Fiber Dynamics part 1

By Venom

We at Hyperplasia are not satisfied with reaching our pre-conceived genetic limitations, but smashing them into smithereens! This article follows the same trend; only now, we will not focus on splitting muscle cells, but dissecting a whole different entity know as dietary fiber.

Dietary Fiber

The conventional definition for dietary fiber is proposed by Trowell et al [41]:

"Plant polysaccharides and lignin which are resistant to hydrolysis [catabolism of larger molecules into smaller ones the body can absorb. Done by enzymes and water]."

But this definition is not without fault. Although it is generally agreed that all the non-starch polysaccharides (another name for dietary fiber) should be regarded as fiber components, frustration occurs because:

·        It fails to include the entire indigestible residue from food that may reach the colon.

·        It uses the ability to be digested as the basis for the definition when undigested food reaching the colon does not necessarily lack the ability to be digested, nor is it necessarily unavailable to the body.

·        Contrary to popular belief, fiber can be broken down, and used as an energy source for the body, you will understand this fully after the article.

To elaborate on this, some starch may reach the large intestine (colon) in an unaltered state along with non-starch polysaccharides (NSP). This starch and much of the NSP’s may then undergo fermentation (anaerobic breakdown) by colonic bacteria;, thereby, producing short-chain fatty acids that may be used for energy by the host. However, despite their presence in colonic residue, starch is not considered by some as dietary fiber because it is digestible by mammalian enzymes, and lignin is not considered by some to be true components of dietary fiber because it is a non-carbohydrate polymer [10,41]. 

Before we move on, lets take a quick look at the digestive track, namely the Large intestine. 

Large Intestine

Intestinal material leaving the ileum, which is the last portion of the small intestines that communicates with the large intestines, empties via the ileocecal valve or sphincter (ring like band of muscle fibers which constrict an opening) into the cecum, at the beginning of the colon. Initially on entering the colon, the intestinal material is still quite watery. Contraction of the muscles in the large intestine is coordinated so as to mix the intestinal contents gently and to keep material in the colon to allow nutrient absorption to occur. The proximal colonic epithelia absorbs sodium, chloride, and water more effectively than does the small intestinal mucosa. For example, about 90% to 95% of the water and sodium entering the colon each day is absorbed. Colonic absorption of sodium is influenced by a number of factors, including hormones. Antidiuretic hormone (ADH), for example, decreases sodium absorption, while glucocorticoids and mineralocorticoids increase sodium absorption in the colon.

Secretions into the lumen of the colon are few. Goblet cells are cells of the epithelial lining in the small intestine that secrete mucus. Mucus protects the colonic mucosa (mucosal cells are any membrane or lining, which contain mucus-secreting glands for lubrication) and acts as a lubricant for fecal matter (waste). Potassium is secreted via an active secretory pathway into the colon. Bicarbonate is also secreted in exchange for chloride absorption. Bicarbonate provides an alkaline (basic; less acidic) environment that helps to neutralize acids produced by colonic anaerobic bacteria. Sodium and hydrogen ion exchanges also occur, permitting electrolyte absorption.

Intestinal bacterial counts of up to 10¹²/g gastrointestinal tract contents in the large intestine. Some examples of bacterial flora that inhabit the large intestine include Bacteroides, Clostridia, Lactobacteria, Bifidobacteria, Coliforms, Methanogens, Eubacteria, and Streptococci. The bacteria in the large intestines are collectively known as colonic bacteria.

These bacteria use primarily dietary carbohydrates (although to a lesser extent amino acids and proteins) as substrates needed for their growth. For example, starch that has not undergone hydrolysis by pancreatic amylase may be used by gram-negative bacteroides or by gram-positive bifidobacteria or eubacteria (types of bacteria). Glycoproteins (proteins containing one or more covalently linked sugar units) found in mucus secretions of the gastrointestinal tract may be broken down and used by bacteria such as bacteroides, bifidobacteria, and clostridia. In addition, sugar alcohols such as sorbitol and xylitol, disaccharides such as lactose, undigested oligosaccharides such as raffinose and stacchyose, and fibers such as some hemicelluloses, pectins, and gums may be degraded by selected bacteria found in the large intestine. Even digestive enzymes themselves may serve as substrates for bacteria such as bacteroides and clostridia. The breakdown of carbohydrates and proteins by bacteria is an anaerobic process (without oxygen) known as fermentation.

As stated above, colonic bacteria primarily degrade carbohydrates, but they also use some amino acids and proteins as substrates necessary for the production of energy, and carbon atoms essential for bacterial growth and maintenance. Acids are one of the principal end products of bacterial carbohydrate fermentation in the large intestine. Specifically, lactate and the short-chain fatty acids-acetate, butyrate, and propionate. These short-chain fatty acids formerly referred to as volatile fatty acids, serve many purposes such as stimulating gastrointestinal cell reproduction. In addition, the presence of the acids lowers luminal pH in the colon, which affects nutrient absorption, and growth of certain bacteria [16,32].

For further explanation on the digestion of starches, and sugars, and full comprehension of future terms to be applied in this editorial, such as hydrogen bonds, read the following article, Dextrose, Maltodextrin, and Sodium an In Depth Analysis.

Dietary Fiber comes from Plants

Dietary fiber (in spite of the controversy to its definition) is derived from plant cells. The plant cell wall includes cellulose, hemicellulose, lignin, pectins, as well as some non-starch polysaccharides. Non-cell wall fiber components consist of gums, mucilage’s, algal polysaccharides, suberin, and cutin (of which will be discussed in detail shortly).

The plant cell wall consists of both a primary and secondary wall. The primary wall is a thin envelope surrounding the contents of the growing cell; the secondary wall develops as the cell matures. The secondary wall of a mature plant contains many strands of cellulose arranged in an orderly fashion within a matrix of non-cellulosic polysaccharides. The primary wall also contains cellulose, but it occurs in smaller amounts and is less well organized. Starch, the energy storage product of the cell, is found within the cell walls.

Lignin deposits form in specialized cells whose function is to provide structural support to the plant. As the plant matures, lignin spreads through the intracellular (inside the cell) spaces, penetrating the pectic substances. Pectic substances function as intercellular cement, and are located between and around the cell walls. Lignin continues dispersing through intracellular spaces, but it also permeates the primary wall, and then spreads into the developing secondary wall. As plant development continues further, suberin is deposited in the cell wall during the later stages. Cutin, a water-impermeable substance, is secreted onto the plant surface.

The consumption of plant foods provides fiber in the diet. Plant species, parts (leaf, root, stem), and maturity all influence the composition (cellulose, hemicellulose, pectin, lignin, etc.) of the fiber that is consumed. For example, consumption of cereal bran such as wheat bran provides primarily hemicellulose as well as lignin. Psyllium provides primarily mucilage’s but also some non-polysaccharides. Consumption of fruits and vegetables provides almost equal quantities (less than 30%) of cellulose and pectin. In contrast, cereals are quite low in cellulose [41].

Next we will discuss how these components, which make up dietary fiber are chemically composed.

Splitting fibers!  

As promised in the introduction, we will now completely dissect fiber, and its many components, starting with the most abundant organic molecule on the earth--cellulose.

Cellulose

Cellulose structure is composed of a long, linear polymer of 1,4 J3-linked glucose units that is found in the plant cell wall. Hydrogen bonding between sugar residues in adjacent parallel running cellulose chains imparts a micro fibril three-dimensional structure to cellulose. Being a large, linear, neutrally charged molecule, cellulose is water-insoluble, although it can be modified chemically (e.g., sodium carboxymethylcellulose) to be more soluble and used as an additive in foods. Degradation of cellulose by colonic bacteria varies, but generally it is poorly fermented (doesn’t digest well). Some examples of foods high in cellulose relative to other fibers include bran, legumes, peas, root vegetables, vegetables of the cabbage family, outer covering of seeds, and apples.

Hemicellulose

Hemicellulose, like cellulose, is found in the plant cell wall. It consists of sugar units containing 5 or 6 carbons; it is insoluble in water, but soluble in alkali (basic solutions). It consists of a heterogeneous (a mixture in which the different components can be visibly seen, such as granite, sand in water, and pizza mmmm) group of polysaccharide substances containing a number of sugars in its backbone and side chains. The sugars, which form a basis for hemicellulose classification, include xylose, mannose, and galactose in the hemicellulose backbone and arabinose, glucuronic acid, and galactose in the hemicellulose side chains. The sugars in the side chains also confer important characteristics on the hemicellulose. For example, hemicelluloses that contain acids in their side chains are slightly charged and water-soluble. Other hemicelluloses are insoluble. Fermentability of the hemicelluloses by intestinal microflora is also influenced by the sugars and positions. For example, hexose and uronic acid components of hemicellulose are more accessible to bacterial enzymes than the other hemicellulose sugars. Some foods that are relatively high in hemicellulose are bran and whole grains.

Lignin

Lignin is the main non-carbohydrate component of fiber. It is a three-dimensional polymer composed of phenol units with strong intermolecular bonding. The primary phenols composing lignin include trans-coniferyl, trans-sinapyl, and trans-p-coumaryl. Lignin forms the structural, rigid components of plants, and is thought to attach to other non-cellulose polysaccharides such as heteroxylans found in plant cell walls. Lignin is insoluble in water, has hydrophobic (water fearing, not adsorbing, or affected by water,) binding capacity, and is not digested (poorly fermented) by colonic bacterial microflora. Mature root vegetables such as carrots, wheat, and fruits with edible seeds such as strawberries are high in lignin.

Gums

Gums are substances dissolved or dispersed in water that give a gelling or thickening effect. In plants, they are secreted at the site of injury by specialized secretory cells and can be excreted from the plants. Gums are composed of a variety of sugars and sugar derivatives. Occurring prominently in the gums are galactose and glucuronic acid as well as uronic acids, arabinose, rhamnose, and mannose, among others. Within the large intestine, gums are highly fermented by colonic bacteria. The structure of the gum arabic contains a J3 1-3 galactose backbone with side chains of galactose, arabinose, rhamnose, glucuronic acid, and/or methyl-glucuronic acid. Gum arabic is the plant hydrocolloid most commonly used as a food additive. Its popularity is due to its physical properties, including high solubility, pH stability, and gelling characteristics. Other water-soluble gums such as guar and locust bean (carob) are referred to as galactomannans. Galactomannans contain a mannose backbone in a 2:1 or 4:1 ratio with galactose present in the side chains. Gums are also found naturally in foods such as oatmeal, barley, and legumes (i.e. peas). Some gums (xanthan gum and gellan gum) can be synthesized by microorganisms.

Pectin

Pectic substances, or more commonly called pectin, are polysaccharides that form gels with sugar and acids, and imparts a crispy texture to food such as freshly picked apples. The backbone structure of pectin is usually an un-branched chain of a 1,4- t linked D-galacturonic acid units. Other carbohydrate moieties (portions) may be linked to the galacturonic acid chain. Additional sugars sometimes found attached as side chains include rhamnose, arabinose, xylose, and fucose. Pectin’s form part of the primary cell wall of plants and part of the middle lamella.

Pectin’s are water-soluble and gel forming and have high ion-binding potential. Moreover, they can be almost completely metabolized by colonic bacteria. Apples, strawberries, and citrus fruits are high in pectin’s. Pectin’s are also added to commercial products and to some internal nutrition formulas administered to tube-fed hospitalized patients.

Mucilages and Algal Polysaccharides

Mucilages and algal polysaccharides are hydrocolloids (colloids are microscopic particles suspended in some sort of liquid medium. Hydrocolloids are gelatinous colloids in an unstable equilibrium with its contained water), and similar to gums in chemical structure. Because of their hydrophilic (water loving, absorbs, and readily interacts with water) property, these substances are excellent stabilizers. Mucilages are synthesized by plant secretory cells to protect the seed endosperm (Tissue present in seeds, they are external to and surrounding the embryo, which it provides with nourishment in the form of starch or other food reserves) from desiccation (dehydration). The algal polysaccharides for example, carrageenan and agar are derived from algae and seaweed and are commonly added to food. Both mucilage’s and algal polysaccharides are degraded by colonic bacteria.

Additional paraphernalia (stuff)

In addition to the aforementioned polysaccharides and lignin, three more substances--cutin, suberin, and waxes, which are plant derived--are considered as dietary fiber by some. Cutin is found on the external surface of the cell wall of plants. It contains long-chain hydroxyaliphatic acids and is impermeable to water. Suberin is found near the plant cell wall external surface, just below the epidermis and skins. Suberin is made up of a variety of substances including phenolic compounds as well as long-chain alcohols and acids. Suberin and cutin are both polymeric esters of fatty acids that are both enzyme and acid resistant. Waxes, complex hydrophobic, hydrocarbon compounds, coat the external surfaces of many plants.

Other substances sometimes considered dietary fiber because they are unable to be digested by human digestive enzymes include Maillard products. Maillard products result from food processing and consist of enzyme-resistant linkages between the amino group (-NH2) of amino acids, especially the amino acid lysine, and the carboxyl groups (-COO-) of reducing sugars. Maillard products are formed during heat treatment, particularly in the baking and frying of foods.

Physiological effects of dietary fiber  

The physiological and metabolic effects of fiber vary based on the types of ingested fiber. Significant characteristics of dietary fiber that affect its physiological and metabolic roles include its solubility in water, its hydration or water-holding capacity and viscosity (thickness of fluid), its adsorptive attraction or ability to bind organic and inorganic molecules, and its degradability or fermentability by intestinal bacteria. Each of these characteristics and their effects on various physiological and metabolic processes will be reviewed.

Water Soluble and Water insoluble Fiber

Dietary fiber may be classified as water soluble or insoluble. Fibers that dissolve in hot water are soluble; those that will not dissolve in hot water are insoluble. In general, water-soluble fibers include some hemicelluloses, pectin, gums, and mucilage’s. Cellulose, lignin, and some hemicellulose are classified as insoluble fibers. Generally, vegetables and wheat, along with most grain products, contain more insoluble fibers than soluble fibers.

Solubility in water also may be used as a basis to broadly divide the characteristics of fibers. For example, generally, soluble fibers delay gastric emptying, increase transit time (slower movement) through the intestine, and decrease nutrient (e.g., glucose) absorption. In contrast, insoluble fibers decrease intestinal transit time and increase fecal bulk. These actions (discussed in the following article) of the soluble and insoluble fibers, in turn, induce other physiological and metabolic effects [36].

Conclusion

This is only the beginning. In the following article, we will step into another stratum! Are you ready? If so, click here to proceed, 

References:

1. Ad Hoc Expert Panel on Dietary Fiber, Federation of American Societies for Experimental Biology. Physiologic and health consequences of dietary fiber. Rockville,MD:FASEB,1987.
2. Aldoori W; Giovannucci E, Rockett H, Sampson L, Rimm E, Willett w: A prospective study of dietary fiber types and symptomatic diverticular disease in men. J Nutr 1998;128:714-9.
3. American Dietetic Association. Position of the American Dietetic Association: health implications of dietary fiber. J Am Diet Assoc. 1997;97:1157-9.
4. Anderson JW; Jones AE, Riddell-Mason S. Ten different dietary fibers have significantly different effects on serum and liver lipids of cholesterol-fed rats. J Nutr 1994;124:78-83.
5. Anderson JW. Whole grains protect against atherosclerotic cardiovascular disease. Proc Nutr Soc. 2003 Feb;62
6. Anderson. W; Bryant CA. Dietary fiber: diabetes and obesity. Am J GastroenteroI1986;81:898-906.
7. Asp NG, Johansson CG. Dietary fiber analysis. Nutr Abstr Rev 1984;54:736-52.
8. Ausman LM. Fiber and colon cancer: does the current evidence justify a preventive policy? Nutr Rev 1993; 51:57-63.
9. Beeebe CA, Pastors JG, Powers MA, Wylie-Rosett J. Nutrition management for individuals with noninsu1independent diabetes mellitus in the 1990s: a review by the Diabetes Care and Education dietetic practice group. J Am Diet Assoc 1991;91:196-202,205- 7.
10. Bingham S. Definitions and intakes of dietary fiber. Am J Clin Nutr 1987;45:1226-31.
11. Butrum RR, Clifford CK, Lanza E. NCI dietary guidelines: rationale. Am J Clin Nutr 1988; 48:888-95.
12. Diabetes Care and Education Practice Group, the American Diabetes Association. Nutritional recommendations and principles for individuals with diabetes mellitus. J Am Diet Assoc1994; 4:504-6.
13. Eastwood MA, Passmore R. A new look at dietary fiber. Nutr Today 1984;19:6-11.
14. E. Raybould and Y. Tache Cholecystokinin inhibits gastric motility and emptying via a capsaicin-sensitive vagal pathway in rats
15. Floch MH, Maryniuk MD, Bryant C, Franz MJ, Tietyen-Clark J, Marrota RB, Wolever T, Maillet J0, Jenkins At. Practical aspects of implementing increased dietary fiber intake. Nutr Today 1986;21:27-30.
16. Gorbach SL. Lactic acid bacteria and human health. Ann Med 1990;22:37-41.
17. Harris PJ, Ferguson LR. Dietary fibre: its composition and role in protection against colorectal cancer. Mut Res 1993;290:97-110.
18. Heaton Kw: Dietary fibre in perspective. Hum Nutr Clin Nutr 1983:37C:151-70.
19. Harris PJ, Roberton AM, Watson ME, Triggs CM, Ferguson LR. The effects of soluble fiber polysaccharides on the adsorption of a hydrophobic carcinogen to an insoluble dietary fiber. Nutr Cancer 1993; 19:43-54.
20. Hill MJ. Bile acids and colorectal cancer. Eur J Canc Prevent 1991;1:69- 72.
21. Howe GR, Benito E, Castelleto R, Cornee J, Esteve J, Gallagher RP, Iscovich JM, Deng-ao J, Kaaks R, Kune GA et al. Dietary intake of fiber and decreased risk of cancers of the colon and rectum: evidence from the combined analysis of 13 case-controlled studies.J Natl cancer Inst 1992;84:1887-96.
22. Ink SL, Hurt HO. Nutritional implications of gums. Food Tech 1987;41:77-82.
23. Jenkins DJA, Jenkins At, Rao AV; et al. Cancer risk: possible role of high carbohydrate, high fiber diets. Am J Gastroenterol1986;81 :931-5.
24. Jenkins D, Jenkins At, Wolever TMS, Rao AV; Thompson ill. Fiber and starchy foods: gut function and implications in disease. Am J Gastroenterol 1986;81 :920-30.
25. Juntunen KS et al High-fiber rye bread and insulin secretion and sensitivity in healthy postmenopausal women. Am J Clin Nutr. 2003 Feb;77(2):385-91.
26. Kritchevsky D. Dietary fiber. Ann Rev Nutr 1988;8:301-28.
27. Lupton JR, Kurtz PP. Relationship of colonic luminal short chain fatty acids and pH to in vivo cell proliferation in rats. J Nutr 1993;123:1522-30.
28. Malara A et al. Effects of alpha- and beta-adrenergic stimulation on free magnesium concentrations in platelets from healthy and obese individuals.
29. Marlett JA, Cheung Tl': Database and quick methods of assessing typical dietary fiber intakes using 228 commonly consumed foods. J Am Diet Assoc 1997;97:1139-48,1151.
30. M. Camilleri, L. J. Colemont, S. F. Phillips, M. L. Brown, G. M. Thomforde, N. Chapman, and A. R. Zinsmeister Human gastric emptying and colonic filling of solids characterized by a new method Am J Physiol Gastrointest Liver Physiol 257: G284-G290, 1989.
31. McKeown NM et al. Whole-grain intake is favorably associated with metabolic risk factors for type 2 diabetes and cardiovascular disease in the Framingham Offspring Study. Am J Clin Nutr. 2002 Aug;76(2):390-8.
32. McNeil N1. The contribution of the large intestine to energy supplies in man.    Am J Clin Nutr 1984;39: 338-42.
33. Oral Magnesium Supplementation Improves Insulin Sensitivity and Metabolic Control in Type 2 Diabetic Subjects: A randomized double-blind controlled trial. Diabetes Care. 2003 Apr;26(4):1147-52.
34. Potter JD. Colon cancer-do the nutritional epidemiology, the gut physiology and the molecular biology tell the same story? Nutr 1993;123:418-23.
35. Rodriguez-Moran M, Guerrero-Romero F. Dietary fiber and type 2 diabetes.
    Clin Excell Nurse Pract. 2000 Sep;4(5):272-6. Review.
36. Russell and P. Bass Canine gastric emptying of fiber meals: influence of meal viscosity and antroduodenal motility Am J Physiol Gastrointest Liver Physiol 249: G662-G667, 1985.
37. Southgate DAT. Minerals, trace elements, and potential hazards. Am J Clin Nutr 1987;45:1256-66.
38. Trock B, Lanza E, Greenwald Po Dietary fiber, vegetables, and colon cancer: critical review and meta-analyses of the epidemiologic evidence. J Natl Cancer Inst 1990;82:650-61.
39. Tomlin T, Read NW Comparison of effects on colonic function caused by feeding rice bran and wheat bran. Eur J Clin Nutr 1988;42:857-61.
40. Klurfeld DM, Dietary fiber-mediated mechanisms in carcinogenesis. Cancer Res (suppl) 1992;52:2055s-9s.
41. Trowell HC, Southgate DAT, Wolever TMS, et al. Dietary fiber redefined. Lancet 1976; 1 :967.
42. U.S. Dept. of Agriculture. The Food Guide pyramid. Washington, DC: Home and Garden Bulletin Number 252,1992.
43. Van Munster IF, Nagengast FM. The influence of dietary fiber on bile acid metabolism. Eur J Cancer Prevent 1991;1:35-44.
44. Zargar AH, Bashir MI. Copper, zinc and magnesium levels in type-1 diabetes mellitus. Saudi Med J. 2002 May;23(5):539-42
45. St. Louis MetroVoice. AinswersinGenesis.com: The Origin of Life
46. God, The Holy Bible