- Describe the different sources of dietary fiber.
- Describe the health benefits of fiber intake.
- Explain the ill effects of excessive and inadequate fiber intake.
- Describe the current state of fiber intake of Americans.
- Discuss the role of fiber in heart health, weight loss, diabetes, cancer prevention and mental health.
Fiber & Digestive Health for Active People of All Ages
As our knowledge of this macronutrient evolves, it continues to reveal myriad benefits.
As a dietitian who specializes in sports, I work with athletes of all ages and abilities. Some athletes eat a stellar diet, while others have much room for improvement. Regardless of their diets or abilities, athletes often arrive at my office with one of two goals in mind: achieve better health and wellness by changing their eating habits, or improve athletic performance by changing their eating habits. In one of these cases, a client asked me to tweak his diet to help him move from a competitive age-group category to elite standing.
As with most clients, I began by reviewing what this athlete typically consumed each day. Unlike most clients, however, he ate no empty calories and his diet provided little room for improvement—it was jam-packed with fruits, vegetables, nuts, seeds, whole grains and legumes. His diet was full of fiber, much more than the diets of any athletes I'd worked with and certainly more than the average American's 15 grams (g) per day. This aspiring professional consumed more than 80 g of fiber each day! This led me to ask if he frequently experienced gastrointestinal (GI) distress? No. Did he have GI distress during race day situations? No. Did he find it easy to incorporate that much fiber? Sort of. Last, and above all, did that much fiber keep him satiated? Absolutely.
His report led me to wonder: Could more fiber be a missing link for athletes? Would adding extra fiber cause most athletes to spend too much time in the Porta Potty on race day? What other changes should athletes make to their diets to accommodate more fiber? Most important, how could all athletes, from amateur to elite, easily add in more fiber and reap the health benefits?
Intoduction to Fiber
Fiber isn't a new subject; our hunter-gatherer ancestors consumed a wide range of fiber-containing plants, and many historical figures have acknowledged and promoted fiber's benefits. Over time, our understanding of the powerful role that fiber plays in keeping the body healthy has developed significantly. Various studies have demonstrated that a diet adequate in fiber is linked to a lower risk of disease (IFIC 2008). And yet, despite strong scientific evidence touting the advantages of fiber, most Americans consume less than the recommended amount and eat too many empty calories.
Although the call for more fiber is not new, the promotional campaign has undergone a face-lift. Originally referred to as "crude," fiber has morphed into a fashionable food additive, and these days "functional fiber" surrounds us. Why the push to eat more of it? Fiber's potential health benefits are prolific, ranging from decreased risk of cardiovascular disease and diabetes to weight maintenance and regular laxation. Because of these many advantages, fiber's star as a health-promoting food component is rising.
Fiber: What Is It Exactly?
Fiber falls into the macronutrient class of carbohydrate. Fibers, like starches, are made mostly of many sugar units bonded together. Unlike in most starches, however, fiber bonds cannot be broken down by digestive enzymes and therefore pass relatively intact into the large intestine. Fiber can eventually be fermented by the colonic microflora into gases such as hydrogen and carbon dioxide, or it can pass through the large intestine and bind with water, increasing stool weight. Although fibers are not converted to glucose, some short-chain fatty acids are produced in the gut as fibers ferment. Short-chain fatty acids are absorbed and can be used for energy in the body (IFIC 2008).
Many definitions for fiber have been proposed. Most have been developed with nutritional labeling in mind (IFIC 2008), with the goal of emphasizing fiber as a measurable food component. Different organizations, countries and professions use different definitions for fiber. Delineations vary based on the origin of fiber components, the analytical methods used to identify and quantify fiber, and whether beneficial health effects are part of the definition (IFIC 2008; Jones, Lineback & Levine 2006).
In the United States, fiber has been defined (for regulatory purposes) primarily by the analytical methods used to isolate and measure it (IFIC 2008). With the ongoing goal of making nutrition labeling universal, the Institute of Medicine (IOM) formed the Panel on the Definition of Dietary Fiber. After much study and debate, the panel proposed moving from a definition based solely on the method used to measure fiber content to one that also recognizes fiber's many positive health effects. The result was a two-part definition for total fiber, which was broken into dietary fiber and functional fiber.
Total fiber is the aggregate amount of fiber in a food product and is displayed on the Nutrition Facts panel on food labels (ADA 2008). In essence, fiber is a nondigestible form of carbohydrate and lignin (defined below) (USDA 2010a; IOM 2002).
Dietary fiber is essentially intact within the plant matrix, and the enzymes within the human digestive tract are unable to hydrolyze or break it down. Dietary fiber is made up of three varying components. The largest component consists of polysaccharides, or plant fibers such as bran; pectins from fruits and vegetables; various gums; and beta-glucans from oats and rye (ADA 2008). The second-largest component is lignin, composed of polyphenylpropane molecules and present in very small amounts in the diet. Lignin is found in stalks and stems. The final component is made up of resistant starches and nondigestible oligosaccharides. These are naturally occurring parts of fiber that resist digestion in the upper digestive tract yet feed and stimulate the growth of friendly bacteria in the lower digestive tract. Resistant starches and oligosaccharides are sometimes added to food as isolated ingredients (e.g., polydextrose) and occur naturally in legumes, various fruits and vegetables, seeds and grains.
Conversely, functional fiber consists of isolated or purified nondigestible carbohydrates. Functional fiber includes animal carbohydrates (chitin and chitosan), pectins or gums, resistant starch formed during processing, psyllium, inulin, and synthetic fibers such as resistant maltodextrin, fructo-oligosaccharides and polydextrose (IFIC 2008; ADA 2008). Functional fiber is sometimes referred to as "isolated" or "novel" fiber.
In the past, the terms soluble and insoluble were used to classify the specific types of fiber within a product. Currently, the IOM recommends that these terms be phased out. The recommendation is based on research that suggests the health benefits of fiber do not hinge solely on whether or not it dissolves in water. For example, it was once believed that insoluble fibers (such as wheat bran) were linked to laxation, while soluble fibers decreased total cholesterol levels. However, scientific support for these postulations is inconsistent at best (ADA 2008). Instead, viscosity and fermentability are now proposed as more meaningful characteristics.
Health Benefits of Fiber
Health and fitness professionals recommend fiber because of its purported health benefits. Besides keeping an athlete or an active client "regular," what are the benefits of adding more fiber to the diet?
Fiber and Heart Health
The evidence for fiber's role in effectively reducing the risk of coronary heart disease (CHD) is so strong that the overall recommended Adequate Intake (AI) for fiber is based on a level observed to protect against CHD. The types of dietary fiber that have been proven effective in preventing CHD include, but are not limited to, cereals and certain viscous fibers such as cereal gums and fruit pectins.
Researchers suggest that the heart health benefit that fiber provides is based on how it effectively interferes with cholesterol and bile acid absorption, consequently lowering total cholesterol and low-density lipoprotein (LDL) cholesterol (IFIC 2008). An alternative mode of action in fiber's fight against heart disease may be the ability fiber has to delay absorption of fat and carbohydrate, which in turn leads to increased insulin sensitivity and decreased levels of circulating triglycerides (IOM 2002).
Fiber has a protective effect against cardiovascular disease (CVD) in the overall population (IOM 2002; ADA 2008), but this effect may be even more pronounced in the elderly. A large, multicenter study following the incidence of CVD among 3,588 men and women aged 65 years or older found that cereal consumption was inversely associated with incident CVD events (while neither fruits nor vegetables had this association) (IFIC 2008). A separate study supported the above findings and found that whole-grain intake and added bran intake were also effective in preventing CVD (while added germ intake was not) (Jensen et al. 2004).
In addition to protecting against CVD, a diet high in fiber has been linked to improved blood pressure parameters (Streppel 2008).
Fiber and Weight Loss
There's no debating that fiber is filling, but is it filling enough to prevent overeating and weight gain? And what about long-term satiety? Current data suggest that dietary and supplemental fiber (intakes of 20–27 g per day [g/dy] from whole foods or up to 20 g/dy from supplements) may help with weight control (ADA 2008). However, it should be noted that research findings on fiber's effects on appetite, energy and food intake have been inconsistent, with results differing according to fiber type and whether the fiber occurs naturally in food sources or is consumed as an isolated supplement (ADA 2008).
How does fiber facilitate weight loss? Research suggests that fiber acts as a physiological obstacle to energy intake by doing at least one of the following: displacing available energy and nutrients from the diet; increasing chewing, which limits intake by promoting secretion of saliva and gastric juices and thereby expanding the stomach and increasing satiety; and/or reducing the absorption efficiency of the small intestine (Heaton 1973).
Regarding satiety, researchers Adam and Westerterp-Plantenga (2005) found that postprandial release of glucagon-like peptide-1 (GLP-1), a hormone shown to increase satiety and suppress appetite in normal-weight subjects, was elevated when a meal included fiber. Worth noting is that in both normal-weight and overweight/obese subjects, more GLP-1 was released following a fiber meal. However, while the normal-weight group reported increased feelings of satiety following the release of GLP-1 and additional fiber, the overweight and obese group displayed a disturbed perception of satiety, with no increased level of satiety following the release of GLP-1 (Adam & Westerterp-Plantenga 2005). The strongest evidence to support fiber's role in weight management comes from multiple studies that have found a clear association between a fiber-rich diet and a lower body mass index (IFIC 2008).
Fiber and Diabetes
Many observational studies have found an association between high-fiber food consumption and reduced risk of type 2 diabetes. Dietary fiber is thought to play a vital role in reducing overall risk by helping to normalize postprandial glucose response and decrease insulin concentration and overall insulin requirements. Higher intakes (13–16 g/dy in one study and as much as 30–50 g/dy in a different study) from whole-food sources—in particular, cereal fiber—have been consistently associated with lower risk of type 2 diabetes and improved insulin sensitivity (IFIC 2008; ADA 2008).
The exact mechanism of fiber's protection against diabetes is not fully elucidated at this time. However, the protective effect may be related to fiber's ability to reduce the glycemic response by delaying gastric emptying and glucose absorption (IFIC 2008). Other high-fiber diet components that may contribute to reduced risk include higher intakes of magnesium, folate, vitamin B6 and vitamin E. While fiber is efficacious in the fight against diabetes, fiber recommendations for people with diabetes are the same as those for the general population: 25 g/dy for women and 38 g/dy for men (USDA 2010).
Fiber and Cancer Prevention
While much of the research linking dietary fiber to cancer prevention is limited or mixed, fiber-rich foods tend to be concentrated sources of cancer-fighting antioxidants, vitamins and minerals. In addition, dietary fiber improves gut transit time, effectively reducing the length of time toxic substances stay in contact with the bowel, which may protect against colon cancers. Because of these facts, the 2007 World Cancer Research Fund and American Institute for Cancer Research report on "Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective" recommends a fiber intake of at least 25 g/dy to reduce the risk of cancer.
Fiber and Mental Health
Occasionally, health and fitness professionals come across a client whose diet is the picture of health. Anecdotal evidence and self-reports tell us that individuals who eat a nutrient-dense, fiber-rich diet feel more energized and report feeling healthier. Researchers at the University of Cardiff, Wales, found that subjects with higher fiber intakes had less emotional distress, fewer cognitive difficulties and less fatigue than their counterparts who consumed less fiber (Smith et al. 2001). In addition, the study found that by increasing overall fiber intake, participants experienced a 10% increase in energy levels and reduced fatigue within 2 weeks. Researchers suggested that this overall improvement was likely related to improvements in digestive health.
How Much Fiber Is Enough?
The IOM established the AI value for fiber in a 2002 report. The recommendation, set at 14 g of fiber per 1,000 kilocalories (kcal), was derived from data on the relationship between fiber consumption and CHD risk. The IOM also considered evidence that fiber decreases the risk of chronic disease and other health-related conditions. The IOM panel felt there was insufficient evidence to set a Tolerable Upper Intake Level (UL) or even a Recommended Dietary Allowance (RDA); therefore an AI was established (IOM 2002).
Because the IOM fiber recommendation is proportional to overall energy intake, total fiber intake should (theoretically) be highest for populations who consume the most calories, namely young males. Fiber recommendations are lower for women and the elderly because their overall calorie needs are generally less than those of active young males. There is one caveat to using the guideline of 14 g of fiber per 1,000 calories consumed; it is not wholly applicable to young children. For example, an intake of 10 g of fiber for an infant who consumes formula as his sole source of nutrition is unfeasible, as is achieving 19 g per day for a 3-year-old. AI for fiber is not established for children under 12 months of age or for children ages 1–18 years old. The "age + 5" rule is therefore recommended (e.g., a 4-year-old child should consume 9 g of fiber per day) (Williams, Bollella & Wynder 1995).
On food packages, dietary fiber is listed on the Nutrition Facts panel, which recommends 25 g of dietary fiber as the daily intake for a person following a 2,000 kcal diet. Manufacturers are permitted to call a food a "good source of fiber" if it contains 10% of the recommended amount (2.5 g/serving) and an "excellent source of fiber" if it contains 20% of the recommended amount (5 g/serving). Dietary fiber listed on food labels is technically total fiber; it includes both dietary fiber and functional fiber. While information detailing the total fiber in a food is readily available, Americans still fall well below the recommendations.
The Current State of Consumption
The benefits of fiber are many, but the average American does not consume enough of it. Dietary fiber is underconsumed across all segments of the population, with usual intakes averaging only 15 g/dy (ADA 2008). According to data compiled during the National Health and Nutrition Examination Survey (NHANES) in 2003–2004, average intakes of dietary fiber from food were 15–18.3 g/dy for adult men and 12.3–13.8 g/dy for adult women (CDC 2003–2004). These averages may be slightly low, owing to the fact that some functional and novel fibers (inulin, oligofructose) were not included in the USDA nutrient database used by the researchers. However, while these estimates may indeed be on the low side, it is still unlikely that the majority of Americans meet the recommended intake.
Fiber and Physical Activity
Adding fiber to the diet of an athlete is not always an easy task. Athletes who are focused on their physique and appearance may already be consuming adequate amounts of fruits, vegetables and filling whole grains. On the other hand, in an attempt to lower their overall caloric intake, athletes who are more concerned with "bulking up," building muscle or simply meeting energy needs may not be as concerned with what they are eating. The "Position Statement of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance" does not suggest that an athlete's fiber intake should differ from the AI. This leads one to believe that athletes, like the rest of the population, should follow the Dietary Guidelines for Americans, eating a wide variety of fruits, vegetables and whole grains and aiming to consume the AI for fiber (ADA, DC & ACSM 2009).
Where does fiber fit into an athlete's diet? Supplemental fiber should be avoided during activity and is not recommended for strenuous activity. According to the joint position statement, the pre-exercise meal should "provide sufficient fluid to maintain hydration, be relatively low in fat and fiber to facilitate gastric emptying and minimize gastrointestinal distress, be relatively high in carbohydrate to maximize maintenance of blood glucose, be moderate in protein, be composed of familiar foods, and be well tolerated by the athlete" (ADA, DC & ACSM 2009).
Athletes who want to increase fiber intake may do so on a rest day or after workouts. If an athlete aspires to include more fiber, she should do so gradually, while keeping fluid intake high. Since a diet high in fiber naturally leads to regularity, and the moisture content of stool is 70%–75% (ADA 2008), an athlete who consumes adequate fiber should pay careful attention to overall fluid intake in order to prevent dehydration and/or constipation.
While some athletes complain that increasing their fiber intake wreaks havoc on their workouts, others report no relief from constipation, regardless of diet or physical activity. Constipation in athletes is not an anomaly; it is one of the most common gastrointestinal complaints in the United States, with over 4 million Americans suffering from it each year (accounting for 2.5 million physician visits) (NIDDK 2007). While healthy eating habits, adequate fluid intake and regular exercise are the best ways to prevent occasional constipation, some clients may need something more. For those who do not consume enough fiber, or for those who consume adequate amounts yet still have difficulty with regularity, a remedy such as Senokot® tablets, which contain a natural vegetable laxative ingredient, can help. Like most over-the-counter products, Senokot tablets can be used to address occasional constipation when additional fiber and diet changes don't work. If athletes complain of chronic constipation (or the opposite, for that matter), a closer look into their diet or overall health may be warranted. These individuals may benefit from a visit to a dietitian or a physician.
Take-Home Messages About Digestive Health
Adding fiber to the diet isn't as hard as it may seem. It just takes a little know-how and some motivation. It can be as easy as choosing nutrient-rich foods—such as legumes, whole grains and fresh and frozen fruits and vegetables—while avoiding nutrient-poor choices, such as soft drinks and processed foods. Luckily, for clients and fitness professionals alike, fiber can be a delicious choice, whether it comes in the form of dried fruit, carrot sticks, whole-grain pita or even popcorn. Review the sidebars for additional and easy ways to add more fiber to everyday dining.
Not all foods are equal when it comes to dietary fiber.
Consider this actual menu showing what an athlete was eating when he first consulted me for dietary advice and how, over time, he added in more and more fiber, finally arriving at an adequate intake for the day. Keep in mind that many food sources on the market contain fiber. To find your favorites, keep an open mind and be willing to try new grains, cereals, legumes, fruits and vegetables. To determine how much fiber you are consuming, consult www.globalrph.com/fiber_content.htm.
Types of Dietary Fiber
Source: USDA 2010a.
High-Quality, High-Fiber Choices
Good high-fiber choices are listed below.
Sources: Palmer 2008; USDA 2010b.
Fiber: Can You Get Too Much?
While the benefits of fiber are great, excessive intake can result in negative side effects, and although overindulgence in fiber is unlikely, it does occur. Side effects include reduced absorption of vitamins and minerals, proteins and energy; diarrhea (and increased time in the Porta Potty line before a race); intestinal obstructions (very rare); and increased amounts of cramping, bloating and flatulence. Athletes with high fiber intake should be aware of the potential need for micronutrient supplementation and nutrient-dense food choices. Various micronutrients, such as zinc and iron, can be lost in the feces.
Adam, T.C.M., & Westerterp-Plantenga, M.S. 2005. Glucagon-like peptide-1 release and satiety after a nutrient challenge in normal-weight and obese subjects. British Journal of Nutrition, 93, 845–51.
ADA (American Dietetic Association). 2008. Position of the American Dietetic Association: Health implications of dietary fiber. Journal of the American Dietetic Association, 108, 1716–31.
ADA, DC & ACSM (American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine). 2009. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. Journal of the American Dietetic Association, 109, 509–527.
CDC (Centers for Disease Control and Prevention). 2003–2004. National Center for Health Statistics (NCHS). National Health and Nutrition Examination Survey Data. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 2003–2004.
IOM (Institute of Medicine). 2002. Food and Nutrition Board. Dietary Reference Intakes: Energy, Carbohydrates, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids. Washington, DC: National Academies Press.
IFIC (International Food Information Council). 2008. Fiber Fact Sheet. www.foodinsight.org/Content/6/FINAL%20IFICFndtnFiberFactSheet%2011%2021%2008.pdf; retrieved Nov. 10, 2010.
Heaton, K.W. 1973. Food fibre as an obstacle to energy intake. The Lancet, 2 (7843), 1418–21.
Jensen, M.K., et al. 2004. Intakes of whole grains, bran, and germ and the risk of coronary heart disease in men. American Journal of Clinical Nutrition, 80 (6), 1492–99.
Jones, J.R., Lineback, D.M., & Levine, M.J. 2006. Dietary Reference Intakes: Implications for fiber labeling and consumption: A summary of the International Life Sciences Institute North America Fiber Workshop, June 1–2, 2004, Washington, DC. Nutrition Reviews, 64 (1), 31–38.
NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases). 2007. National Digestive Diseases Information Clearing House: Constipation. http://digestive.niddk.nih.gov/ddiseases/pubs/constipation/Constipation.pdf; retrieved Nov. 20, 2010.
Palmer S. 2008. The top fiber-rich foods list. Today's Dietitian, 10 (7), 28.
Smith, A., et al. 2001. High fiber cereals reduce fatigue. Appetite 37, 1–3. USDA (United States Department of Agriculture). 2010a.
Report of the DGAC on the Dietary Guidelines for Americans, 2010. www.cnpp.usda.gov/Publications/DietaryGuidelines/2010/DGAC/Report/D-5-Carbohydrates.pdf; retrieved Nov. 10, 2010.
USDA. 2010b. USDA Nutrient Analysis Library. National nutrient database for standard reference, release 23: Fiber. www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/SR23/nutrlist/sr23a291.pdf; retrieved Nov. 10, 2010.
Williams, C.L., Bollella, M., & Wynder, E.L. 1995. A new recommendation for dietary fiber intake in childhood. Pediatrics, 96 (5), 985–88.
World Cancer Research Fund and American Institute for Cancer Research. 2007. Food, nutrition, physical activity, and the prevention of cancer: A global perspective. www.dietandcancerreport.org.
IDEA Fitness Journal, Volume 8, Number 3
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