Medically Reviewed by Sabrina Felson, MD on August 05, 2022
3 min read

Fiber is the general name for certain carbohydrates -- usually parts of vegetables, plants, and grains -- that the body can't fully digest. While fiber isn't broken down and absorbed like nutrients, it still plays a key role in good health.

There are two main types of fiber. They are soluble fiber (which dissolves in water) and insoluble fiber (which does not). Combined, they're called total fiber.

A number of studies have found that a high intake of total fiber, from foods and supplements, lowers the risk of heart disease. High-fiber diets have also been associated with a reduced risk of type 2 diabetes.

Insoluble fiber adds bulk to stools. It helps treat constipation and diverticular disease and may benefit people with some types of IBS (irritable bowel syndrome). Recent research has shown that increased fiber is also linked to increased survival in people with colon cancer.

Soluble fiber seems to lower cholesterol levels. It binds with cholesterol in the intestines and prevents it from being absorbed. Soluble fiber may also be useful in treating diabetes and insulin resistance (prediabetes). It can slow the absorption of carbohydrates, helping to improve blood sugar levels.

Since fiber is filling and has very few calories, high-fiber foods may also help with weight loss.

Fiber that comes from whole foods is called dietary fiber. Fiber that's sold in supplements, or added to fortified foods, is called functional fiber. The Institute of Medicine has set an adequate intake (AI) for total fiber, which includes all sources. Getting this amount of fiber should be enough to stay healthy. Doctors may recommend higher doses of fiber.

Adequate Intake (AI)
1-3 years19 g/day
4-8 years25 g/day
9-18 years26 g/day
19-50 years25 g/day
51 years and up21 g/day
Pregnant28 g/day
Breastfeeding29 g/day
9-13 years31 g/day
14-50 years38 g/day
51 years and up30 g/day

Even in high amounts, fiber appears to be safe. Experts have not discovered an amount of fiber that's harmful.

Most people in the U.S. take in much less fiber than they should. The best way to get it is from food, like a variety of fruits, vegetables, and grains. Some good sources of soluble fiber include:

  • Oatmeal and oat bran
  • Apples, citrus fruits, and strawberries
  • Beans, peas, and lentils
  • Barley
  • Rice bran

And some sources of insoluble fiber are:

  • Cereal brans
  • Whole grains, like barley
  • Whole-wheat breads, wheat cereals, and wheat bran
  • Vegetables like carrots, cabbage, beets, and cauliflower

Some foods, like nuts, contain both soluble and insoluble fiber.

  • Side effects. Fiber does not have serious side effects. At high levels, it can cause bloating, cramping, gas, and perhaps worsening constipation. Drinking more water -- 2 liters a day -- may help.
  • Interactions. If you take any regular medications, talk to a doctor before you start using a fiber supplement. It may block the absorption of some drugs.
  • Risks. Rarely, fiber supplements have caused intestinal blockages. If you have any chronic disease, talk to a doctor before you start using a fiber supplement. The sugar and salt in some supplements, particularly powders, might be risky for people with diabetes or high blood pressure. People with diabetes may want to choose a sugar-free powder or another form of fiber. Blond psyllium is the most common type of fiber supplement on the market.