Fiber
Eat more!
What is it?
Fiber is a diverse set of compounds that cannot be absorbed or digested by enzymes in the small intestines. As a whole category, fiber is generally considered a carbohydrate however, unlike other carbohydrates like sugars or starch, it passes through the body and reaches the large intestine generally undigested.
What are the different kinds?
In short, there are several different kinds of fiber. Some sources online may list the fiber sources simply as soluble and insoluble. According to those definitions:
Insoluble fiber: does not dissolve in water, helps move material/food waste through your digestive system, can also help with disease prevention.
Soluble fiber: dissolves in water, can help with disease prevention.
Other sources may add something called ‘fermentable fiber’ to the list of definitions.
Fermentable fibers are those compounds that can become food for the bacteria in the gut.
These definitions are definitely not wrong. Where it gets confusing for consumers is the lack of consensus on the definition of fiber.
In 2001, a panel of experts convened by the Institute of Medicine (now the National Academy of Medicine) developed distinct definitions for fiber that occurs naturally in plant foods (dietary fiber) and synthetic or isolated fibers that may be added to foods or dietary supplements (functional fiber). These definitions are not widely adopted; however, it's important to share this view of fiber.
Dietary fibers are non-digestible carbohydrates and lignans that occur naturally in plants and includes the below types. Note: having a “laxative effect” as mentioned in some of the bullets below is not a bad thing, this is how fiber can help to prevent constipation.
Lignans – insoluble fiber found in corn and wheat bran, flaxseed, nuts, vegetables, unripe bananas; it can trigger a mucus secretion in the colon and add bulk to the stool, has a laxative effect.
Cellulose, hemicellulos – an insoluble fiber that is found in the cell walls of fruits and vegetables and in cereal grains. It absorbs water and adds bulk to stool which may result in a laxative effect.
Beta-glucans – soluble and fermentable fiber found in barley and oats that is metabolized and fermented in the small intestine. It can act as a prebiotic (feeds the good bacteria in the gut) and can add bulk to the stool but generally does not have a laxative effect. It may help to normalize cholesterol levels and blood sugar.
Guar gum – a soluble and fermentable fiber that is both found in and isolated from seeds. It has a viscous texture and is usually added to foods as a thickener. It is metabolized and fermented in the small intestine. While it does not have a laxative effect it can help to normalize cholesterol and blood sugar levels.
Pectins – soluble and fermentable fiber found in berries, apples, and other fruits. Its bulking and laxative effects are minimal. Because it has gelling-like properties it can slow-down digestion and help to normalize cholesterol and blood sugar.
Inulin, oligofructose, oligosaccharides, fructooligosaccharides – soluble and fermentable fibers found in onions, asparagus, chicory root, and Jerusalem artichokes. They may provide bulk to the stool, a laxative effect, act as a prebiotic, and normalize blood sugar. Some may be sensitive to these fibers as they’ve been known to cause stomach upset and/or bloating.
Resistant Starch – soluble and fermentable fiber found in unripe bananas, legumes, cooked and cooled pasta, and potatoes. It acts as a prebiotic, adds bulk to the stool, and may help to normalize cholesterol and blood sugar.
Functional Fibers are manufactured fibers, some of which are extracted from plants or synthetically made and are nondigestible fibers that offer a beneficial health effect. They include the below types, some of which are also listed above.
Psyllium – a soluble, viscous, non-fermentable fiber that is extract from psyllium seeds. It holds onto water and can both soften and bulk up stools. It has a laxative effect and can be found as an ingredient in high fiber cereals and over-the-counter laxative products. It can help to normalize blood sugar and cholesterol.
Polydextrose and polyols – a soluble fiber made from glucose and sorbitol (a sugar alcohol). It can both increase bulk of stool and have a minor laxative effect. It does not have a large health benefit but is added to foods to improve texture, moisture, increase fiber content, and add sweetness.
Inulin, oligosaccharides, pectins, resistant starch, gums – soluble fibers that are derived from plant foods but are isolated and modified into a form that is more concentrated which can then be added to foods or supplements marketed for fiber.
Other ways that fiber can be classified:
Soluble, viscous/gel forming, readily fermented – (e.g., beta-glucans, guar gum) – can feed the good bacteria, may improve blood sugar and cholesterol levels, do not have a laxative effect.
Soluble, viscous/gel forming, non-fermented – (e.g., psyllium) – can lower blood cholesterol and sugar, and offers a laxative effect allowing it to prevent constipation and normalize stool.
Soluble, non-viscous, readily fermented – (e.g., inulin, oligosaccharides, resistant starches) – not a strong laxative effect but may feed the good gut bacteria.
Insoluble, poorly fermented – (cellulose, lignan) – large particles can have a laxative effect and irritate the intestines causing water to be added to the stool. Whereas small particles do not have a laxative effect and can actually cause constipation.
Now that these definitions have been shared, let’s summarize.
Fiber can be classified simply as soluble vs. insoluble and broken down even further into natural dietary fibers vs. functional or manufactured fibers.
They can be broken down even further and classified as viscous/gel-forming, non-viscous/non-gel-forming, fermentable and non-fermentable.
Insoluble fibers are those that do not dissolve in water, do not generally trap water (non-viscous/non-gel-forming) and are not readily used as food for good gut bacteria (non-fermentable). They may irritate the bowel and can either contribute to constipation or diarrhea. This doesn’t make them sound good, but be advised, they are still an important fiber.
Soluble fibers can be viscous and gel-forming which means they pick up water in the intestine and can help to produce a more solid stool.
Soluble fibers can also be non-viscous or non-gel-forming meaning they don’t provide a benefit associated with improving stool consistency. These fibers are however fermentable.
Soluble and fermentable fibers: These fibers readily feed the good bacteria in the gut.
Soluble and non-fermentable: These do not readily feed the good bacteria in the gut.
What does this all mean? The variety of fibers out there exist to give us a variety of health benefits. That means that focusing on one food group only, or one fiber supplement only, may exclude the benefits that other fiber containing foods and supplements could provide. Bottom line, try to get a variety of foods that will provide a variety of the different types of fibers, and go for a food first approach as trying to increase fiber using an over-the-counter fiber supplement alone, may actually be detrimental to health.
Why is fiber important?
As part of a healthy diet, fiber can provide a range of health benefits. As already mentioned it can help to regulate blood sugar and cholesterol levels. In addition, it can do the following:
Lower the risk of developing heart disease, diabetes, cancers, stroke, high blood pressure, and metabolic syndrome.
Improve diverticular disease.
Improve constipation and diarrhea.
Maintain a healthy digestive tract.
Help maintain a healthy weight.
Ease hemorrhoids.
Decrease symptoms associated with bowel diseases.
May help to support healthy skin.
An important note is that an NIH funded, longitudinal, randomized crossover design study at Stanford School of Medicine in 2022 showed that fiber benefits may be unique to different individuals. Meaning one person may not get the same benefits from fiber as their friend who eats the same foods or takes the same supplements. The study included 18 participants and had them take different fiber supplements (arabinoxylan (AX), long chain inulin (LCI), and a mixture of fibers consisting of equal parts AX, LCI, and other soluble fibers) over the course of three weeks.
The researchers found that not only did the different fibers have different health effects for different people, but the individual fibers also had some not so beneficial health effects. For example, the LCI was associated with increased inflammation and liver enzymes. In addition, the diversity of the gut microbiota decreased when taking the purified fiber supplements.
This supports the need to get a variety of fiber from a variety of real food sources.
What are sources of fiber?
A variety of foods offer fiber, these include the following:
Soluble: oats, chia seeds, nuts, beans, lentils, apples, blueberries, bananas, peas, lima beans, brussels sprouts, avocados, carrots, barley, citrus fruits, psyllium.
Insoluble: whole wheat/bran, quinoa, brown rice, legumes, leafy greens (kale), almonds, walnuts, seeds, fruits with edible skins, cauliflower, green beans, potatoes.
The above is not an exhaustive list and the below chart provides some more foods with their fiber content per serving size. Generally speaking, including more whole grains, fruits, vegetables, beans, peas, legumes, nuts, and seeds in the diet will help to improve fiber intake.
How much fiber is needed?
The recommendation for daily fiber intake is below. Unfortunately, the existing recommendations do not count for gender diversity.
Studies show that only 5% of Americans are getting enough fiber in their diet with the average consumption being only 16 grams per day. In my private practice, 100% of the nutrition patients I see do not get enough fiber in their diet.
How can I increase fiber in my diet?
Try to include more whole grain foods along with nuts, seeds, fruits, and vegetables. When purchasing bread and other packaged goods look for items that offer 3 grams of fiber or more per serving on the food label. If it doesn't, put it back and opt for a higher fiber choice.
In addition, try the below ideas to add more fiber to your day!
Breakfast: add ½ cup of raspberries to your breakfast meal and if you’re eating toast eat 1-2 pieces with 3+ grams of fiber each = 7-10g
Lunch: Eat a lentil soup with a mixed vegetable salad = 11g
Dinner: Make ½ of your plate brussels sprouts (1 cup) = 6g
Snacks: 1 oz almonds and a small pear = 8g
Total = 32 – 35 grams.
References
Centers for Disease Control and Prevention. (2022, June). Fiber: The Carb That Helps You Manage Diabetes. Retrieved from https://www.cdc.gov/diabetes/library/features/role-of-fiber.html
Cleveland Clinic. (2023, March 14). How Much Fiber You Need and Why It Matters. Health Essentials. Retrieved from https://health.clevelandclinic.org/fiber/
Harvard T.H. Chan School of Public Health. (2022, April). Fiber. Retrieved from https://www.hsph.harvard.edu/nutritionsource/carbohydrates/fiber/
Lancaster, S. M., Lee-McMullen, B., Abbott, C. W., Quijada, J. V., Hornburg, D., Park, H., Perelman, D., Peterson, D. J., Tang, M., Robinson, A., Ahadi, S., Contrepois, K., Hung, C. J., Ashland, M., McLaughlin, T., Boonyanit, A., Horning, A., Sonnenburg, J. L., & Snyder, M. P. (2022). Global, distinctive, and personal changes in molecular and microbial profiles by specific fibers in humans. Cell host & microbe, 30(6), 848–862.e7. https://doi.org/10.1016/j.chom.2022.03.036
Mayo Clinic. (2022, November). Dietary Fiber: Essential for a healthy diet. Retrieved from https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/fiber/art-20043983
Oregon State University, Linus Pauling Institute. (n.d). Fiber. Retrieved from https://lpi.oregonstate.edu/mic/other-nutrients/fiber#classification-systems
US Department of Health and Human Services, National Institutes of Health. (2022, May 24). Health benefits of dietary fibers vary. Retrieved from https://www.nih.gov/news-events/nih-research-matters/health-benefits-dietary-fibers-vary