Dietary Fiber: A Healthy Discussion - Part One

Health claims
Defining moments
Insolubles categorized
Soluble sources

Dietary fiber constitutes an important part of a healthful diet. While well-known to health professionals, this is perhaps not yet sufficiently apparent to many consumers - despite the craze of several years ago, during which fiber from oat bran could be found in everything from granola bars to beer.

How much fiber is needed? Generally, nutritionists recommend 20 to 35 grams per day, or 10 to 13 grams per 1,000 kilocalories. The Nutrition Facts panel provides a good reference, stating as a goal 25 grams of dietary fiber for a 2,000 kilocalorie per day diet, or 30 grams for a 2,500 kilocalorie per day diet. According to the National Center for Health Statistics, Hyattsville, MD, U.S. consumers average only 14 to 15 grams of fiber intake per day, far below recommended levels. This is due in part to the fact that many fruits, vegetables, and grains considered by consumers to be high in fiber actually contain low to moderate levels of dietary fiber.

This deficit in fiber intake may prove difficult to erase - many consumers were turned off during the oat-bran craze by attempts to load a poorly functional ingredient into a wide range of products for no other reason than to provide a fiber source. In order to bulk up fiber intake, the consumer will need to better understand the benefits of soluble versus insoluble fiber, learn where to find these fibers, and discover how to incorporate various types of fiber into their diets. The scientific community is still in the learning process as well, discovering the active components' effects on disease, how these components must be delivered, and what interactions to expect.

FDA recognizes the importance of dietary fiber in several ways. First, fiber occupies a prominent position on the Nutrition Facts panel on food labels. Second, FDA has approved a number of claims relating fiber intake to lowered risk of heart disease and cancer. And third, as evidenced by discussions at the September 1998 American Association of Cereal Chemists (AACC) annual meeting in Minneapolis, FDA is willing to work with the food industry to find the best working definition of dietary fiber.

The Nutrition Labeling and Education Act (NLEA) defines three main health claims for dietary fiber:
Diets low in fat and rich in fiber-containing grain products, fruits, and vegetables may reduce the risk of some types of cancer.,
2) Diets low in saturated fat and cholesterol and rich in fruits, vegetables, and grain products that contain fiber, particularly soluble fiber, may reduce the risk of coronary heart disease.
3) Diets low in fat and rich in fruits and vegetables, which are low-fat foods and may contain fiber or vitamin A (as beta-carotene) and vitamin C, may reduce the risk of some cancers.

In addition, in March 1995, the Quaker Oats Company, Chicago, petitioned FDA for the right to make fiber health claims based on the fiber found in oat bran and rolled oats. FDA reviewed the available scientific evidence, and concluded that the beta-glucan soluble fiber of whole oats is the primary component responsible for the total and LDL blood-cholesterol-lowering effects in diets that incorporate whole-oat-containing foods at appropriate levels. In January 1997, FDA approved a health claim for oat bran and rolled oats, such as oatmeal and whole oat flour. A food must provide at least 0.75 grams of soluble fiber per serving to make a claim such as "soluble fiber from foods such as oat bran (or oatmeal, rolled oats, or whole oat flour), as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease." The words "diet low in saturated fat and cholesterol" are required in the health claim.

Quaker publicized this claim through its Smart Heart Challenge™, challenging residents of three communities to eat a bowl of oatmeal every day for 30 days, and measuring their cholesterol levels before and after the test period. Of 100 Lafayette, CO residents, 98 lowered their cholesterol an average of 25 mg/dl. Residents of Middletown, CT and www.thirdage.com, an online community of older adults, have also taken the challenge, with the online site garnering over 500 participants.

In response to a petition from Kellogg Company, Battle Creek, MI, FDA has amended this regulation to also include soluble fiber from psyllium seed husk (PSH). In its analysis of the Kellogg petition, FDA stated that "since soluble dietary fibers are a family of very heterogeneous substances that vary greatly in their effect on risk of CHD (coronary heart disease), a case-by-case approach is necessary to evaluate petitions for health claims on fiber." In order to make a claim, foods must provide at least 1.7 grams of soluble fiber from PSH per serving. Kellogg studies were based on an intake of 7 grams of PSH per day. The company has followed up by introducing a line of healthy meals under the Ensemble(tm) name that feature PSH as a source of soluble fiber.

India grows most of the psyllium available on the world market, but the plant is also grown in the Mediterranean, southwestern United States, Mexico and China. Plantago ovara is an annual herb harvested March through April. The husk-free seed is marketed as a health food, but psyllium has historically been targeted for pharmaceutical use. Psyllium muciloid contains a mixture of polysaccharides with varying proportions of xylose, arabinose, rhamnose, galactose, galacturonic acid and other mixed polysaccharides. They form viscous dispersions at up to 1% solids and form a clear gelatinous mass at 2% solids. Psyllium muciloid is well-known as a component of Metamucil® and Fiberall®.

Back to top

The FDA currently has no formal definition of dietary fiber. From a physiological point of view, dietary fiber is the fibrous or gummy non-digestible portion of food that can affect the health of the digestive tract and the output of the bowel. Chemically, fiber is the remnant of plant cell walls, lignin, polysaccharides and similar substances that resist hydrolysis in the human digestive tract. These substances have been further defined as "detectable by defined AOAC methodology." This definition is now being questioned, and FDA is considering a broadening of the definition.

"Dietary fiber" as a classification encompasses a very wide range of fiber sources. Two sub-classes are recognized - soluble and insoluble. These types of fiber are very different in chemistry and in physiological effects. Insoluble dietary fiber, the predominant class, is insoluble in an aqueous enzyme solution. About two-thirds to three-fourths of the dietary fiber in a typical diet is insoluble. Soluble dietary fiber is soluble in an aqueous enzyme system, but can be precipitated with four parts of ethanol to one part of the aqueous mixture. A point of controversy is that a number of resistant polysaccharides, as well as resistant oligosaccharides, such as inulin and polydextrose, are not precipitated in this step.

Back to top

Cellulose is the most abundant source of insoluble dietary fiber, and also is the most abundant carbohydrate in nature, making up a significant part of the mass of a plant. Cellulose, a linear polymer of beta-1,4-linked D-glucose, is indigestible by the human gastrointestinal (GI) tract. (In contrast, the alpha-1,4-linked glucose in starch is highly digestible). Foods high in insoluble fiber include whole grains, cereals, seeds and skins from fruits and vegetables.

One of the most common cellulose products found in foods is microcrystalline cellulose (MCC). The first MCC product was marketed in this country as Avicel™ by FMC Corporation, Philadelphia, but other companies now sell the product in the United States as well. Many forms of MCC are available in a range of particle sizes - MCC alone, or with carboxymethylcellulose (CMC) added, or in combination with other gums such as guar and sodium alginate. MCC and its derivatives have been used extensively to replace high-calorie ingredients, especially fat. For example, Opta Food Ingredients, Inc., Bedford, MA markets Blanver's Best™ MCC products, which are made by co-processing microcrystalline cellulose with 10% to 20% CMC. The products are generally used at levels up to 3%, and thus do contribute to both the insoluble (MCC) and soluble (CMC) fiber content of the food.

Other cellulose derivatives, such as methylcellulose and hydroxypropylmethylcellulose (HPMC), have been used for years as multipurpose thickeners because of their ability to hydrate and build viscosity quickly to form clear gels of varying strengths. Powdered cellulose, at 99+% total dietary fiber (TDF), is available from several sources. Because it is almost pure TDF, powdered cellulose is considered to be non-caloric. One commonly used version, marketed in various fiber lengths by Fiber Sales and Development, a division of Protein Technologies, Inc., St. Louis, MO, is derived from wood pulp. The raw materials are treated to remove lignin and other impurities, then milled to lengths ranging from 22 to 120 microns. Chemically, this product is 90% beta-1,4 glucan plus approximately 10% hemicelluloses.

Other sources besides wood can be processed into food-grade cellulose. Fiber derived from oat hulls is produced by Opta Food Ingredients. This product is predominantly composed of cellulose, approximately 80%. It also consists of about 8% hemicellulose, 4% to 5% moisture and 1%, or less, lignin. This fiber analyzes by the Prosky method as approximately 93% total dietary fiber, on a dry basis.

"As an insoluble fiber, the benefits of oat fiber are primarily on the health of the gastrointestinal system," says Guy Crosby, Opta's vice president, research and development. "This would include improved stool formation, and potentially reduced incidences of colon cancer and ulcerative colitis. However, the FDA has not approved the labeling of foods containing oat fiber as an aid to reducing these diseases, even though studies have shown these benefits of insoluble fiber in the diet."

Oat fiber can be used at up to about 4% to 5% in a formulated food, says Crosby. Higher levels might result in unacceptable textural effects such as unsatisfactory mouthfeel and dryness. Typical applications include baked goods, certain meat products and nutritional beverages.

Another form of cellulose is derived from cottonseed linters, and can therefore be called by the label-friendly term "cottonseed fiber." Because its composition is much the same as wood-derived cellulosic fiber, it shares the same functions and applications, according to Larry McKee, vice president, technical services, International Filler Corporation, North Tonawanda, NY. "Cottonseed fiber is predominately used in formulations for baked goods to enhance fiber content as well as reducing fat and calorie content," he says. "Powdered cellulose is also used as an anticaking agent for shredded and grated cheese.

"The most common length for most applications is 35 microns," says McKee. "In bakery applications, for example, we've found that this particular fiber provides good mouthfeel. We've also used a smaller size range - 25 microns - for applications such as fortified beverages. At this size range the fibers are almost as wide as they are long, so they act more like a particle."

Depending on fiber length, cellulosic fiber can retain 3.5 to 10 times its weight in water. Since longer fiber lengths retain more water, they also increase the viscosity of the food system. "The water uptake is proportional to the length of the capillary in the fiber," McKee explains. "With other particles, such as cocoa, the smaller the particle, the more surface area, and the greater its absorptive capacity. So the smaller the particle, the greater the increase in viscosity. Due to its capillary properties, the opposite holds true for cellulose - the longer the fiber, the greater the water or oil absorption. The fineness of the cellulosic fiber can be matched for the properties required."

Another interesting glucosidic polymer is curdlan, a beta-1,3 glucan linear polymer produced by Alcaligenes faecalis var. myxogenes and available from Takeda USA, Inc., Orangeburg, NY. This was approved as a food additive in 1996 for use as a formulation aid, processing aid, stabilizer, thickener and texturizer. Heating an aqueous suspension of curdlan above 80°C forms a high-set irreversible gel. A low-set gel can be formed by heating to 60°C and then cooling.

Hemicellulose is insoluble in water and dilute acid, but is soluble in dilute alkali solutions. It is a heterogeneous grouping of small saccharine polymers containing xylose, mannose, glucose, and galactose. Many hemicelluloses are a mixture of insoluble and soluble dietary fiber. The pentosans in wheat flour are a well-known example of hemicellulose. Since isolation of hemicelluloses would be an expensive process, no purified products are commercially available. Flour pentosans can be made available for their functional properties through use of enzymes.

Lignin, a complex molecule of polyphenylpropane sub-units, is a class of compounds constituting the "glue" that holds cell walls together. Lignin is not easily hydrated, and is very poorly digested. Technically, lignin is not a fiber, but serves as a "functional fiber" due to its poor digestibility.

Resistant starches (RS) are starches that escape digestion in the small intestine, but are fermented in the large intestine (most starches are easily digested in the digestive tract). Resistant starches are classified as RS1, RS2 and RS3. The RS1 starches are physically entrapped in partially milled seeds, grains or legumes. RS2 starches are intact ungelatinized granules that are not easily gelatinized, and cannot be attacked by amylases until they have been gelatinized. RS3 starches have been retrograded by processing, so that they are not easily gelatinized or attacked by enzymes.

Resistant starches do analyze as total dietary fiber by the AOAC methods. Although technically these starches are not fiber, they act as a "functional" fiber. While they could be viewed as insoluble fiber as analyzed by the methodology, RS have many physiological similarities to soluble fiber. They increase transit time in the GI tract, increase fecal bulk, and are fermented in the colon, releasing short-chain fatty acids.

RS are available in many foods as RS1 or RS2, but are also available commercially. For example, National Starch and Chemical Company, Bridgewater, NJ, has introduced an RS2 starch containing approximately 40% total dietary fiber. CrystaLean®, an ingredient based on RS3 starch, is available from Opta Food Ingredients. As a source of dietary fiber, it can be added to baked and extruded food products at levels of up to 30% without adversely affecting texture. It is based on crystallized, debranched, high-amylose corn starch, and appears on label statements as "maltodextrin."

Back to top

Soluble dietary fibers are, in the traditional sense, food gums. Pectins, the most widely available soluble fibers, are polygalacturonic acids with side chains made up of arabinose, xylose, rhamnose, glucose and galactose. Pectin substances are widely found in fruits, vegetables, legumes and roots. Commercial sources generally include citrus peels, apple pumice, or sugar beet pulp. As a water-binder, pectin has a wide range of functionalities. Commercial varieties are esterified with methyl groups to varying degrees. High-methoxy (HM) varieties are used in fast-setting firm gels, and low-methoxy (LM) varieties are used in slower-setting, softer gels.

Konjac is a very high molecular-weight polysaccharide made of glucose and mannose sugars. Konjac analyzes as approximately 95% to 96% TDF. More than 95% is soluble and a minor amount (less than 2%) of insoluble cellulose occurs in konjac flour. "Konjac flour produces viscosities which are significantly higher than guar gum or pectin," says Crosby. "On a practical level this means that konjac can provide useful texturizing properties at very low concentrations." In addition, he points out, "Konjac flour has been proven in numerous published clinical studies to significantly lower glucose and cholesterol levels. Konjac flour's ability to reduce both glucose and cholesterol levels in the blood has been linked to its high molecular weight." Crosby cites an unpublished 1989 study by David Jenkins at the University of Toronto, where the consumption of konjac-enriched biscuits reduced the glycemic response to the control biscuits by 70% to 75% as one of the most meaningful glucose-level reduction studies. Also among the positive studies is one published in the American Journal of Clinical Nutrition in 1995 by Arvill and Bodin, "Effect of Short-Term Ingestion of Konjac Glucomannan on Serum Cholesterol in Healthy Men." In this experiment, 3.9 grams of konjac eaten daily reduced total cholesterol by 10% and LDL cholesterol by 7.2%.

Other gums that act as fiber include carrageenan, guar, locust bean gum, gum acacia and xanthan gum. The fiber content of these ingredients ranges from approximately 80% to 90%, all or mostly (98%) soluble fiber. Most of these ingredients have limited fiber fortification benefits, since at high levels they can produce unacceptable textures, but manufacturers have found several ways to overcome this.
One ingredient resulting from hydrolyzing guar gum is Benefiber(r), a soluble dietary fiber manufactured by Novartis Nutrition Corporation, Minneapolis. This product consists of 80% minimum soluble dietary fiber as analyzed by the AOAC method, and is very low in viscosity (less than 12 cps in 5% solution). It is soluble in cold water and delivers no color or off-flavor at 5% solids. As with all gums, although it is fiber, it is considered to contain 4 kilocalories per gram. Total carbohydrate content is 92.5% typically, with 8% to 9% reducing sugars. There is evidence that partially hydrolyzed guar gum, like inulin, favors the growth of bifidobacteria in the intestines. A GRAS petition for hydrolyzed guar gum was filed in the United States in 1995, and is under review by the FDA. These gums are currently available on an assumed-GRAS status.

Subjecting gum acacia to a physical process results in a product with 85% soluble fiber, marketed as Fibregum by Colloides Naturels Inc., Brigdewater, NJ. This ingredient can be used in solution at relatively high levels with minimal effect on viscosity. At a 10% solution it maintains a viscosity close to 0 cps, and at 40% the viscosity rises to about 1000 cps. The molecules consist of a galactan main chain carrying heavily branched galactose/arabinose side chains. The side chain terminations may contain some rhamnose and/or glucuronic acid.

Beta-glucans are glucose polymers with varying ratios of beta-1,4 and beta-1,3 linkages. They are found in the largest quantities in barley (3.0% to 6.0%), oats (2.5% to 6.6%), and rye (1.9% to 2.9%). Smaller amounts are found in other grain products. In oat bran, beta-glucans form the largest part of the soluble dietary-fiber content, which represents about half of the TDF. This fact, combined with beta-glucan's effects on human health (supported by a large number of studies), led to the 1997 FDA approval of the oat-bran health claim.

A product that has used beta-glucan to advantage is oatrim, an enzymatically hydrolyzed oat flour (this portion is fully digestible) which contains 5% beta-glucan soluble fiber. Oatrim was developed by George Inglett, a USDA chemist, who developed it primarily as a fat replacer. Oatrim has been commercially developed by Quaker Oats, and marketed by A. E. Staley Manufacturing Company, Decatur, IL, and Rhodia, Cranbury, NJ.

Inglett also developed an insoluble fiber designated as Z-Trim for its zero calories, and has recently announced the development of another fat replacer, Nu-Trim. This ingredient contains 5% beta-glucan, but has properties differing from those of oatrim. Neither Z-Trim nor Nu-Trim have been commercially developed to date.

A combination of steel-cut oat groat, a sterilized high-fiber fraction from oat hulls, and an oat-bran fraction results in an ingredient that contains 3% to 5% beta-glucan, with a TDF content of 16% and a caloric density of 3.2 kilocalories per gram. The Canadian Harvest Dietary Fibers division of DCV Food Ingredients, Cambridge, MN, markets this product as Stabilized Oatex®, and also markets a line of primarily insoluble oat fibers with greater than 90% TDF. This utilizes the USDA-derived alkaline peroxide process for the digestion and bleaching of oat hulls.

Beta-glucans were also isolated from yeast cell walls and marketed in the 1980's by Anheuser-Busch, St. Louis, and more recently by Alpha-Beta Technology, Worcester, MA, but are not commercially available for food products at this time.

Click here to read Dietary Fiber: A Healthy Discussion - Part Two

• Photo: National Starch and Chemical Company

• Photo: A.E. Staley Manufacturing Company

© 1999 by Weeks Publishing Company

Weeks Publishing Company
3400 Dundee Rd. Suite #100
Northbrook, IL 60062
Phone: 847-559-0385
Fax: 847-559-0389