With consumers still vague about its benefits, scientists and regulatory bodies wrestle over its meaning in debates marked by acrimony and politics. For food product developers, this means confusion. That is unfortunate, because much is at stake, both in terms of public health and the food industry’s ability to provide dietary fiber to consumers in safe, practical and cost-effective ways.
What follows is a summary of dietary fiber’s definition and recognition as an essential food ingredient today.
In the EndHumans owe a good part of their health to large colonies of digestive bacteria that cheerfully and symbiotically inhabit the lower intestines. This bacteria is known as “probiotic.” “Pro” is the positive prefix that connotes how these prolific bacterial colonies produce nutrients essential to continued health. Some microbes are more probiotic than others, so they are added to yogurts and supplements.
Probiotic bacteria are tasked with the final stages of gastrointestinal (GI) digestion, processing the most-difficult-to-digest food components into molecules that provide numerous benefits. These benefits include vitamins (such as vitamin K) and short-chain fatty acids (SCFAs) such as butyrate. A significant body of research indicates that butyrate promotes tissue repair, improves serum lipid profiles and possibly protects against certain cancers. It is likely that many additional nutraceutical and nutritional benefits of probiotic activity will come to light in time.
To be healthy, active and productive, probiotic bacteria need food, and dietary fiber fits that need. This bacteria food must be able to negotiate the acid-hydrolytic, absorptive and enzymatic hazards of the GI system. Along the way, dietary fiber contributes additional nutritional and physical benefits in the form of laxation and the mediation of GI absorption of both nutrients and toxins. This probiotic food is known as “prebiotic.”
Virtually all dietary fiber derives from the cell walls of plant materials (chitin, from shellfish, and fungi are exceptions), especially seeds. This provides a compelling argument for consuming brans and whole seeds (grains).
All dietary fibers are prebiotic, but some fibers are more prebiotic than others. Soluble dietary fibers, for example, tend to be highly prebiotic. So, what does all this have to do with food ingredients and labeling?
A Glossary of Dietary FiberLike “whole grains,” the definition of dietary fiber is a contentious work-in-progress involving competing interest groups (see sidebar). It is important, because the U.S. National Academy of Science (NAS) has established essential nutrient Daily Reference Intakes (DRIs) for “total fiber” ranging between 21g/day for women (51 years and up) and 38g/day for men aged 19 to 50. Mostly, American consumers ingest only a small portion of the DRI for fiber, so the pressure is on food companies to find innovative ways to increase dietary fiber consumption.
If only all could agree on what is meant by “total dietary fiber.” Part of the confusion stems from the divergent needs of food scientists, food analysts and nutritionists. Like the three blind men describing an elephant, one group emphasizes the physical and chemical behaviors of dietary fiber in foods; the second is fixated on its measurement; and the third sees only nutritional benefits. Consequently, current definitions are replete with exceptions, omissions and contradictions.
n Carbohydrates: Food carbohydrates are composed of five- and/or six-carbon sugars and combinations thereof. Single sugar molecules (saccharides) are called “simple carbohydrates” and are digestible. Digestibility depends upon whether the human system can cleave the bonds in linked saccharides or complex polysaccharides to release sugar molecules to be absorbed into the bloodstream. Most disaccharides are digestible. Many oligosaccharides (3-9 saccharides in length) are not. Polysaccharides may or may not be, depending upon the type of linkage (α or β) between the sugars and whether those linkages are susceptible to enzyme activity. If not susceptible to enzyme activity, it could be argued that they represent “dietary fiber.” Not all agree.
n Digestible Carbohydrates: Also referred to as “available carbohydrates,” this refers to those carbohydrates that are fully digested in the upper regions of the gastrointestinal tract into sugars—ergo energy. Digestible carbohydrates contribute their full energy value of 4Kcal/g. We call the most recognizable complex digestible carbohydrates “starches.”
n Indigestible Carbohydrates: This gets closer to a definition for “fiber.” These carbohydrates pass into the colon largely intact. However, this term is not quite correct, as many of these “indigestible carbohydrates” are often partially digested in the upper colon and further digested in the lower colon. Determination of their caloric value can be problematic.
This is because, in addition to partially releasing sugar molecules into the upper intestine, colonic bacteria further metabolize “indigestible” carbohydrates into short-chain fatty acids (SCFAs), which are absorbed into the bloodstream and contribute caloric value. Thus, it is not unusual to see caloric designations for these ingredients that range from 0 to 2.5Kcal/g.
n Crude Fiber: Early agricultural scientists crudely defined fiber as indigestible roughage. It was measured as the residue from sequentially boiling plant matter in acid and alkali. Naturally, not much survives this abuse, so reported crude fiber values are very low. Crude fiber still occasionally appears in ingredient specifications and USDA Handbook composition charts, but it is pretty much obsolete for everything but livestock feeds.
Analytical techniques improved to distinguish between water-“soluble” and “insoluble” fiber—terms common today because they help elucidate a fiber’s physical, chemical and nutritional properties.
n Insoluble Dietary Fiber: The majority of dietary fiber in foods is insoluble—primarily cellulose, hemicellulose and lignin (although lignin is not, technically, a carbohydrate).
Insoluble dietary fiber is the residue left from subjecting carbohydrate materials to amylase digestion followed by a water-wash. Insoluble dietary fiber comprises the large majority (typically 75%) of the dietary fiber in most foods. It offers well-defined nutritional benefits and tends to have a very low water absorption capacity, unless further modified. It helps regulate transit times of foods through the upper intestine. Major food ingredient sources of insoluble dietary fiber include cellulose from wood, wheat stalk, oat hulls and corn bran.
In order to increase dietary fiber content of foods, an insoluble fiber is used, as insoluble fiber binds relatively little water and thus interferes less with the available water in food formulations. Insoluble fibers also tend to be less expensive. However, insoluble fibers can be modified to increase their water absorption capacity—either by physical treatment to increase capillary and colloidal activity or by chemical treatment to increase water solubility (e.g., cellulose gum).
n Soluble Dietary Fiber: The water-soluble fractions, washed from the α-amylase-treated insoluble carbohydrate fractions, contain soluble dietary fibers. Most (not all) are precipitated from water with ethanol. One very major source of complication in dietary fiber analysis and definition is those remaining carbohydrate components not precipitated in ethanol, which require additional analysis.
Soluble dietary fibers are fibers that readily solubilize or form hydrocolloids in water. In sum, they are highly interactive with water, which provides interesting nutritional and nutraceutical benefits. Soluble dietary fibers include vegetable gums, β-glucans, inulin, oligosaccharides, pectins, polydextrose, resistant starches and dextrins, and chemically modified cellulose fibers.
Soluble dietary fibers, unless specially treated, tend to be major water hogs. Very low levels of use significantly affect available water, texture, thickness, gelling and flavor-release properties in foods. This can be good. Exceptions to this include resistant starches, resistant dextrins and hydrolyzed gums that have been treated to minimize their water absorption capacities.
n Total Dietary Fiber (TDF): This is a useful but increasingly complicated term. It refers to the analytical sum of soluble and insoluble dietary fiber. This is the value that appears on food labels. However, to be accurate, scientists must be able to measure all soluble fiber components: insoluble fiber + soluble fiber = total dietary fiber.
The problem with many fiber analyses is that they miss certain soluble fiber components. Consequently, very specific analyses are required to detect and quantify oligosaccharides and resistant starches. If a product contains sugar alcohols, they, too, behave as dietary fibers. If it is suspected that any of these components may be naturally or deliberately present in food or food ingredients, they must be specified for analysis. Otherwise, the risk is a significant understating of the true dietary fiber content of the product.
The Caloric ConundrumFinally, all dietary fiber contributes some dietary calories through partial upper-GI digestion or as short-chain fatty acids (SCFAs). This is confusing when calculating nutrition labels. Manufacturers can exclude insoluble fiber from the caloric count, but the FDA specifies that soluble fiber be assessed 4Kcal/g, unless otherwise documented. Sugar alcohols come with their own recommended caloric value, but what about resistant starches? Some resistant starches’ contents vary according to processing conditions. A comprehensive dietary fiber definition will likely reference “digestibility,” but what about caloric impact? Scientists still debate whether that counts or not.
Storm Clouds on the HorizonPROGRESS IN DIETARY FIBER DEFINITION TRENDS MOVES BY LURCHES, RATHER THAN LEAPS. NO DEFINITION THUS FAR APPEARS TO BE WHOLLY SATISFACTORY. DEBATE RAGES ON HOW TO ADDRESS NON-PLANT FIBERS (CHITIN), PURIFIED VS. WHOLE FOOD DIETARY FIBERS, SUGAR ALCOHOLS, RESISTANT STARCHES AND MORE. HOWEVER, ONE RECENT DEVELOPMENT, IF NOT COUNTERED, POSES A REAL THREAT TO THE FOOD AND NUTRITIONAL PRODUCTS INDUSTRIES.
HERE IS AN EARLIER DEFINITION AGREED UPON BY THE AMERICAN ASSOCIATION OF CEREAL CHEMISTS (NOW AACC INTERNATIONAL) IN 2001:
“DIETARY FIBER IS THE EDIBLE PART OF PLANTS OR ANALOGOUS CARBOHYDRATES THAT ARE RESISTANT TO DIGESTION AND ABSORPTION IN THE HUMAN SMALL INTESTINE WITH COMPLETE OR PARTIAL FERMENTATION IN THE LARGE INTESTINE. DIETARY FIBER INCLUDES POLYSACCHARIDES, OLIGOSACCHARIDES, LIGNIN AND ASSOCIATED PLANT SUBSTANCES. DIETARY FIBERS PROMOTE BENEFICIAL PHYSIOLOGICAL EFFECTS, INCLUDE LAXATION, AND/OR BLOOD CHOLESTEROL ATTENUATION, AND/OR BLOOD GLUCOSE ATTENUATION.”
IS THIS A FOOD, ANALYTICAL OR NUTRITION-FOCUSED DEFINITION? NOTE THE PASSING REFERENCE TO “ASSOCIATED PLANT SUBSTANCES” AND THE OMISSION OF NUTRITIONAL BENEFITS ASSOCIATED WITH SCFAS.
A YEAR LATER, THE U.S. FOOD AND NUTRITION BOARD OF THE NATIONAL ACADEMY OF SCIENCES (FNB/NAS) PUBLISHED ANOTHER DEFINITION:
“DIETARY FIBER CONSISTS OF NON-DIGESTIBLE CARBOHYDRATES AND LIGNIN THAT ARE INTRINSIC AND INTACT IN PLANTS. FUNCTIONAL FIBER CONSISTS OF ISOLATED, NON-DIGESTIBLE CARBOHYDRATES AND LIGNIN THAT HAVE BENEFICIAL PHYSIOLOGICAL EFFECTS IN HUMANS. TOTAL FIBER IS THE SUM OF DIETARY FIBER AND FUNCTIONAL FIBER.”
EXCEPT, AS NOTED, VIRTUALLY ALL DIETARY FIBERS ARE AT LEAST PARTIALLY DIGESTIBLE, WITH IMPORTANT IMPLICATIONS FOR BOTH HUMAN HEALTH AND CALORIC VALUE. MOREOVER, THIS DEFINITION HELPS NEITHER FOOD ANALYSTS NOR PRODUCT DEVELOPERS VERY MUCH IN QUANTIFYING THE DIETARY FIBER CONTENT OF FOODS. IT DOES, HOWEVER, MAKE AN IMPORTANT DISTINCTION BETWEEN “INTRINSIC” FIBERS AND “ISOLATED” FIBERS. WITH THIS IN MIND, NOTE THE FOLLOWING:
IN THE MAY-JUNE ISSUE OF CEREAL FOODS WORLD, A PUBLICATION OF THE AACC INTERNATIONAL, NOTED CEREAL SCIENTIST DENNIS GORDON ALERTED THE INDUSTRY TO A RECENT, UNILATERAL INITIATIVE BY THE UNITED NATIONS FAO/WHO TO FURTHER RESTRICT THE DEFINITION OF DIETARY FIBER TO “INTRINSIC PLANT CELL WALL POLYSACCHARIDES.”
ACCORDING TO GORDON, THIS DEFINITION WOULD REMOVE ANY PURIFIED OR MODIFIED DIETARY FIBER INGREDIENTS FROM THE “ACCEPTED” LIST OF RECOGNIZED DIETARY FIBER INGREDIENTS IN FAVOR OF “WHOLE FOOD” INGREDIENTS SUCH AS GRAINS AND VEGETABLES.
SHOULD THIS PROPOSED, U.N.-ENDORSED DEFINITION BE ALLOWED TO STAND AND AFFECT INTERNATIONAL CODEX STANDARDS, THE INTERNATIONAL FOOD INDUSTRY WOULD FACE AN ENORMOUS CHALLENGE. COUNTRIES THAT FEEL OBLIGATED TO HEED THIS STANDARD FOR POLITICAL REASONS WOULD CLOSE THE DOOR ON MANY FIBER-ENHANCED FOODS. GIVEN THE CURRENT CHALLENGES FACED BY COMPANIES IN DEVELOPING NEW, DIETARY FIBER-ENHANCED PRODUCTS TO ADDRESS A RECOGNIZED NUTRIENT DEFICIENCY, THE FAO/WHO PROPOSAL HARDLY BODES WELL FOR THE HEALTH OF EITHER CONSUMERS OR THE FOOD INDUSTRY. ON THIS ISSUE, THE FOOD INGREDIENT INDUSTRY NEEDS TO BE HEARD.