The Latest Basics on Fiber
I’m sharing a primer on the latest basics on fiber facts, including terms, categories, labeling, health, and filling the fiber gap.
The many health benefits of dietary fiber—including digestive health, lowering total and LDL cholesterol levels, lowering risk of type 2 diabetes, metabolic syndrome, and cardiovascular disease are well established. Yet, only about five percent of Americans meet the daily recommended amount of dietary fiber.1 As one of the under-consumed nutrients of public health concern, according to the Dietary Guidelines for Americans 2015-2020, “low intakes are associated with health concerns.” 2
The role of dietary fiber—in particular prebiotic fiber—has intensified with the huge global attention to the gut microbiome. With more than 5,000 research articles published over the past six years, evidence supports the influence of the gut microbiota, and the prebiotic fibers that feed it, on the entire body—from the lungs, muscles, brain, liver, and bones. It is crucial to overall health and well-being.
Research supports the benefits of prebiotic fibers on the “immune system, weight management, mineral absorption, systemic inflammation, autoimmune disorders, and cognition via the gut-brain axis, in addition to emerging science on the gut-muscle axis,” says Denisse Colindres, Manager, Nutritional Communication, North America Region for BENEO, Institute, an ingredients company which produces prebiotic fibers. Colindres, who regularly communicates with dietitians on clearing up fiber confusions, spoke at a 2020 nutrition conference on the latest innovations and terminology related to dietary fiber.
Terms and categories of dietary fiber have evolved over the years. Dietary fiber includes the parts of plant foods—mainly fruits, vegetables, whole grains, and legumes—the body can’t digest or absorb. It passes mostly intact through the stomach and small intestine on its way to the colon where it may or may not be broken down by the microorganisms living there. However, not all fibers are the same. Whether they are broken down in the colon, explains Colindres, “depends on the chemical nature of the dietary fibers and the microbiota composition.”
Types of Fiber
As the understanding of fiber continues to grow, it’s important to stay on top of the different types of fibers and their functions. The categorization of dietary fibers is based on how they differ in solubility, viscosity, fermentability, and prebiotic properties.
Insoluble fiber does not dissolve in water, but remains mostly intact and, therefore, is not a source of calories. Food sources high in insoluble fiber include fruits and vegetables, whole grain foods—such as brown rice, whole grain breads, cereals, and pasta—and nuts and seeds. Some fibers may be extracted from their food source and added as an ingredient to supplement the fiber content of foods. Common sources come from the hull or bran layer of edible grains, such as oat fiber, wheat bran, barley fiber, and rice bran. It also comes from plant sources other than grains, including corn bran, soy hull fiber, bamboo, and sugar beet. It benefits the digestive system by adding bulk to the stool and keeping things moving through the bowels, preventing constipation.
Soluble fiber dissolves in water to form a gel-like substance. Food sources include apples, citrus fruits, carrots, peas, beans, oats, and barley. It can help lower blood cholesterol and glucose levels. Soluble fiber is either viscous or non-viscous. The thickness of the gel formed by some types of soluble fiber is known as viscosity. When a thick gel forms, it is viscous. Viscous fibers slow the digestion and absorption of nutrients, which creates a feeling of fullness which can reduce food intake and lead to weight loss. Viscous fibers include psyllium, pectins, guar gum, glucomannan, and beta-glucans. Whole food sources include flax seeds, oats, legumes, asparagus, and Brussels sprouts. Non-viscous soluble fibers are fermented.
Fermentability refers to whether a fiber is digestible by gut bacteria in the colon. Most fermentable fibers are soluble and include pectins, inulin, oligofructose, and beta-glucans. Whole food sources include beans and legumes. But there are some fermentable insoluble fibers, such as resistant starches, in foods like seeds, legumes, and whole grains. Fermentable fibers slow digestion and increase satiety, which can help support weight management by encouraging the consumption of fewer calories, and it may help with diabetes management by increasing insulin sensitivity and decreasing glycemic response.
Prebiotic properties of different fermentable fibers uniquely benefit the microbiome by feeding the good gut bacteria to help them produce nutrients for cells in the colon for a healthy digestive system. For example, chicory root and garlic each contain the prebiotic fiber inulin, which helps improve digestion, relieve constipation, and improve fat digestion. Garlic also contains the prebiotic fiber fructooligosaccharides, which helps promote the growth of beneficial Bifidobacteria in the gut and prevents harmful bacteria from growing. Other food sources with prebiotic properties include leeks, onions, asparagus, and bananas.
According to the Food and Drug Administration (FDA), dietary fibers that can be declared on Nutrition and Supplement Facts labels include those that are either naturally occurring and are intrinsic and intact in plants, or they are added isolated or synthetic non-digestible soluble and insoluble carbohydrates that are scientifically proven to benefit human health,3 such as lowering blood glucose and cholesterol levels, reducing calorie intake and increasing the frequency of bowel movements.
Naturally occurring, or intrinsic, fiber refers to that found in foods like vegetables and fruits, whole grains, cereal bran, flaked cereal, and flours. Intact fibers simply mean they have not been removed from the food. These foods have been scientifically proven to benefit human health, so manufacturers don’t have to call them out. Isolated and synthetic non-digestible carbohydrates that are added to foods must be evaluated to determine beneficial physiological effect.
Chicory root fibers—inulin and oligofructose—are FDA-approved dietary fibers, according to Colindres. “As a result of their extensive scientific review process,” Colindres explains, “the FDA Nutrition Science Review Team concluded that chicory root fibers are in compliance with the dietary fiber definition as laid down in the nutrition labelling regulations. This means that chicory root fibers remain classified as dietary fibers in the Nutrition Facts panel and that the requirements for a proven health benefit of these fibers are fulfilled.”
FDA-Approved Isolated/Synthetic Fibers3
- Beta-glucan soluble fiber
- Psyllium husk
- Guar gum
- Locust bean gum
- Mixed plant cell wall fibers (a broad category that includes fibers like sugar cane fiber and apple fiber, among many others)
- Inulin and inulin-type fructans
- High amylose starch (resistant starch 2)
- Resistant maltodextrin/dextrin
- Cross linked phosphorylated RS4
Filling the Fiber Gap
The USDA daily fiber recommendation for men and women up to age 50 is 38 grams and 25 grams, respectively; for men and for women older than 50, it’s 30 grams and 21 grams.4 However, studies show that more fiber is better. According to a review of nearly 250 studies in the February 2019 journal Lancet,5 eating fiber from a variety of whole food sources, like vegetables, fruits, and whole grains, can decrease risk of death from heart disease and cancer. Those who ate the most fiber reduced their risk of death from heart disease, stroke, type 2 diabetes, and/or colon cancer up to 24 percent, compared to people who ate very little fiber. Each additional eight grams of fiber, disease risk went down between five and 27 percent. Greatest reductions were seen with daily intake between 25 and 29 grams.
Consuming enough intrinsic fiber can be challenging for some, considering the volume that would be needed to reach daily recommendations. Added fibers can help make up the difference, while keeping intake and calories at sensible levels. Because most Americans don’t meet their daily fiber recommendation, isolated fibers can definitely fit into one’s diet to help make up the difference, says Massachusetts-based Liz Weiss, MS, RDN.
Added fibers are a concentrated source of dietary fiber, which can enhance foods in important ways. Chicory root, for example, can make a food that’s low in fiber a good (2.8 g) or excellent (5.6 g) source, without compromising taste, texture, or appearance, and while adding a prebiotic soluble dietary fiber that provides a valuable health benefit. It can also lower sugar, fat, or both, which results in lower calories as well. “In the case of chicory root fiber,” explains Colindres, “because of the amount and quality of supportive science, some food manufacturers are able to make a prebiotic claim on the package to communicate this benefit to consumers, as long as there is sufficient soluble fiber added to the product (more than 1.25 grams per serving).” While consuming prebiotic fibers as an added fiber in food products such as protein bars, can provide a benefit, says Amy Gorin, MS, RDN, a registered dietitian in the New York City area, “If you’re not used to consuming fiber-rich foods, you’ll want to increase your intake slowly to minimize potential side effects such as bloating and gas.”
It’s important to get a variety of fiber, both soluble and insoluble, by consuming a variety of foods that provide them, which can include food products with isolated fibers, says Toby Amidor, MS, RD, CDN, FAND, of the Tri-State Area. “However,” she explains, “you do want to consume foods that are nutrient-dense to meet your fiber needs. So do read food labels to ensure the food you choose to eat that contains isolated fibers is also a nutrient-dense food.”
- Quagliani D, Felt-Gunderson P. Closing America’s Fiber Intake Gap. American Journal of Lifestyle Medicine. 2016;11(1):80-85. doi:10.1177/1559827615588079
- Chapter 2 Shifts Needed To Align With Healthy Eating Patterns. A Closer Look at Current Intakes and Recommended Shifts – 2015-2020 Dietary Guidelines. https://health.gov/dietaryguidelines/2015/guidelines/chapter-2/a-closer-look-at-current-intakes-and-recommended-shifts/. Accessed May 1, 2020.
- Center for Food Safety and Applied Nutrition. Questions and Answers on Dietary Fiber. U.S. Food and Drug Administration. https://www.fda.gov/food/food-labeling-nutrition/questions-and-answers-dietary-fiber. Accessed May 1, 2020.
- Fiber. NAL. https://www.nal.usda.gov/fnic/fiber. Accessed May 1, 2020.
- Reynolds A, Mann J, Cummings J, Winter N, Mete E, Morenga LT. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet. 2019;393(10170):434-445. doi:10.1016/s0140-6736(18)31809-9
Written by Lori Zanteson and Sharon Palmer, MSFS, RDN
Images by Sharon Palmer, MSFS, RDN
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