Soluble Fiber vs. Insoluble Fiber: How Do They Differ?

You’ve likely heard that you should be including more fiber in your diet, but did you know that different types of fiber exert distinct health benefits? Fiber is a type of indigestible carbohydrate found in plant-based foods. It is separated into two types, soluble and insoluble, which can be distinguished by their solubility and fermentability characteristics. Although most plant-based foods contain both types of fiber in varying amounts, certain foods contain larger amounts of either soluble or insoluble fiber. Continue reading to learn about the best sources of fiber, as well as the health benefits and distinct characteristics of both types of fiber.

Plant-based foods, such as fruits, vegetables, beans, and grains, are excellent sources of soluble and insoluble fiber.

What is soluble fiber?

As the name suggests, soluble fiber dissolves in water, creating a gel-like substance during the digestion process. Several types of soluble (digestible) fibers, such as beta-glucan, guar gum, pectin, and psyllium, are found in dietary and supplement sources. (3) Dietary sources of soluble fiber include:

• Beans
• Fruit (e.g., citrus, apples, strawberries)
• Lentils
• Oats
• Peas
• Psyllium husk
• Seeds (e.g., sunflower seeds, flaxseed) (6)(20)

Benefits of soluble fiber

The benefits of soluble fiber include increased satiety (feeling full after eating), as well as improved gut and cardiometabolic health.

Satiety

The gel-forming effects of soluble fiber help delay gastric (stomach) emptying, meaning that food travels through the intestines at a slower rate. Delayed gastric emptying can help enhance satiety, helping you feel full for a longer period of time after eating. (3) Increased feelings of satiety may prevent overeating and reduce hunger pangs between meals. (18)

Gut health and regularity

Soluble fibers bypass the small intestine and reach the colon, where some begin to undergo fermentation. When certain soluble fibers (e.g., polysaccharides and resistant starch) reach the colon, they begin to ferment. The fermentation process of these prebiotic fibers produces metabolites, such as short-chain fatty acids (SCFAs), which promote gut health by protecting against inflammation and enhancing intestinal barrier function. (6)(7)(17)

Soluble fiber also promotes bowel regularity. As soluble fiber draws water into the gut, stools become softer and, therefore, easier to pass. (14) A systematic review of six randomised controlled trials determined that soluble fiber is beneficial in the management of chronic constipation. This review demonstrated that soluble fiber can improve stool consistency, increase the average number of weekly bowel movements, and reduce the number of days between bowel movements. (19)

Did you know? As a general rule, adult women should consume 25 g, and men are advised to consume 38 g of fiber each day. (1)

Cardiometabolic health

Beyond the demonstrated gut health benefits, diets high in soluble fiber are also associated with numerous cardiometabolic benefits, including a reduced risk of heart disease, obesity, hypertension, and diabetes. (3) Certain soluble fibers, such as beta-glucan and psyllium, have been shown to improve blood glucose (sugar) regulation, and may reduce your risk of developing type 2 diabetes. (14) Soluble fiber also effectively improves blood glucose levels and insulin sensitivity in both diabetics and non-diabetics. (4) Because soluble fiber stays in the digestive tract longer and delays gastric emptying, it can help slow down the absorption of glucose into the blood, therefore reducing spikes and drops in blood sugar that increase diabetes risk. (4)

Several meta-analyses have determined that soluble fiber may reduce cardiovascular disease incidence and mortality by lowering total blood cholesterol and low-density lipoprotein (LDL) levels. (12) Furthermore, the fermentation process of soluble fibers in the colon produces propionate, a short-chain fatty acid, that has been shown to inhibit cholesterol production and is believed to be partly responsible for the hypolipidemic effects of soluble fiber. (22)

What is insoluble fiber?

Unlike soluble fiber, most insoluble fibers have limited fermentability and don’t dissolve in water. (11) Instead, insoluble (or indigestible) fibers, such as lignin and cellulose, are responsible for adding bulk to stool, helping food move quickly through the intestines and promoting digestive regularity. (6)(14)

Dietary sources of insoluble fiber include:

• Wheat bran
• Whole wheat
• Vegetables (e.g., beets, spinach, carrots, broccoli) (6)(20)

Benefits of insoluble fiber

Soluble and insoluble fiber provide many of the same benefits, however, insoluble fiber promotes digestive regularity by increasing the speed at which food moves through the digestive system and may help prevent certain gastrointestinal conditions.

Digestive regularity

Insoluble fiber is best known for its ability to regulate digestion and its laxative effects. (14) While soluble fiber slows digestion, insoluble fiber adds bulk to stools and reduces the time it takes for food to move through the digestive system. (8)

Coarse insoluble fibers, like those found in wheat bran, promote regularity and stimulate a laxative effect by irritating the mucosa of the large intestine, thus triggering water and mucous secretion. (14)

Did you know? It’s essential to stay hydrated when consuming fiber-rich foods. Consuming too much fiber and not enough fluid can cause nausea or constipation. (1)

Reduced risk of diverticulitis and colon cancer

Some research has indicated that insoluble fiber may help prevent diverticulitis, a condition characterized by inflammation or infection of small pouches in the colon that can result from certain modifiable and non-modifiable factors, including genetic predisposition, smoking, obesity, lack of exercise, and use of non-steroidal anti-inflammatory drugs (NSAIDs). (15)(21)

Additional research suggests a link between high-fiber diets and reduced risk of colon cancer. (2) Current evidence is limited, however, it’s hypothesized that insoluble fiber may be protective against colon cancer because it can bind to carcinogens and other harmful substances, thus inhibiting the carcinogenic process. (9)(16)

To learn more about fiber benefits and daily fiber recommendations, visit the Fullscript blog.

Fiber supplements

It’s best to meet your daily fiber requirements through dietary sources. Research has shown that many fiber supplements may provide some health benefits, but not to the same degree as dietary fiber. (10) However, if you struggle to eat enough fiber each day, a fiber supplement may help you reach your daily fiber goal or mitigate occasional gastrointestinal symptoms. (10) The table below outlines some of the common ingredients found in fiber supplements.

Fiber supplements contain varying amounts of different types of soluble and insoluble fibers. (13)(18)Fiber supplements may cause unwanted effects such as gas, bloating, constipation, or diarrhea, especially if you increase your intake of fiber too rapidly. (1) It’s important to note that fiber supplements, as well as high-fiber diets, may interact with certain medications and affect their absorption. (5) Speak to your integrative healthcare practitioner before using fiber supplements or making significant changes to your diet.Fiber supplements contain varying amounts of different types of soluble and insoluble fibers. (13)(18)

The bottom line

Plant-based foods, such as fruits, vegetables, beans, and whole grains, are excellent sources of dietary fiber. Consuming a balanced diet with a variety of foods can help ensure you’re obtaining both soluble and insoluble types of fiber. It’s best to reach your daily fiber goals through your diet, however, adding a fiber supplement can help boost your intake and may yield similar health benefits to dietary fiber. If you’re a patient, speak to your integrative healthcare practitioner to see if adding supplemental fiber to your treatment plan is right for you.