In the 1970s, a paradigm shift occurred in medical and nutritional research circles that profoundly impacted their respective scientific communities to the present day. No longer was dietary fiber considered nonnutritive and unimportant; instead, fiber played a significant physiological role, as simply lowering fat intake was insufficient for a well-rounded, healthy diet.
Fast forward to today and we know dietary fiber is the fibrous, coarse-textured, or gummy nondigestible portion of our food that affects the comfort of our digestive system and the makeup of our bowel output. We also know dietary fiber can help modulate the level of glucose in the blood and slow down the production of insulin for individuals with diabetes.
Common sense and decades of research tell us dietary fiber is a necessary component to a balanced, nutritious diet. But from a scientific standpoint, it’s important to understand the research methods and physiological effects of insoluble and soluble dietary fiber.
6 Sources of Insoluble Dietary Fiber
Insoluble dietary fiber (IDF) makes up about 66% of the fiber in foods. It’s the “roughage” found in whole cereal grains and whole grain products, fruits, nuts, and vegetables—specifically stalks, seeds, and skins—that humans can’t digest. The most obvious benefit of increased intake of IDF (particularly wheat bran) is the feeling of wellbeing resulting from increased laxation.
IDF typically has a high water binding capacity which results in the formation of softer stools that pass through the system faster. The softer stools reduce the pressure necessary for elimination, thus, less constipation, and lower incidence of maladies such as diverticular disease, varicose veins, hemorrhoids, hernias, appendicitis, and phlebitis. The more rapid movement of the fecal bulk through the colon (reduced transit time) results in increased “scrubbing action” within the colon reducing the likelihood of stagnation and temporary sepsis setting in at any particular spot.
Cellulose, the primary component of IDF, is insoluble in cold and hot water, dilute acid, and dilute alkali. It is the most abundant carbohydrate structural material in nature, making up the cell walls of most plant materials, typically one half of the plant’s mass. This linear polymer of beta-D-glucose molecules linked in the 1,4-positions is undigestible in the human system because humans do not have enzymes that hydrolyze beta linkages.
In contrast, starch, the major source of energy for humans, is also a pure glucose polymer with mostly 1,4-linkages (and occasional 1,6-linkages) and has alpha linkages. Starch is very digestible and its particular linkages result in the molecule of helical conformation as opposed to being flat and linear. This makes starch water soluble and interactive with other molecules such as free fatty acids.
Those unfamiliar with fiber might incorrectly assume that hemicellulose is similar to cellulose. The term hemicellulose appears to be a historical usage relating to solubility properties. Both cellulose and hemicellulose are insoluble in cold and hot water and dilute acid, therefore, hemicellulose was probably thought to be related to cellulose. Hemicellulose is distinguished from cellulose by its solubility in dilute alkali.
The name hemicellulose applies to a variety of heterosaccharridic polymers which tend to be small (50-200 saccharide units) with branching present usually consisting of more than two sugars. Predominant monomers are xylose, arabinose, mannose, glucose, and galactose. Arabinoxylans found in cereal grains are an excellent example of hemicellulose. Part of hemicellulose is quantitated as IDF; part of it as SDF.
3. Resistant Starch
Resistant starch, when eaten by humans passes undigested through the small intestine and into the large bowel where it is fermented or excreted. This fermentation energy source may be significant for maintaining colonic health. The relative amount of undigested starch can vary from food to food and person to person, however, the “resistant starch” quantitated with dietary fiber using the official methods for fiber is resistant in all cases.
Lignin results when polyfunctional phenols are polymerized with ether and ester linkages during plant growth, intimately forming with and infiltrating the cellulose of cell walls, resulting in a hard, rigid matrix of tremendous strength. At sufficient lignin concentration, plant tissues become “lignified” or “woody” to the point of being inedible (wood, a highly lignified tissue has a strength greater than steel on a weight basis).
Lignin is an important component of dietary fiber, making the fiber hydrophobic, resistant to enzymatic breakdown in the small intestine and bacterial breakdown in the large intestine. It is almost completely recovered in the feces. Lignified tissue in food offers unique textural properties, although they are not always considered desirable.
This waxy hydrophobic layer made up of highly hydrophobic long chain hydroxy aliphatic fatty acids polymerized by ester linkages is resistant to digestion and can be recovered in fecal material. A number of the fatty acids have trifunctionality, resulting in polymer crosslinking and branching. Ester linkages also occur between the cutin and other cell wall polymers such as hemicelluloses.1
Kolattukudy indicates the scant evidence available allows only conjecture regarding the structure of suberin as a highly branched and crosslinked (by ester linkages) combination of polyfunctional phenolics and polyfunctional hydroxyacids and dicarboxylic acids.2
Like cutin, it is chemically linked to cell wall carbohydrate polymers especially through its lignin forming components (p-coumaric and ferulic acids). As evidence of the intimate interaction between suberin and other dietary fiber components, only suberin-enriched preparations, not pure ones, have been obtained in the laboratory.
3 Sources of Soluble Dietary Fiber
Soluble dietary fiber (SDF), on the other hand, is soft, gummy, and highly water-absorbent. The most common dietary sources of it include beans, peas, barley, oats, and avocados. While not as effective as IDF in promoting laxation, SDF does exert a positive effect through a different mechanism.
SDF is fermented in the colon, a substantial quantity of bacterial mass accumulates which is soft, bulky, and water retaining, which helps reduce intestinal transit time and produces an environment healthier for colonic structure. SDF fermentation also generates significant quantities of gases which exercise the colon during transit.
Decreased risk of coronary heart disease is correlated with increases in consumption of dietary fiber, typically SDF. Increased risk of coronary heart disease is also correlated with a significant number of other risk factors which are reduced by dietary fiber, such as diabetes, high serum cholesterol, high levels of low density lipoprotein (LDL) associated cholesterol, low and low levels of high density lipoprotein (HDL) associated cholesterol, obesity, and possibly hyperinsulinemia.
Increased intake of SDFs such as guar gum, locust bean gum, oat gum and pectin can significantly decrease total and LDL serum cholesterol while maintaining or increasing HDL cholesterol levels. SDFs increase fermentation in the large intestine, increasing the production of short-chain fatty acids, helping to remove bile salts from the system and depressing cholesterol production.
The most widespread SDFs in foods are pectins—or, polygalacturonic acids—found in fruits, vegetables, legumes, and roots (i.e., sugar beets and potatoes) as storage polysaccharides. Commercial pectin is isolated from either apple pumice or from citrus peels—with levels reaching up to 30% of daily recommended value on a dry weight basis. The functional groups of the polymer are present either as free carboxylic acids, methyl esters, or carboxylate anions (i.e., sodium, potassium, or calcium salts).
The degree of esterification is significant in defining the properties of pectins. But, there are still some differences of opinion regarding the molecular makeup of pectin.
DeVries et. al. propose that pectin’s unique properties result from a backbone of long sections of galacturonic acids, interrupted with a section of rhamnose having side chains of arabinose, galactose, glucose, and xylose. These chain interruptions result in a soft, water-soluble molecule rather than a linear polymer with high intermolecular hydrogen bonding, which has properties similar to cellulose.3, 4
beta-Glucans are undigestible mixed beta linkage—beta 1,3 interspersed with beta 1,4—glucose polymers less well known than the glucose polymers starch and cellulose. Adding the alternate positional linkages gives water soluble (mostly) food gums, which upon hydration with water give high viscosity solutions with relatively little shear and tensile resistance compared to cellulose. Cellulose is essentially water-insoluble and has tremendous shear and tensile resistance; in fact, it’s strong, rugged, and durable enough to use for clothing and shelter.
In the large intestine, beta-glucans undergo extensive fermentation whereas cellulose passes through essentially unchanged. Grains are the primary source of beta-glucans:
|Table 1: Percentage of beta-Glucans in Common Grains|
|Barley||2-9% though typically 3-6%|
Ripsin et. al. showed that oat products consistently display hypocholesterolemic effects in controlled human studies.6 The relationship between oat consumption and heart health effects is strong enough that the U.S. Food and Drug Administration (USFDA) allows a cardiovascular health claim on the label of oat-based foods. Beta-glucans may be playing a significant role in the observed effects and are proposed as a marker entity for oats.
3. Galactomannan Gums
Soluble galactomannan gums are part of the hemicellulose fraction of the food, originating in leguminous plants such as guar and locust beans (also known as carob). These gums consist of a mannose polymer backbone to which are attached galactose side chains.
Insoluble & Soluble Dietary Fiber on Nutrition Labels
Despite the clear benefits of knowing which type of dietary fiber one is consuming, the USDA and USFDA developed regulations for labeling only “dietary fiber,” while IDF and SDF disclosure remains optional. Moreover, despite decades of research data, the Nutrition Labeling and Education Act (NLEA) regulations remain cautious about health claims regarding dietary fiber consumption.
There are three specific claims that can be made relating improved health status with increased consumption of high fiber foods. All claims must use the terms “may” or “might” reduce risk, apply to foods containing grain products, fruits, and vegetables (which contain fiber) and must promote low fat/high fiber food diets. The claims are:
- Fiber-containing grain products, fruits, and vegetables and cancer (the food must qualify as a “good source” (2.5 g/serving) of fiber without fortification)
- Fruits, vegetables, and grain products that contain fiber, particularly SDF, and risk of coronary heart disease (the food must contain at least 0.6 g/serving of SDF without fortification)
- Fruits and vegetables and cancer—emphasis is on antioxidant vitamins, rather than fiber, but most fruits and vegetables contain significant fiber (the food must be a “good source” of vitamin A, vitamin C, or dietary fiber)
What is the future of dietary fiber research and method development? Will scientists be looking for further physiological fractionation of total dietary fiber beyond the physiological effects associated with soluble versus insoluble fiber? Will additional physiological effects be discovered?
Physicochemical characteristics, such as molecular structure, types of bonding, chain length, ion exchange capacity, water holding capacity, and fermentability critically affect the physiological behaviors of dietary fiber. In future research efforts, it will be important to characterize these physicochemical/physiological relationships. This will help the food industry and the public choose dietary fiber sources of maximum potential health benefit for food formulation and consumption.
- Deas, A.H. and Holloway, P.J. 1977. The Intermolecular structure of some plant cutins in Lipids and Lipid Polymers in Higher Plants. ed. M. Tevini, H.K. Lichtenhaler, 293-300.
- Kolattukudy, P.E. 1981. Structure, biosynthesis, and biodegradation of cutin and suberin. Ann. Rev. Plant Physiol. 32:539-67.
- DeVries, J.A., Rombouts, F.M., Voragen, A.G.J., and Pilnik, W., 1982, Enzymatic degradation of apple pectins, Carbohydr. Polym., 2, 25.
- DeVries, J.A., den Vijl, C.H., Voragen, A.G.J., Rombouts, F.M., and Pilnik, W., 1983, Structural features of the neutral sugar side chains of apple pectic substances, Carbohydr. Polym., 3, 193.
- Stuart, et. al. 1987.
- Ripsin, C.M., Keenan, J.M., Jacobs, D.R., Elmer, P.J., Welch, R.R., Van Horn, L., Liu, K., Turnbull, W.H., Thye, F.W., Kestin, M., Hegstad, M., Davidson, D.M., Davidson, M.H., Dugan, L.D., Demark-Wahnefried, W., and Beling, S., 1992, Oat Products and Lipid Lowering-A Metaanalysis, Journal of the American Medical Association, 267:3317-3325.