Dietary fiber, the indigestible part of plant material, is made up of two main types. Soluble fiber easily dissolves in water and is broken down into a gel-like substance in the part of the gut known as the colon. Insoluble fiber does not dissolve in water and is left intact as food moves through the gastrointestinal tract.
The term fiber refers to all the parts of plant-based foods that cannot be digested or absorbed by the body. Unlike simple carbohydrates, including most breads and sugars, fiber is a complex carbohydrate and does not raise blood sugar levels.
Fiber is commonly found in vegetables, fruits, whole grains, and legumes. It is also sometimes called roughage or bulk. It is an essential nutrient, which means it must be eaten in the diet.
Fast facts on soluble and insoluble fiber:
- Soluble and insoluble are the two main types of fiber. Many fiber-rich foods contain some of both.
- Both forms of fiber have health benefits.
- Humans have been using fiber as a dietary aid since ancient times.
- In a society built on refined carbohydrates, or white breads, pastas, and sugar sweeteners, getting enough fiber can take effort.
Soluble fiber dissolves in water and gastrointestinal fluids when it enters the stomach and intestines. It is transformed into a gel-like substance, which is digested by bacteria in the large intestine, releasing gases and a few calories.
Insoluble fiber does not dissolve in water or gastrointestinal fluids and remains more or less unchanged as it moves through the digestive tract. Because it is not digested at all, insoluble fiber is not a source of calories.
The health benefits of dietary fiber are plentiful. Some of the main ones are listed here.
- Lowering fat absorption and helping weight management: 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 digested. Over time, soluble fiber can help lower cholesterol levels or the amount of free cholesterol in the blood.
- Stabilizing blood sugar (glucose) levels: Just as it prevents fats from being absorbed, soluble fiber slows down the digestion rate of other nutrients, including carbohydrates. This means meals containing soluble fiber are less likely to cause sharp spikes in blood sugar levels and may prevent them.
- 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 it is fermentable in the colon, and so it helps the bacteria thrive longer.
- Preventing constipation: As an indigestible material, insoluble fiber sits in the gastrointestinal tract, absorbing fluid and sticking 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.
Soluble and insoluble fiber
- 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. Insoluble fiber physically fills up space in the stomach and intestines, furthering the sensation of being full. These properties can help people manage their weight.
- 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.
Good sources of fiber
The nutrition label on food packaging lists the amount of dietary fiber found in each serving of the product.
If a product is marketed as being high in fiber or having associated health benefits, the amount of soluble and insoluble fiber in grams (g) per serving must be listed under the dietary fiber heading. Some manufacturers may also voluntarily give the soluble and insoluble content of the fiber element of the product.
According to the FDA, foods that are considered high in fiber contain at least 20 percent of the recommended daily value (DV) of dietary fiber per serving. Foods that have 5 percent or less are considered poor sources of dietary fiber.
Beans, peas, and whole grains are high in fiber. Some fruits and vegetables are also relatively high in fiber. Common foods that are good sources of fiber include:
- cooked navy beans (1/2 cup contains 9.5 g)
- 100 percent ready-to-eat bran (1/2 cup contains 8.8 g)
- canned kidney beans (1/2 cup contains 8.2 g)
- cooked split peas (1/2 cup contains 8.1 g)
- cooked lentils (1/2 cup contains 7.8 g)
- cooked pinto/black beans (1/2 cup contains 7.8/7.5 g)
- cooked artichoke (one whole artichoke contains 6.5 g)
- cooked white beans/chickpeas/great northern beans (1/2 cup contains 6.3-6.2 g)
- mature soybeans (1/2 cup cooked contains 5.2 g)
- plain rye wafers or crackers (2 crackers contain 5.0 g)
- baked sweet potato with the peel (1 medium potato contains 4.8 g)
- raw pear or Asian pear (1 small pear contains 4.3-4.4 g)
- cooked green peas (1/2 cup contains 4.4 g)
- whole wheat English muffin/bread (1 muffin or 2 slices contains 4.4 g)
- cooked bulgur wheat (1/2 cup contains 4.1 g)
- raw raspberries (1/2 cup contains 4.0 g)
- boiled sweet potato without the peel (1 medium potato contains 3.9 g)
- baked potato with the peel (1 medium potato contains 3.8 g)
- stewed prunes (1/2 cup contains 3.8 g)
- dried figs or dates (1/2 cup contains 3.7-3.8 g)
- raw oat bran (1/2 cup contains 3.6 g)
- canned pumpkin (1/2 cup contains 3.6 g)
- cooked spinach (1/2 cup contains 3.5 g)
- shredded ready-to-eat wheat cereals (1 ounce contains 2.8-3.4 g)
- raw almonds (1 oz. contains 3.3 g)
- raw apple with the skin (1 medium apple includes 3.3 g)
- cooked whole wheat spaghetti (1/2 cup contains 3.1 g)
- raw banana or orange (1 fruit contains 3.1 g)
A healthful diet contains a mix of both soluble and insoluble fiber. Soluble fibers are more common in foods, such as 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 vitamin B and iron, found in fiber-rich foods. Supplements may also not be, as easily or fully absorbed by the body.
It is helpful to keep some simple rules in mind when shopping or preparing meals. 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, such as Metamucil, Citrucel, and others.
- Eating beans, peas, or lentils on a daily basis.
- Eating at least one food daily that contains 20 percent DV per serving.
- Consuming fruits and vegetables with their skins or peels intact when possible.
- Looking up the best way to eat specific foods. The amount of dietary fiber in many foods changes, depending on whether they are raw, cooked, stewed, steamed, fried or baked.
- 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 making 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.