Dietary Fiber: A Healthy Discussion - Part Two

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

Your nutritionist knows
A regulatory dilemma
Soluble fiber
A variety of choices

Why is fiber so important to the diet? A number of population-based studies have indicated that diets low in saturated fat and cholesterol and high in fiber lead to a reduced risk of heart disease, certain cancers, digestive disorders and diabetes. As a general rule, insoluble dietary fibers contribute to bulk and reduced transit times in the GI tract, while soluble dietary fibers are fermented, and contribute to changes in metabolism.

Let's look at the benefits of fiber related to the health claims allowed by the FDA:
• Low-fat, fiber-containing grain products, and cancer: As with all types of epidemiological studies, direct associations between fiber, fat and cancer are hazy, but studies based on consumption and disease in different countries suggest that increased fiber intake may protect against colon cancer, breast cancer, and prostate cancer.
• Low saturated fat, low cholesterol, high soluble fiber, and coronary heart disease: Some soluble fibers, such as beta-glucan or psyllium, can lower blood cholesterol 5% or more - the greater the initial level, the greater the potential decrease. Studies indicate that soluble fibers interfere with the absorption of bile acids. This causes the liver to remove cholesterol from the blood to replace them. A number of studies have shown an inverse relationship between heart attacks and total fiber intake.
• Fruits and vegetables (containing low levels of fat and high levels of fiber, vitamin A and vitamin C) and cancer: This claim is very broad, but contains the association between fat reduction, plus the high dietary-fiber content and antioxidants found in fruits and vegetables, and the inverse relationship with cancer incidence in correlative studies.

Dietary fiber has a number of other physiologically beneficial effects interrelated to reduced risk of cancer and heart disease. Soluble fiber can slow the digestion and absorption of carbohydrates, preventing wide swings in blood sugar level. This aids in the control of diabetes. Certain types of fiber may be helpful in decreasing the risk of type II diabetes. Insoluble fiber, which can increase laxation and add fecal bulk, can also help to lessen the constipation and abdominal pain associated with diverticulosis. Also, the increased bulk from insoluble and soluble fiber contributes to a feeling of fullness, and because fiber is low in calories, this may help contribute to weight loss.

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The definition of dietary fiber was discussed at last September's AACC meeting. Dietary fiber is traditionally defined as polysaccharides and remnants of plant cell walls that are resistant to human alimentary enzymes. Dietary fiber is quantified by AOAC official methods 985.29 and 991.43. But, as noted by Dennis Gordon, chair of the department of cereal science, North Dakota State University, Fargo, certain non-absorbable, non-digestible saccharides are not precipitated in alcohol. Therefore they are not counted as soluble fiber in the current method, even though they contribute to physiological functions and may be beneficial to health. Gordon specifically cited insoluble resistant starches, polydextrose, Fibersol®-2, fructooligosaccharides (FOS), inulin, polyols, and D-tagatose as examples.

This runs counter to thought in Japan, according to K. Ohkuma of Matsutani Chemical Industry Co., Ltd., Hyozo, Japan. There, dietary fiber is defined as "all the indigestible components in foods that cannot be hydrolyzed by human digestive enzymes." The Japanese government, through a program called Foods for Specified Health Use, recognizes the importance of consuming low-molecular-weight soluble dietary fibers. Ohkuma noted that the government adopted an enzyme-HPLC method as part of the Nutritional Improvement Act. This combines the AOAC enzymatic digestion with HPLC procedures to include insoluble dietary fiber, soluble dietary fiber typically precipitated by ethanol, and low-molecular-weight soluble dietary fibers. These lower-molecular-weight oligosaccharides are fermented in the lower intestine, producing short-chain fatty acids. They also provide bulk, laxation and lowered cholesterol and blood glucose levels.

Michael A. McLaughlin, senior research scientist, FDA, acknowledged the dilemma caused by the specificity of the definition and the methods, but asked for help from the industry to better define dietary fiber. He asked that a thorough review be undertaken to determine where to expand the definition - "Should it include chemical and physical properties?" He listed a number of classifications to be considered, including compound classification, source, digestibility, molecular weight, solution viscosity and binding capacity, and asked which of these, at what ranges, might be considered.
McLaughlin presented a number of pros and cons to changing the definition. He noted that currently some amount of regulatory flexibility exists, and asked whether expansion of the definition would have a significant impact on public health. He feels that any adopted definition would need to be flexible, and would need to change along with increasing knowledge. McLaughlin urged comments from industry and asked for help in further understanding the relationship of dietary fiber and complex carbohydrates. He urged industry to continue petitioning for health claims and structure/function claims.

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A number of the compounds currently not recognized as fiber in the United States were discussed at length in other sessions at the 1998 AACC meeting. One example is Fibersol-2, produced by Matsutani Chemical. Claimed as 40% to 50% soluble fiber, this ingredient appears on an ingredient legend as "maltodextrin." It is an indigestible dextrin produced by acid and enzyme hydrolysis of cornstarch, containing alpha-1,4; alpha-1,6; beta-1,2; beta-1,3; and beta-1,6 glucosidic bonds. It is soluble at up to 70% dry solids at 20°C, and possesses low viscosity.

Inulin and oligofructose are natural components of plants such as leek, chicory, onions and garlic. Inulin is a beta-2,1 fructan with the basic structure Gf_n, where G = glucosyl unit; f = fructosyl unit; and n = number of units linked.

The DP (degree of polymerization) of inulin is primarily 2 to 60, with over half of the dry weight at a DP less than 20. Oligofructoses, or FOS, are a mixture of beta-2,1 fructans with the same structure noted for inulin. However, the DP ranges from 2 to 20, with the primary weight as 2 to 8. Inulin is generally available commercially as an extract of chicory root.

The nutritional properties of inulin and FOS have been reported by a number of sources. They are low in calories: 1 kcal/gram for inulin, slightly higher for FOS. They are also suitable for diabetics since they don't result in an increase in blood glucose. Inulin and FOS stimulate the growth of bifidobacteria in the GI tract, inhibiting the growth of harmful bacteria and positively affecting the immune system. Other benefits include aiding better resorption of minerals, production of nutrients and prevention of constipation. As dietary fiber, they decrease fecal transit time, reduce glycemic response, lower blood cholesterol, and increase stool weight. Unfortunately, they do not test as soluble dietary fiber under the current U.S.-accepted AOAC methods.

Polydextrose suffers the same fate. Polydextrose is a randomly bonded, branched glucose polymer containing a predominance of alpha-1,6 glucosidic bonds. Its highly branched complexity prevents hydrolysis by alimentary enzymes, resulting in a caloric density of 1.0 kcal/gram. In Japan, polydextrose is widely used in fiber-fortified beverages. It is considered a food, and is approved for a label claim reading "provides improved intestinal function." Because it is fermented in the colon, as are a number of soluble fibers, polydextrose has a laxative effect in sensitive individuals. In the United States, and in four other countries, a laxation statement is required on the label if the food contains more than 15 grams per serving. Polydextrose is not precipitated by ethanol in the AOAC procedure for dietary fiber. Cultor Food Science, Ardsley, NY, is working to coordinate an AOAC collaborative study to change the methodology to include polydextrose as TDF.

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Choice of fiber is dependent on the goal of product development. If the ingredient is mainly yielding viscosity, water-binding or film-forming properties, the level of fiber probably will not be high enough for fiber claims. Products labeled "good source" or "fiber fortified" must contain 2.5 grams per serving, or 10% of the Daily Recommended Value (DRV) of 25 grams per day. Products labeled "high fiber" must contain a minimum of 5 grams per serving, or 20% of the DRV. Other considerations for selection are ingredient legend fit (what would look best to a consumer), particle size, flavor masking or flavor contribution, and processing considerations, including additional ingredients required. A careful look at the percent TDF, insoluble fiber and soluble fiber is warranted. Many grain fibers might look attractive on the label, but are comparatively low in TDF. Cost, of course, is often a major factor in ingredient selection. In some cases, certain types of fiber provide consumer appeal, or a health claim benefit. Fortification alone might be the goal, or perhaps low viscosity, low flavor impact or high TDF might be the determining factors. A wide range of fiber types is available, and current discussions on fiber definitions will likely expand this range even further, particularly in the soluble fiber group.

"I see the importance of fiber in foods increasing in the future," says Crosby. "Studies continue to show the benefits of both soluble and insoluble fiber in the diet. This is especially true in developed countries where the amount of fiber in the diet has significantly decreased since the turn of the century. Some of the hurdles facing food formulators are cost and FDA limits on labeling the benefits. Without the ability to promote the health benefits through labeling, there is little justification to increase the cost of a formulated food by adding more fiber. Consumer education on the benefits of fiber is also critical to justifying the cost of adding fiber to food, or demanding foods with more fiber."

• Photo: National Starch and Chemical Company

• Photo: A.E. Staley Manufacturing Company

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