake (at least six glasses a day) as you increase the amount of fiber you eat.

Moderation, the rule in good nutrition, applies to fiber as well. Fiber can bind minerals, such as calcium, iron, magnesium and zinc. You are unlikely to consume too much fiber — more than the maximum recommended 35 grams a day — if you rely on food sources alone. But fiber supplements, such as unprocessed bran or commercial products, may put you over the top and sweep these important minerals out of your digestive system. This is a potential problem for elderly people whose mineral intake may be low and who are likely to take fiber supplements. The precise nutritional and clinical significance of fiber remains elusive. A broad range of food substances is involved and the properties of these indigestible components can vary widely. Here are some simple tips to help you increase your daily fiber intake:

• Choose whole-grain breads and pasta. (Flour from whole wheat contains three times as much fiber as refined white flour.)
• Choose brown rice instead of white. (Brown rice has three times as much fiber).
• Eat the whole fruit instead of juice. (One orange contains about six times as much fiber as a 4 ounce glass of juice).
• Stir a few tablespoons of wheat germ into yogurt or hot cereal; add a couple of tablespoons of bran to pancake or waffle batter.
• Start your morning with cold or hot cereals that contain at least 4 grams of fiber in a serving. Read package labels and compare fiber content. Add fruit to your breakfast cereal.
• Pack cold cereal, fruit or cut-up vegetables in your briefcase or backpack for nibbles.
• Buy cut-up raw veggies at a salad bar or vegetable stand if time is short.
• Use beans (black, white, pink, red, kidney, pinto, etc.) or lentils in salads, as a side dish and in soups and stews.

Any high-fiber food contains both soluble and insoluble fibers. Some foods, however, are particularly good sources of one or the other, as listed below. Individual needs vary, but most people should aim for 25 to 35 grams of total fiber a day.

Food Total Fiber Grams