August/September 2020 Issue
Dietary Fiber: An Update on Fiber
By Sharon Palmer, MSFS, RDN, and Lori Zanteson
Vol. 22, No. 7, P. 24
A Primer on the Latest Terms, Categories, Labeling, and Filling the Fiber Gap
The many health benefits of dietary fiber—including boosting digestive health, lowering total and LDL cholesterol levels, and decreasing risk of type 2 diabetes, metabolic syndrome, and CVD—are well established. Yet, only about 5% of Americans meet the daily recommended amount of dietary fiber.1 As one of the underconsumed nutrients of public health concern, “low intakes are associated with health concerns,” according to the Dietary Guidelines for Americans 2015–2020.2
With global attention being paid to the gut microbiome, the role of dietary fiber—in particular prebiotic fiber—has intensified. 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—including the lungs, muscles, brain, liver, and bones. In short, dietary fiber 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 of nutritional communication, North America region for BENEO Institute, an ingredients company that 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’re broken down in the colon, Colindres explains, “depends on the chemical nature of the dietary fibers and the microbiota composition.”
Types of Fiber
As understanding of fiber continues to grow, it’s important to stay on top of the different types of fibers and their functions. Dietary fibers are categorized based on differences in solubility, viscosity, fermentability, and prebiotic properties.
Insoluble fiber remains mostly intact in water without dissolving and therefore isn’t a source of calories. Food sources high in insoluble fiber include fruits and vegetables, whole grain foods—such as brown rice and 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 are the hull or bran layer of edible grains, such as oat fiber, wheat bran, barley fiber, and rice bran. Insoluble fiber also derives 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 gellike 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 nonviscous, terms that describe the thickness of the gel formed by some types of soluble fiber. When a thick gel forms, it’s viscous. Viscous fibers slow the digestion and absorption of nutrients, which creates a feeling of fullness, possibly reducing food intake and leading to weight loss. These fibers include psyllium, pectins, guar gum, glucomannan, and beta-glucans. Whole food sources include flaxseeds, oats, legumes, asparagus, and Brussels sprouts. Nonviscous 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, including resistant starches, in foods such as 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. They also 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 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 fructooligosaccharide, 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 FDA, dietary fibers that can be declared on Nutrition and Supplement Facts labels include those that are naturally occurring, and are intrinsic, or those intact in plants. Others that can be listed on labels include fibers added to foods that are isolated or synthetic nondigestible soluble and insoluble carbohydrates that have a scientifically proven benefit to human health, such as lowering blood glucose and cholesterol levels, reducing calorie intake, and increasing the frequency of bowel movements.3
Naturally occurring, or intrinsic, fiber refers to that found in vegetables and fruits, whole grains, cereal bran, flaked cereal, and flours. Intact fibers simply are those that haven’t been removed from the food. These foods have been scientifically proven to benefit human health, so manufacturers don’t have to call them out. On the other hand, isolated and synthetic nondigestible carbohydrates added to foods must be evaluated to determine beneficial physiological effects.
Chicory root fibers—inulin and oligofructose—are FDA-approved dietary fibers, according to Colindres. “As a result of their extensive scientific review process, 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 labeling regulations,” she explains. “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.”
As of this writing, the FDA has approved the following isolated and synthetic fibers for use in food and supplements3:
• beta-glucan soluble fiber;
• psyllium husk;
• guar gum;
• locust bean gum;
• mixed plant cell wall fibers (a broad category that includes fibers such as 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; and
Filling the Fiber Gap
The USDA daily fiber recommendation for men and women younger than 50 is 38 g and 25 g, respectively; for men and women older than 50, it’s 30 g and 21 g, respectively.4
However, studies show that more fiber is better. According to a review of nearly 250 studies in the February 2019 issue of The Lancet, eating fiber from a variety of whole food sources, such as 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 by as much as 24% compared with people who ate little fiber. With each additional 8 g fiber, disease risk decreased between 5% and 27%. Greatest reductions were seen with daily intake between 25 and 29 g.5
Consuming enough intrinsic fiber can be challenging for some, considering the volume 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 quota, isolated fibers can fit into one’s diet to help make up the difference, says Massachusetts-based dietitian Liz Weiss, MS, RDN, the voice behind the family food podcast and blog Liz’s Healthy Table, and author of several cookbooks.
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 also can lower sugar, fat, or both, which results in fewer calories as well.
“In the case of chicory root fiber,” Colindres says, “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 g per serving).”
While consuming prebiotic fibers as an added fiber in food products, such as protein bars, can provide a benefit, “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,” says Amy Gorin, MS, RDN, a media RD and owner of Amy Gorin Nutrition, LLC, in the New York City area.
It’s important to consume both soluble and insoluble fiber from a variety of foods, including those with isolated fibers, says New York–based dietitian Toby Amidor, MS, RD, CDN, FAND, founder of Toby Amidor Nutrition and a Wall Street Journal best-selling author. “However, you do want to consume foods that are nutrient-dense to meet your fiber needs,” she says. “So do read food labels to ensure the food you choose to eat that contains isolated fibers is also a nutrient-dense food.”
— Sharon Palmer, MSFS, RDN, is known as The Plant-Powered Dietitian and has her MS in Sustainable Food Systems. She’s the nutrition editor of Today’s Dietitian, associate faculty at Prescott University’s MS in Sustainable Food Systems program, author, and blogger at SharonPalmer.com.
— Lori Zanteson is a food, nutrition, and health writer based in Southern California.
1. Quagliani D, Felt-Gunderson P. Closing America’s fiber intake gap: communication strategies from a food and fiber summit. Am J Lifestyle Med. 2017;11(1):80-85.
2. Department of Health and Human Services. Dietary Guidelines for Americans 2015–2020: chapter 2: shifts needed to align with healthy eating patterns. https://health.gov/dietaryguidelines/2015/guidelines/chapter-2/a-closer-look-at-current-intakes-and-recommended-shifts/. Published January 7, 2016. Accessed May 1, 2020.
3. Questions and answers on dietary fiber. U.S. Food and Drug Administration website. https://www.fda.gov/food/food-labeling-nutrition/questions-and-answers-dietary-fiber. Updated January 30, 2020. Accessed May 1, 2020.
4. Fiber. US Department of Agriculture, National Agricultural Library website. https://www.nal.usda.gov/fnic/fiber. Accessed May 1, 2020.
5. 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. Lancet. 2019;393(10170):434-445.