There are two main types of dietary fiber: soluble and insoluble. The body processes these forms in different ways, and each carries different health benefits.

Dietary fiber is a complex carbohydrate that the body cannot digest. Dietary fiber is commonly found in vegetables, fruits, whole grains, and legumes, sometimes called roughage or bulk. It is an essential nutrient and does not cause a spike in blood sugar levels.

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As the name implies, soluble fiber dissolves in water and gastrointestinal fluids when it enters the stomach and intestines. It is transformed into a gel-like substance, which bacteria in the large intestine ferment.

Insoluble fiber does not dissolve in water or gastrointestinal fluids and remains more or less unchanged as it moves through the digestive tract.

The health benefits of dietary fiber are plentiful. Both soluble and insoluble fiber can help:

  • Feeling satiated or full longer after meals: Soluble fiber slows down how quickly foods are digested, meaning most people feel full longer after fiber-rich meals.
  • Helping lower disease risk: Due to fiber’s many health benefits, a high fiber diet is associated with a lower risk of many diseases, including obesity, cardiovascular disease, diabetes, metabolic syndrome, and others.

Soluble fiber

Soluble fiber slows down digestion and can help:

  • Lowering fat absorption: As a thick, spread-out gel, soluble fiber blocks fats that would otherwise be digested and absorbed.
  • Lowering cholesterol: Soluble fiber prevents some dietary cholesterol from being broken down and absorbed. Over time, soluble fiber can help lower cholesterol levels or the amount of free cholesterol in the blood.
  • Stabilizing blood sugar (glucose) levels: Soluble fiber slows down the digestion rate of other nutrients, including carbohydrates, making blood sugar spikes less likely.
  • Reducing the risk of cardiovascular disease: By lowering cholesterol levels, stabilizing blood sugars, and decreasing fat absorption, regularly eating soluble fiber may reduce the risk of heart disease and circulatory conditions.
  • Feeding healthy gut bacteria: Some soluble fiber-rich foods feed gut bacteria, as they are fermentable in the colon, and so they help the bacteria thrive longer.

Insoluble fiber

Insoluble fiber aids digestion and adds roughage to stool. This may help by:

  • Preventing constipation: Insoluble fiber draws fluid into the gut and sticks to other byproducts of digestion that are ready to be formed into the stool. Its presence speeds up the movement and processing of waste, helping prevent gastrointestinal blockage and constipation or reduced bowel movements.
  • Lowering the risk of diverticular disease: By preventing constipation and intestinal blockages, insoluble fiber helps reduce the risk of developing small folds and hemorrhoids in the colon. It may also reduce the risk of colorectal cancer.

In the United States, more than 90% of women and 97% of men do not meet their daily recommended fiber intake.

Prioritizing whole fruits, vegetables, and whole grains can help people increase their fiber intake. Common foods that are good sources of fiber include:

  • beans and legumes such as lentils, soybeans, and pinto beans
  • artichokes
  • potatoes, parsnips, and other root vegetables
  • apples
  • oranges
  • pears
  • bananas
  • raspberries, blackberries, and other berries
  • bulgur, freekeh, and other grains
  • chia and pumpkin seeds

Learn more about the best sources of dietary fiber here.

A healthful diet contains a mix of both soluble and insoluble fiber. Soluble fibers are more common in beans, peas, oats, barley, apples, and citrus fruits. Good sources of insoluble fiber include beans, whole wheat or bran products, green beans, potatoes, cauliflowers, and nuts.

While many fiber supplements exist, most do not contain the additional vitamins and minerals, including B vitamins and iron, found in fiber-rich foods. Supplements may also not be as easily or fully absorbed by the body.

Keeping some simple rules in mind when shopping or preparing meals is helpful. Good tips for increasing fiber intake include:

  • Picking products that have whole grains close to the start of their ingredients list.
  • Choosing foods naturally rich in fiber over supplements.
  • Consuming fruits and vegetables with their skins or peels intact when possible.
  • Picking unrefined grain and cereal products to include regularly in a diet.
  • Picking whole fruits and vegetables rather than juices.
  • Adding beans, peas, and lentils to soups and salads
  • Adding more beans, peas, or lentils than meat, or make them the main ingredient when preparing pasta dishes, casseroles, or stir-fry.
  • Making dips or spreads out of chickpeas, beans, peas, lentils, and other pulses.
  • Eating unsalted nuts, seeds, or dried fruits as snacks, or sprinkling them over cereals, salads, or yogurt.
  • Starting the day with whole grain breakfast options, especially 100 percent ready-to-eat bran.
  • Picking brown rice above the white variety.

Dietary fiber is a vital part of a balanced diet and provides numerous health benefits. Dietary fiber either dissolves in water and gastrointestinal fluids (soluble) or remains solid throughout digestion (insoluble).

Soluble and insoluble fibers have different health benefits, but are both crucial to good digestive, and overall, health.