July 2018 Issue

Dietary Fiber: Fiber — Increase Amount and Variety
By Karen Collins, MS, RDN, CDN, FAND
Today's Dietitian
Vol. 20, No. 7, P. 11

Major nutrition recommendations generally refer to total dietary fiber, and unless claims are made relevant to specific fiber types such as soluble or insoluble, Nutrition Facts panels on food labels are required to list only total dietary fiber. Yet research that goes beyond categorizing fiber as simply soluble or insoluble demonstrates that different types of fiber protect health in different ways.1-3

Dietitians have an important role in helping people meet recommendations for dietary fiber (which call for considerably more fiber than is consumed by most Americans) and guiding them to consume it from a variety of foods to maximize the carbohydrate's range of benefits. 

Defining Fiber
Total fiber, which is the sum of dietary fiber and functional fiber, refers to nondigestible carbohydrates and lignin that are intrinsic and intact in plants. This includes the fiber found naturally in vegetables, fruits, whole grains, legumes, nuts, and seeds.1,4

Functional fiber identifies as isolated nondigestible carbohydrates with beneficial physiological effects in humans.1,4 Examples include psyllium, beta-glucans, and wheat dextrin in supplements or an added ingredient in foods such as cereal, bread, snack bars, and yogurt. Some functional fibers occur naturally, isolated from plants (such as psyllium from the husk of psyllium seed or beta-glucan from oats or barley), while others are artificially created, including polydextrose, wheat dextrin formed by heat and acid treatment of wheat starch, and methylcellulose from chemically treated wood pulp.5

Evaluating dietary fiber's potential to promote health is especially challenging. Observational studies link high dietary fiber intake with health benefits such as reduced risk of colorectal cancer, CVD, and type 2 diabetes.1,6-8 However, these studies can't establish cause and effect, and it's difficult to determine how much benefit relates to dietary fiber vs other health-promoting nutrients and phytochemicals contained in high-fiber foods. Therefore, most randomized controlled trials (RCTs) turn to isolated fiber supplements.

Different Types, Different Roles
Previously, fiber was categorized as either soluble or insoluble based on its solubility in water. Now research has moved forward, identifying fiber more specifically based on its physiological effects.1 Although it's often heard that soluble fibers lower elevated serum cholesterol or improve glycemic control, that's not true of all soluble fibers.9

Viscous fibers form a gel in the intestinal tract, which can lower LDL cholesterol and slow absorption of carbohydrate, thus reducing the rise in blood sugar after meals.2,10,11 Limited evidence suggests that these gel-forming fibers may provide short-term support for weight management by increasing satiety stemming from slowed gastric emptying and small bowel transit.2,12,13

Gel forming is viscous fibers' critical feature for reducing LDL cholesterol. As water is reabsorbed during passage through the intestines, bile is trapped by gel-forming fiber and eliminated with the stool. This reduces the bile acid pool normally available for reabsorption, increasing clearance of LDL cholesterol from the blood for liver production of the bile acids needed for digestion.

In addition, gel formation also is critical for improved glycemic control. Increased viscosity slows digestive enzymes' degradation of nutrients and thus slows glucose absorption, reducing peak postprandial blood glucose after a meal.1,5 Glucose absorption slows, but this doesn't reduce the total amount of carbohydrate absorption.

Some evidence suggests that these effects on digestion and absorption can lead to the release of peptides that can improve insulin production and sensitivity and decrease glucagon secretion.9 In a meta-analysis that combined RCTs of high-fiber diets and soluble fiber supplements, changes in glycated hemoglobin and fasting glucose showed that increased fiber can improve overall glycemic control.14 However, studies show inconsistent results that need further investigation.2

Individual results from increasing viscous fibers vary. Actual gel-forming ability of a particular viscous fiber, such as oat bran, beta-glucan, or guar gum, is reduced by processing that involves heat or pressure.5,9 Moreover, greatest reduction of LDL cholesterol and glycemia occurs in people with elevated baseline levels of LDL cholesterol or glucose, respectively, and in unrestricted (possibly less ideal) diets.5,9

Fermentable fibers, often referred to as prebiotics, are used by health-promoting gut bacteria to produce short-chain fatty acids such as butyrate, which have demonstrated cancer-protective effects in the colon.15 Extent of fermentation varies with transit time and gut bacterial community.1

Through promotion of a healthy gut microbiome, research suggests that fermentable fiber could suppress development of chronic inflammation, which could reduce risk of a wide range of chronic diseases, including cancer, type 2 diabetes, and CVD.1,2

Fermentable fibers don't remain intact in the large bowel, thus providing little water-holding capacity. As a result, they don't provide stool-softening benefits.9

"Bulking" fibers create larger, softer stools. Most often this is viewed as a way to help prevent or relieve constipation. However, this role might be better viewed as stool normalizing, since it also can help firm up loose or liquid stools.5 In addition, it may provide long-term health protection by speeding passage of waste through the gut, diluting potential carcinogens, and reducing exposure to the colon.

How Much Is Enough?
Dietary reference intakes for dietary fiber are based on evidence that fiber reduces heart disease risk. The basic recommendation calls for 14 g/1,000 kcal.4 This has been translated into Adequate Intake recommendations of 25 g/day for women and 38 g/day for men up to age 50, and 21 g/day for women and 30 g/day for men over 50. Adequate Intakes for youth range from 19 to 38 g/day based on age and gender.

The latest American Institute for Cancer Research/World Cancer Research Fund Expert Report on diet and cancer shows the most dramatic reduction in colorectal cancer risk linked to current average US intake compared with very low intake.6 However, the lowest colorectal cancer risk is associated with dietary fiber intake of at least 30 g/day.6

Theoretically, each type of fiber provides mechanisms that could play a role in reducing risk of colorectal cancer. Risk of other cancers also may be reduced by fibers that affect hormone levels, inflammation, and carcinogen exposure. Thus far, however—amidst challenges in defining and assessing consumption of total and specific fiber types—evidence doesn't clearly tie fiber to reduced risk of other cancers.

Current dietary fiber consumption in US adults averages about 17 g/day.16 Take note, however, that popular low-carbohydrate diets reportedly lower fiber intake to an average of 10 g/day.5

Helping People Find Fiber
Dietary fiber is found in plant foods such as grains, vegetables, fruits, legumes, nuts, and seeds. In general, the least-processed choices are those that provide the most fiber.

Foods are always a mixture of several different types of dietary fiber. However, many high-fiber foods are better sources of some particular types of fiber than other types. Table 1 illustrates why it's important to eat a variety of fiber-rich foods to obtain the widest range of potential benefits.

Dietary patterns such as those provided in the 2015–2020 Dietary Guidelines for Americans and the American Heart Association Scientific Statement on dietary patterns demonstrate how recommended levels of dietary fiber can be reached as part of an overall healthful diet.17,18

These dietary patterns provide a baseline; consumers can further increase fiber intake by choosing higher-fiber options and including more than the recommended 1 cup of legumes per week. Including a daily serving of pulses (dried beans, peas, and lentils) has been shown to reduce LDL cholesterol by at least 5% (about 6.6 mg/dL), sometimes in as little as two weeks, according to an analysis of 25 RCTs.19 (Appendix 13 of the Dietary Guidelines for Americans lists dietary fiber content in food within the context of its calorie content.17)

Making Sense of Labels
Current food labels base content information on the DV of 25 g dietary fiber. However, as part of the update on food label format, the dietary fiber DV will be listed as 28 g, based on the recommendation of 14 g/1,000 kcal. The deadline for compliance has been postponed, but many food labels already have begun to use the updated DV.

When reading labels, it's important to note that the definition of functional fiber requires clinical evidence of beneficial effects in humans. Therefore, although research shows that inulin (produced from chicory root) is a nondigestible carbohydrate that's fermentable and seems to function as a prebiotic that can nurture healthy gut bacteria,2 the FDA doesn't currently accept inulin as providing physiological benefit, and therefore it can't be included in fiber content on food or supplement labels.20

If fiber intake for a particular benefit is higher than someone can reach with basic healthful eating choices, fiber intake can be increased with foods containing added functional fiber or a fiber supplement. In advising patients about these options, it's important to explain that choices should be made with recognition that different types of added or supplemental fiber have different effects.

Another essential message is that such options provide health benefits specific to those fibers, whereas high-fiber foods also supply a wide range of protective nutrients and phytochemicals. Working toward an overall healthful eating pattern should remain the primary strategy.

— Karen Collins, MS, RDN, CDN, FAND, is a nutrition consultant specializing in cancer prevention and cardiometabolic health, and nutrition advisor to the American Institute for Cancer Research.

1. Dahl WJ, Stewart ML. Position of the Academy of Nutrition and Dietetics: health implications of dietary fiber. J Acad Nutr Diet. 2015;115(11):1861-1870.

2. Slavin J. Fiber and prebiotics: mechanisms and health benefits. Nutrients. 2013;5(4):1417-1435.

 3. Fuller S, Beck E, Salman H, Tapsell L. New horizons for the study of dietary fiber and health: a review. Plant Foods Hum Nutr. 2016;71(1):1-12.

4. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids: chapter 7: dietary, functional, and total fiber. https://www.nap.edu/read/10490/chapter/9. Published 2005.

5. Lambeau KV, McRorie JW Jr. Fiber supplements and clinically proven health benefits: how to recognize and recommend an effective fiber therapy. J Am Assoc Nurse Pract. 2017;29(4):216-223.

6. World Cancer Research Fund; American Institute for Cancer Research. Diet, nutrition, physical activity and cancer: a global perspective. https://www.wcrf.org/dietandcancer/about. Published 2018. Accessed June 14, 2018.

7. Threapleton DE, Greenwood DC, Evans CE, et al.  Dietary fibre intake and risk of cardiovascular disease: systematic review and meta-analysis. BMJ. 2013;347:f6879.

8. Yao B, Fang H, Xu W, et al. Dietary fiber intake and risk of type 2 diabetes: a dose-response analysis of prospective studies. Eur J Epidemiol. 2014;29(2):79-88.

9. McRorie JW Jr, McKeown NM. Understanding the physics of functional fibers in the gastrointestinal tract: an evidence-based approach to resolving enduring misconceptions about insoluble and soluble fiber. J Acad Nutr Diet. 2017;117(2):251-264.

10. Ho HV, Sievenpiper JL, Zurbau A, et al. A systematic review and meta-analysis of randomized controlled trials of the effect of barley β-glucan on LDL-C, non-HDL-C and apoB for cardiovascular disease risk reduction. Eur J Clin Nutr. 2016;70(11):1239-1245.

11. Ho HV, Sievenpiper JL, Zurbau A, et al. The effect of oat β-glucan on LDL-cholesterol, non-HDL-cholesterol and apoB for CVD risk reduction: a systematic review and meta-analysis of randomised-controlled trials. Br J Nutr. 2016;116(8):1369-1382.

12. Wanders AJ, van den Borne JJ, de Graaf C, et al. Effects of dietary fibre on subjective appetite, energy intake and body weight: a systematic review of randomized controlled trials. Obes Rev. 2011;12(9):724-739.

13. Rebello CJ, O'Neil CE, Greenway FL. Dietary fiber and satiety: the effects of oats on satiety. Nutr Rev. 2016;74(2):131-147.

14. Silva FM, Kramer CK, de Almeida JC, Steemburgo T, Gross JL, Azevedo MJ. Fiber intake and glycemic control in patients with type 2 diabetes mellitus: a systematic review with meta-analysis of randomized controlled trials. Nutr Rev. 2013;71(12):790-801.

15. Bultman SJ. The microbiome and its potential as a cancer preventive intervention. Semin Oncol. 2016;43(1):97-106.

16. US Department of Agriculture, Agricultural Research Service. What We Eat in America, NHANES 2013-2014: nutrient intakes from food and beverages: mean amounts consumed per individual, by gender and age, in the United States, 2013-2014. https://www.ars.usda.gov/ARSUserFiles/80400530/pdf/1314/Table_1_NIN_GEN_13.pdf. Published 2016.

17. US Department of Agriculture; US Department of Health and Human Services. 2015–2020 Dietary Guidelines for Americans, 8th Edition. https://health.gov/dietaryguidelines/2015/guidelines/. Published January 7, 2016. Accessed April 30, 2018.

18. Van Horn L, Carson JA, Appel LJ, et al. Recommended dietary pattern to achieve adherence to the American Heart Association/American College of Cardiology (AHA/ACC) guidelines: a scientific statement from the American Heart Association. Circulation. 2016;134(22):e505-e529.

19. Ha V, Sievenpiper JL, de Souza RJ, et al. Effect of dietary pulse intake on established therapeutic lipid targets for cardiovascular risk reduction: a systematic review and meta-analysis of randomized controlled trials. CMAJ. 2014;186(8):E252-E262.

20.  Questions and answers for industry on dietary fiber. US Food and Drug Administration website. https://www.fda.gov/Food/LabelingNutrition/ucm528582.htm#define_dietary_fiber. Updated March 1, 2018. Accessed May 9, 2018.