Quick viewing(Text Mode)

What Is Dietary Fiber?

What Is Dietary Fiber?

NUTRITIONAL VALUE OF SOYBEAN FIBER

J.L. Slavin Associate Professor Extension Nutritionist Department of Food Science and Nutrition University of Minnesota

WHAT IS ?

Dietary fiber has been defined in many ways. The most abundant compounds identified as fiber are in the wall; other fibers are part of the intracellular cement; and others are secreted by the plant in response to injury (I). Thus, dietary fiber cannot be equated to the plant . Trowell (2) has defined dietary fiber as plant cellular material resistant to by human beings. that resists pancreatic action and passes to the colon would be considered dietary fiber, according to Trowell's definition (3). Also, according to Trowell's definition, the would be considered liquid fiber, and escaping digestion in -deficient individuals would be considered dietary fiber (4).

Dietary fiber has been defined physiologically as the sum of and not digested by the endogenous secretions of the human (5). Such a definition excludes the gums and mucilages and used in food processing (4). Further, accepting a physiological definition for dietary fiber rather than a chemical definition makes it difficult to measure dietary fiber in foods. Thus, the most popular definition of dietary fiber is the portion of plant cells that cannot be digested by human alimentary and therefore cannot be absorbed from human small bowels. Dietary fiber is not an accurate term because many of its components are not fibrous. Gums and mucilages, for example, are classified as dietary fiber because they are not digested by mammalian enzymes or secretion. Only one component of dietary fiber, namely , is truly fibrous; yet dietary fiber is the accepted nomenclature when describing the roughage or in the human diet.

179 COMPONENTS OF DIETARY RBER

Just as different vitamins have different functions in the body, different dietary fibers have different physiological effects (4). Thus, attempts have been made to describe components of dietary fiber. On the basis of intra-plant functions, dietary fiber can be divided into the following three major functions (I):

Structural polysaccharides - associated with the cell wall - include noncellulose polysaccharides and cellulose Structural nonpolysaccharides - predominantly lignin Nonstructural polysaccharides - include gums and mucilages secreted by cells - include polysaccharides from and seaweed

Dietary fiber chemistry can also be used to describe components of dietary fiber. Extension reviews of fiber chemistry are available (6,7). Chemical classification of dietary fiber is summarized in Table I. fiber chemical components main chain side chain description polysaccharides cellulose none main structural component of plant cell wall noncellulose cell wall ; backbone of 1-4 galactmc linked pyranoside glucose pectic substances galacturonic part of primary cell wall and middle lamella arabinose xylose mucilages -mannose galactose synthesized by plant secretory cells; used in glucose-man nose food industry arabinose-xylose galacturonic acid-rhamnose gums galactose xylose secreted at site of plant injury by -mannose specialized secretory cells galacturonic acid-rhamnose galactose algal polysaccharides mannose galactose derived from algae and seaweed xylose glucuronic acid glucose nonpolysaccharide lignin sinapyl alcohol 3-dimensional structure non-; phenyl propane coniferyl alcohol p-coumaryl alcohol

For simplicity, dietary fiber can be divided into noncellulosic polysaccharides, cellulose, and lignin. Human foodstuffs contain mainly 180 noncellulosic polysaccharides, some cellulose, and little lignin (I). The average content of noncellulosic polysaccharides, cellulose, and lignin for cereals is 75%, 17% and 7% respectively; for raw vegetables, 66%, 31%, and 3%, respectively; and for fruits, 63%, 20%, and 17%, respectively (I).

Overall, fruits and vegetables tend to be higher in cellulose than cereals. Lignin is highest in fruits with edible seeds or in mature vegetables such as carrots or other root vegetables. Dietary fiber composition of a plant depends upon plant species, maturity, and components (i.e. leaf, root, or stem). Dietary fibers have also been classified by in , since water-soluble and water-insoluble fiber have distinct physiological effects.

METHODS TO MEASURE DIETARY FIBER

Because the most widely accepted definition for dietary fiber is physiological, it has been difficult to find methods to measure dietary fiber and its components accurately. Historically, values were given as crude fiber. The crude fiber procedure treats food with acid and alkali to isolate the indigestible component. Although the method is appropriate for feeds, it seriously underestimates the dietary fiber content of food, recovering only 50% to 80% of the cellulose, 10% to 50% of the lignin, and 20% of the hemicellulose (8).

Crude fiber values have no consistent relationship to the dietary fiber values of human foods. Dietary fiber values usually are 3 to 5 times higher than crude fiber values, but no correction factors can be used because the relationship between crude fiber and dietary fiber varies depending upon the various chemical components (9). Bran flakes, for example, contain six times more dietary fiber than crude fiber, yet strawberries contain only 1.6 times more dietary fiber than crude fiber. Crude fiber has little physiological significance in and should not be used to report the fiber content of human diets.

Despite the enormous amount of scientific and pseudo-scientific discussion concerning the benefits of a high fiber diet, few reliable values exist in the literature for dietary fiber content of foods. The difficulty in devising a method for assessing total dietary fiber can be appreciated if one considers the diverse nature of dietary fiber. A simple, reproducible method to remove , fat, and soluble sugars and starch from food while retaining both water-soluble and insoluble components of dietary fiber is not presently available. Many methods to measure dietary fiber have been tried, but no ideal method exists (10,11).

181 The most common methods currently used to measure dietary fiber are the Van Soest detergent methods. Neutral detergent fiber (NDF) methods are one-step processes that remove the cytoplasmic and water-soluble plant cell-wall components leaving only the water-insoluble fiber. Thus, the NDF procedure underestimates total dietary fiber because water-soluble fiber are lost in the procedure. NDF has been measured with in selected fruits and vegetables to give estimates of total dietary fiber (12). USDA Agriculture Handbook 8-8 (13) includes insoluble fibers, as analyzed by the NDF prodecure. Thus many of the dietary fiber values commonly in use are NDF values or combinations including NDF values.

Heckman and Lane (14) compared dietary fiber methods on a range of food samples. Foods included cereals, fruits, vegetables, pectin, locust bean gum, and soybean polysaccharide. Methods compared were the detergent fiber methods, an enzyme-modified NDF procedure (15), a 2-fraction enzyme method for soluble and insoluble fiber proposed by Furda (16), and a I-fraction enzyme method for total dietary fiber proposed by Hellendoorn et al (17). Not surprisingly, the detergent methods gave lower results, since water-soluble fibers are lost in the detergent extraction. The authors concluded that the Furda method agrees well with Southgate procedure (18) and is a simpler procedure analytically. They recommend that the Furda method be used for analysis of dietary fiber in human foodstuffs.

The Southgate procedure is still used for dietary fiber analysis. Many of the existing dietary fiber values were generated with the Southgate procedure (19). In the Southgate procedure, nonfibrous components of food are removed sequentially with chemical and enzymatic extractions, leaving total dietary fiber. Hemicellulose, pectin, cellulose, and lignin are also extracted with chemical reagents, and the fiber fractions are measured gravimetrically. High starch foods, such as peas and beans, are very high in dietary fiber as measured by the Southgate procedure. Much of the dietary fiber may actually be starch and thus Southgate values of high starch items should be considered overestimates of actual dietary fiber content.

Recently, the Food and Drug Administration conducted a large collaborative effort to measure total dietary fiber in foods by an enzymatic gravimetric procedure based on the methods of Asp, Englyst, Furda, Schweitzer, Southgate, Theander, and Van Soest (20). Large interlaboratory variation was found. Coefficients of variation for some samples were greater than 100%. Incomplete starch and protein removal were found to be problems. The method was modified, and a second collaborative study was conducted (21). The modified method is now accepted as an official first 182 action method by the Association of Official Analytical Chemists.

Few foods have been analyzed with the method. Most fiber values currently available were generated with a modified NDF procedure, the Southgate procedure, or various chemical and enzymatic extraction procedures (22). Lanza and Butrum (20) have compiled an extensive list of dietary fiber values from many sources. The values, summarized in Table 2, were generated with different fiber methods and therefore should be considered estimates.

food group <1gm 1-1.9 gm 2-2.9gm 3-3.9gm 4-4.9 gm 5-5.9 gm >6gm breads (1 slice) bagel whole- bran muffin (1) NA• NA NA NA white French cereals (1 oz) Krispies oatmeal Wheaties Most Bran Chex Corn Bran All-Bran Special K Nutri-Grain Shredded Wheat Honey Bran 40% Bran Flakes Bran Buds cornflakes Cheerios Raisin Bran 100% Bran (1 c) NA macaroni NA whole- NA NA NA spaghetti wheat spaghetti rice ('Ii c) white brown NA NA NA NA NA legumes (½ c) NA NA NA lentils lima beans NA beans cooked dried peas baked bean5 navy beans vegetables (½ c cucumber asparagus broccoli peas NA NA NA unless stated) lettuce green beans brussel sprouts (1 c) cabbage carrots green pepper cauliflower corn w/out potato w/ skin skin (1) (1) celery spinach fruits (1 medium grapes (20) apricots (3) apple, w/out apple, wl skin NA NA NA unless stated) watermelon grapefruit(½) skin pear, wl skin (1 c) peach w/ skin banana raspberries pineapple orange (½ c) (½ C)

•Not applicable.

Most commonly consumed cereals have less than 2 grams dietary fiber per serving. Wheat cereals contain between 2 and 4 grams per serving. Concentrated bran cereals contain more than 6 grams dietary fiber per serving and are the most concentrated fiber sources generally consumed in the American diet.

Bran fiber is effective in increasing stool weight but has little effect on serum or blood glucose. Oatmeal is low in dietary fiber, but much of that fiber may be gums. Gums have been shown to decrease serum cholesterol (23) and stabilize or reduce blood glucose (24). Fruits and vegetables are generally low in fiber, containing between I and 3 grams per serving. Certain fruits, such apples, citrus fruits, and strawberries, are high in pectin, which, like gums, has been shown to decrease serum cholesterol and stabilize blood glucose.

Legumes appear to be the champion of fiber sources, although figures are 183 undoubtedly inflated because of residual starch and protein. Legumes, like , are a concentrated source of water-soluble fiber. Breads and pasta, unless they are made from whole wheat, contribute little dietary fiber to the diet.

HOW MUCH FIBER DO WE EAT AND HOW MUCH SHOULD WE EAT?

Because of epidemiological evidence linking a low fiber diet with various degenerative diseases, many organizations have recommended an increase in fiber intake. The National Cancer Institute reports that Americans consume 10 to 20 grams of dietary fiber daily and should consume 25 to 35 grams (25). Other estimates of desirable fiber intake range from 25 to 50 grams (26,27) Unless consumers are willing to adopt more vegetarian diets, it is unlikely they will be able to consume the recommended amounts of dietary fiber. Therefore, purified fiber supplements have become popular as means to increase the fiber content of the diet.

WHAT IS soy FIBER?

Soy fiber used in human foods is primarily cell wall material of soybean cotyledon derived from processing dehulled and defatted soybean flakes. It is neither soybean hull nor soy bran. It is bland, odorless, and contains approximately 75% dietary fiber, as measured by the AOAC method. Soy fiber can be added to many food systems and is available in various types of fiber which are tailored to specific applications. Currently different soy fiber sources are used in liquid diets, specifically enteral nutrition formulas, low calorie breads, and baked goods. Because soy fiber is a concentrated fiber source compared to other fibers consumed by humans, it only takes 13 grams of soy fiber to provide 10 grams of dietary fiber. In contrast, it takes 58 grams of bran, 23 grams of wheat bran, 735 grams of lettuce, and 502 grams of apple to provide 10 grams of fiber. The extraction process to obtain soy fiber is shown below:

FIGURE 1 PRODUCTS FROM THE SOYBEAN

I Soy~ans I Dehulling Hulls I Oil Extraction For Animal Feeds 1--,,.------il Soybean Oil I

Soy Soy Protein Isolated ~ Concentrate Soy Protein ~ COMPONENTS OF SOY RBER

Soy fiber is a mixture of cellulosic and non-cellulosic internal cell wall structural components (28). Its major fractions are non-cellulosic and consist of acidic polysaccharides, arabinogalactan, arabinan chains and about 10% cellulosic components. Acidic polysaccharides are highly-branched made of a backbone of D-galacturonic acid and D-galactose interspersed with L-rhamnose. Side chains associated with acidic polysaccharides are composed of galactose, xylose, and fucose residues.

Typical analysis of soy fiber yields 6% moisture, 12% protein, 0.2% fat, 4.5% ash, and 75% dietary fiber. The dietary fiber content of soy fiber varies greatly depending on the method used for analysis. According to the Furda method, soy fiber is 54% total dietary fiber, with 32% soluble and 22% insoluble. The Southgate method yields 66% total dietary fiber, 1% soluble, 50% insoluble, 14% cellulose, 1% lignin. The AOAC method finds soy fiber to be 75% total dietary fiber, 4% soluble, and 71% insoluble. Thus, it is difficult to conclude the actual fiber content of soy fiber, although the AOAC method is gaining acceptance as an official method for dietary fiber. Generally, it thought that soluble fiber is associated with cholesterol lowering and improved diabetic control, while insoluble fiber is associated with enhanced bowel . Yet, as explained below, soy fiber seems to have all these beneficial physiological effects.

PHYSIOLOGICAL EFFECTS OF soy FIBER

A summary of the physiological benefits of soy fiber is presented below.

Effects on gastrointestinal responses - decreased mouth to cecum transit time (29) - increased fecal bulk (29) - increased fecal moisture (29) Effects on blood cholesterol in humans - reduced plasma cholesterol levels in subjects with mildly elevated plasma cholesterol levels (30) - reduced plasma cholesterol levels in Type IIA hypercholesterolemic patients (31 ) Effects on blood glucose and insulin concentration - reduced hypoglycemic rebound in normals (32) - lowered insulin concentration in normals (32) - improved glucose tolerance in obese diabetics (32) - long-term supplementation of soy fiber to patients with hyperlipidemia and impaired glucose tolerance tests significantly improved their glucose 185 and insulin responses during standard glucose tolerance tests (31) Effects on utilization - mineral balance was not affected (33) - macronutrient utilization was not affected (33)

When compared to other fiber sources available as supplements for human foods, soy fiber provides a broad range of positive functional, nutritional, and physiological effects. Soluble fibers, such as gums and , lower plasma cholesterol and normalize blood glucose, but are difficult to incorporate into foods and are not well accepted by human subjects. Fibers such as wheat bran are effective in the prevention and treatment of constipation, but are also not well accepted long term by humans. Soy fiber appears to be a good compromise since it offers the physiological effects of both soluble and insoluble fibers, yet is well accepted and easily incorporated into foods. If you accept the premise that all Americans should consume 25 - 35 grams of dietary fiber each day, then it seems likely that supplemental fiber will be needed for humans to reach these levels unless they are willing to adopt more vegetarian eating patterns. Soy fiber appears to be one of the best fiber supplements currently on the market.

REFERENCES

I. Schneeman, 8.0. (1986) Dietary fiber: Physical and chemical properties, methods of analysis, and physiological effects. Food Technol 40:I04.

2. Trowell, H. (1972) lschemic heart disease and fiber. Am. J. Clin. Nutr. 25:926.

3. Stephan, A.M., Hadda, A.C. & Phillips, S.F. (1983) Passage of carbohydrate into the colon: Direct measurement in humans. Gastroenterology 85:589.

4. Eastwood, M.A. (1986) What does measurement of dietary fibre mean? Lancet I :1487.

5. Trowell, H.C. (1974) Definitions of fibre. Lancet 1:503.

6. Selvendren, R.R. (1984) The plant cell wall as a source of dietary fiber. Chemistry and structure. Am. J. Clin. Nutr. 39:320.

7. Kay, R.M. (1982) Dietary fiber. J. Res. 23:221.

8. Van Soest, P.J. (1978) Dietary fibers: Their definition and nutritional properties. Am. J. Clin. Nutr. 3I:Sl2. 186 9. Slavin, J.L. (1983) Dietary fiber. Diet. Curr. 10:29.

10. Southgate, D.A.T., Hudson, G.J. & Englyst, H. (1978) The analysis of dietary fibre - choices for the analyst. J. Sci. Fd. Agre. 29:979.

II. Asp, N.G. & Johansson, C.G. (1984) Dietary fibre analysis. Nutr. Abstr. Rev. 54:736.

12. Ross, J.K., English, C. & Perlmutter, C.A. (1985) Dietary fibre constituents of selected fruits and vegetables. J. Am. Diet. Assoc. 85:1111.

13. Douglass, J.S., Matthews, R.H. & Hepburn, F.N. (1982) Composition of Foods. Breakfast Cereals - Raw, Processed, Prepared. Rev. USDA Handbook, No. 8-9.

14. Heckman, M.M. & Lane, S. (1981) Comparison of dietary fiber methods for foods. J. Assoc. Off. Anal. Chem. 64:1339.

15. Approved methods of the MCC, St. Paul, American Association of Cereal Chemists, 1983.

16. Furda, I. (1977) Fractionation and examination of from dietary fiber. Cer. Foods World 22:252.

17. Hellendoorn, E.W., Nordhoff, M.G. & Slagman, J. (1975) Enzymatic determination of the indgestible residue (dietary fibre) content of human food. J. Sci Fd. Agric. 26:1461.

18. Southgate, D.A.T. (1969) Determination of in foods. II. Unavailable carbohydrates. J. Sci Fd. Agric. 20:331.

19. Paul, A.A. & Southgate, D.A.T. (1978) Mccance and Widdowson's The Composition of Foods. London, Her Majesty's Stationery Office.

20. Prosky, L., Asp, N., Furda, I., Devries, J., Schweizer, T. & Harland, B. (1984) Determination of total dietary fiber in foods, food products, and total diets. lnterlaboratory Study. J. Assoc. Off. Anal. Chem. 67:1044.

21. Prosky, L., Asp, N., Furda, I., Devries, J., Schweizer, T. & Harland, B. (1985) Determination of total dietary fiber in foods and food products. Collaborative study. J. Assoc. Off. Anal. Chem. 68:677.

22. Southgate, D.A.T. (1978) Dietary fiber: Analysis and food sources. Am. J. 187 Clin. Nutr. 31:SI07.

23. Vahouny, G.V. (1982) Dietary fibers and intestinal absorption of lipds. In: Vahouny, G.V. & Kritchevsky, D., eds., Dietary Fiber in Health and Disease, New York, Plenum Press.

24. Anderson, J.W. (1982) Dietary fiber and diabetes. In: Vahouny, G.V. & Kritchevsky, D., eds. Dietary Fiber in Health and Disease. New York, Plenum Press.

25. Diet, nutrition and cancer prevention: A guide to food choices. NIH Publ. No (NCI) 85-2711, 1984.

26. Stephen, A.M. (1981) Should we eat more fibre? J. Hum. Nutr. 35:403.

27. Kelsay, J.L. (1982) Effects of fiber on mineral and vitamin bioavailability. In: Vahouny, G.V. & Kritchevsky, D., eds., Dietary Fiber in Health and Disease, New York, Plenum Press.

28. Aspinall, G.O., Bigbie, R. & Mckay, J.E. (1967) Polysaccharide components of soybeans. Cereal Sci. Today 12:224.

29. Slavin, J.L., Nelson, N.L., McNamara, E.A. & Cashmere, K. (1985) Bowel function of healthy men consuming liquid diets with and without dietary fiber. JPEN 9:317.

30. Shorey, R.L. & Day, P.J. (1985) Effects of soybean polysaccharide on plasma . J. Am. Diet. Assoc. 85:1461.

31. Lo, G.S. & Goldberg, A.P. (1987) Effect of soy fiber on human plasma lipoprotein lipid and glucose tolerance. Atherosclerosis (in press).

32. Tsai, A.C., Mott, E.L. et al. (1983) Effect of soy polysaccharide on gastrointestinal functions, nutrient balance, steroid excretion, glucose tolerance, serum lipids, and other parameters in humans. Am. J. Clin. Nutr. 38:504.

33. Bowen, P.E., Taper, LI. et al (1982) Mineral absorption using fiber-augmented diets. JPEN 6:575.

188

Top View