Most of us know the basics on fiber: It helps us “stay regular,” and if things aren’t as “regular” as usual, we try to eat more foods like bran cereals and prunes.
But there is far more to know about fiber, especially in relation to health and wellness. What exactly is fiber? Why is it so good for our digestive systems? Can it fend off disease? How much should we consume? How does the recommended intake compare with how much the typical American eats?
Fortunately, the abundant health benefits of fiber have been thoroughly investigated, and emerging areas of research are yielding a host of insights that are useful to fitness professionals. Read on to get your fill on fiber, find out how much fiber you and your clients need, and learn even more from our translation of some of the latest science. (See Tables 1, 2 and 3, respectively, for grams of dietary fiber in standard portions of some common foods; easy swaps to help you choose foods that provide more dietary fiber; and how much fiber we need in our diets, by age and gender, versus how much we’re getting on average.)
What Is Fiber?
Dietary fibers are parts of plants (storage and cell wall polysaccharides) that the human body’s digestive enzymes cannot break down. Broadly, they fall into two categories: soluble fibers, which dissolve in water, and insoluble fibers, which do not dissolve in water. For example, oat bran contains mostly soluble fiber, while wheat bran has mainly insoluble fiber (Marlett et al. 2002). Soluble fibers typically form a gel in water and/or are fermented in the colon (Slavin 2013). Fiber can occur naturally in plant-based foods like beans and whole grains, or it can be isolated from plants and then added to foods (USDA
Health Benefits of Fiber
Fiber’s documented health benefits range from satiety and weight control to reduction of cardiovascular disease risk. The Institute of Medicine identifies three primary health benefits of fiber: reduced risk of coronary heart disease, assistance in the maintenance of blood glucose levels and improved laxation (IOM 2005).
Coronary Heart Disease
Studies have associated high intakes of dietary fiber and the consumption of whole grains with a reduced risk for coronary heart disease. Research indicates that this happens because fiber can help to lower levels of LDL (“bad cholesterol”) in the blood (Slavin 2013). Fiber has also been shown to reduce other risk factors associated with CHD, including obesity, blood pressure and elevated cholesterol.
Blood Glucose Levels
Research shows that people who eat high-fiber diets are less likely to have diabetes. Studies also show that higher-fiber diets may play a significant role in preventing diabetes or in halting the progression of prediabetes to diabetes (Anderson et al. 2009). Current evidence suggests that a daily diet with 30–50 g of fiber results in lower blood glucose levels than a low-fiber diet does (Marlett et al. 2002). This may be relevant to people who need to manage their blood glucose.
Health Benefits for the Gut and Beyond
Emerging research is revealing specific ways that fiber improves gastrointestinal health and promotes changes in the gut (including stomach, small intestine and large intestine) that can improve our overall well-being.
Laxation and regularity are the most widely recognized benefits of consuming enough fiber. Combining a high-fiber diet with adequate fluid intake and adequate exercise (such as walking or running) helps maintain regularity and may prevent constipation (ACG 2010). Fiber assists in maintaining normal bowel movements by increasing the weight and size of stools; it also promotes a softer stool consistency.
Stool weight and size can increase for several reasons: the presence of fiber itself, the water absorbed by the fiber and the rising quantity of stool microbes that result from fermentation of fiber in the intestine (Marlett et al. 2002). Cereal fibers, especially wheat bran, are considered most effective in increasing stool weight (Slavin 2013).
Fiber fermentation may be key to gut health. As fibers—especially soluble fibers—get fermented by certain gut bacteria, they are converted into acids, including short-chain fatty acids (SCFAs), and gas. This makes the environment acidic, discouraging the growth of pathogens and putrefactive bacteria.
SCFAs also promote the growth of certain intestinal cells, thickening the intestinal walls and possibly reducing the risk of bacteria migrating out of their physiologic boundaries (Meyer 2015). Fermentable fibers are found in foods like wheat, oats, barley, chicory root, artichokes, peaches, apples and carrots.
Fiber and the Microbiota
Emerging research has shown that fiber has a strong influence on the diversity and composition of the GI microbiota—the microorganisms that live in the gastrointestinal tract. This diversity and composition can play an important role in overall physical well-being. Microorganisms in the gut interact with each other and with the human body, influencing health and disease (Clemente 2012). A healthy human’s GI microbiota typically contains several hundred species, which account for trillions of microorganisms. Overall, high-fiber plant-based diets are associated with a more diverse GI microbiota and higher production of SCFAs than are diets low in fiber (Simpson
Certain fermentable fibers can act as prebiotics. A dietary
is “a selectively fermented ingredient that results in specific changes in the composition and/or activity of the GI microbiota, thus conferring benefit(s) upon host health” (Gibson et al. 2010). In other words, prebiotics selectively stimulate the growth and/or activity of certain beneficial bacteria.
Suggested benefits of consuming prebiotics include reducing the prevalence and duration of diarrhea, enhancing the uptake of minerals like calcium and magnesium, and protecting against chronic diseases like colon cancer and heart disease (Slavin 2013; Anderson et al. 2009).
Most prebiotic fibers are
—complex carbohydrates made up of chains of simple sugars—that get fermented by the microbiota in the colon (Slavin 2013).
(also known as fructooligosaccharides, or FOS), the most widely studied prebiotic fiber, has beneficial effects at relatively low doses (5 g per day). Inulin is a fiber found in many plants, like garlic, onions, asparagus, chicory, artichokes, bananas, wheat and leeks (Gibson et al. 2010). It can be readily extracted from chicory root and incorporated into products like snack bars and breakfast cereal. Other prebiotic fibers include
from milk, polydextrose and soluble corn fiber (Slavin 2013; Meyer 2015).
Reduced Colon Cancer Risk
Early evidence from studies of dietary patterns and health outcomes around the world found that populations with higher dietary fiber intakes had a reduced risk for developing colon cancer (Marlett et al. 2002; Meyer 2015). A study two decades ago estimated that colorectal cancer risk in the United States could drop by 31% if people increased their fiber intake by an average of 13 g per day (Howe et al. 1992). However, intervention studies and more recent epidemiological evidence have been mixed and cannot support this hypothesis, so further research is needed (Marlett et al. 2002; Anderson et al. 2009).
Much Fiber Do You and Your Clients Need?
When the Institutes of Medicine established the Dietary Reference Intake system, there was not enough evidence to create a Recommended Dietary Allowance (commonly known as RDA) for fiber. An Adequate Intake, or AI, for fiber was determined based on the amount needed to reach the lowest risk for coronary heart disease.
The DRI recommends 14 g of dietary fiber per 1,000 calories. For adult women eating a 2,000-calorie-per-day diet, that would be 28 g of fiber; for adult men eating a 2,600-calorie-per-day diet, the amount would be 36 g of fiber (USDA
Are Americans Eating Enough Fiber?
Although the health benefits of fiber are widely understood, 9 out of 10 Americans are not getting enough fiber in their diets: Only 5% of adults and less than 1% of children and teens meet their daily fiber recommendations (see Table 3). In the United States, the average fiber intake is 15 g per day, less than half the recommended amount (IOM 2005). Women over 50 years of age get the most fiber in their diets, but only 17% of them meet the recommendation (USDA 2015a).
Foods With Fiber
With so many Americans missing out on the numerous benefits of fiber, personal trainers can play a key role in knowing and recommending food sources of fiber (see Tables 1 and 2). To get more fiber into your clients’ diets, recommend that they consume more whole grains, fruits, vegetables and legumes (USDA 2015b).
Whole-grain foods include oatmeal, ready-to-eat cereal, oat-based cereal bars and whole-grain bread. Other sources of fiber include fruit such as bananas, apples and strawberries, and legumes and vegetables like peas, lentils and spinach.
Understanding Food Labels
A product can be labeled a “good source of fiber” if it has at least 2.5 g of fiber per serving. It can be labeled an “excellent source of fiber” if it has 5 g or more of fiber per serving.
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Anderson, J.W., et al. 2009. Health benefits of dietary fiber. Nutrition Reviews, 67 (4), 188-205.ÔÇ®
Clemente, J.C., et al. 2012. The impact of the gut microbiota on human health: An integrative view. Cell, 148 (6), 1258-70.ÔÇ®
Gibson, G.R., et al. 2010. Dietary prebiotics: current status and new definition. Food Science & Technology Bulletin: Functional Foods, 7 (1), 1-19.ÔÇ®
Howe, G.R., et al. 1992. Dietary intake of fiber and decreased risk of cancers of the colon and rectum: Evidence from the combined analysis of 13 case-control studies. Journal of the National Cancer Institute, 84 (24), 1887-96.ÔÇ®
IOM (U.S. Institute of Medicine). 2005. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Washington, DC: National Academies Press.ÔÇ®
Marlett, J.A., et al. 2002. Position of the American Dietetic Association: Health implications of dietary fiber. Journal of the American Dietetic Association, 102 (7), 993-1000.ÔÇ®
Meyer, D. 2015. Health benefits of prebiotic fibers. In J. Henry (Ed.), Advances in Food and Nutrition Research (Vol. 74, pp. 47-91). Waltham, MA: Academic Press.ÔÇ®
Simpson, H.L., & Campbell, B.J. 2015. Review article: Dietary fibre-microbiota interactions. Alimentary Pharmacology & Therapeutics, 42 (2), 158-79.ÔÇ®
Slavin, J. 2013. Fiber and prebiotics: Mechanisms and health benefits. Nutrients, 5 (4), 1417-35.ÔÇ®
USDA (U.S. Department of Agriculture). 2015a. Individuals 1 year and over (excluding breast-fed children and pregnant or lactating females), dietary intake data. What We Eat in America, NHANES 2007-2010. Scientific Report of the 2015 Dietary Guidelines Advisory Committee.ÔÇ®
USDA. 2015b. What We Eat in America, NHANES 09-10 (ages 2+). Scientific Report of the 2015 Dietary Guidelines Advisory Committee, Part E. Section 2: Supplementary Documentation to the 2015 DGAC Report.ÔÇ®
USDA & HHS (U.S. Department of Agriculture & Department of Health and Human Services). 2010. Dietary Guidelines for Americans (7th ed.). Washington, DC: U.S. Government Printing Office.