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Newsgroups: sci.med.nutrition From: altar@beaufort.sfu.ca (Ted Wayn Altar) Subject: Dietary Fibre Message-ID: <altar.727322081@sfu.ca> Organization: Simon Fraser University, Burnaby, B.C., Canada Date: Mon, 18 Jan 1993 01:54:41 GMT Lines: 710

"Things sweet to taste prove in digestion sour" Shakespeare (from "The Winter's Tale, III:2)

I. INTRODUCTION

Some recent queries about fibre arose on rec.food.veg. To help sort things out a bit, I earlier posted this information on r.f.v. I thought maybe some people here would also be interested.

I've taken most of what follows from Hunt & Groff's text, ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Ted

II. IMPORTANCE OF FIBRE

An adequate intake of fibre has great importance for health as indicated by its demonstrated physiologic effects. Among these are:

- the hypoglycemic effect of soluble fibre - the hypolipidemic effect of soluble fibre - the lowering of serum cholesterol levels. Such a lowering, as we know, presently appears to have a significant benefit in the prevention of atherosclerosis - slowing the absorption of carbohydrate can be very useful to the diabetic in regulating blood sugar levels. - anti-toxic effects. Most international epidemiological studies show an inverse relationship between colon cancer mortality and fibre content of diet. While these studies often fail to disentangle the known effects of fat and energy intake on colorectal cancer, some studies have still found a inverse relationship after these factors have been statistically adjusted for. Besides the anti-toxic effects discussed below, the reduced intestinal transit time is also thought to be a key factor. - apparent reduction or control of gastrointestinal disorders that include diverticular disease, gallstones, irritable-bowel syndrome, inflammatory bowel disease and constipation. - the satiety effect that can help *some* individuals better maintain their ideal body weight (also helps a little with reducing certain dietary utilization of some sugars and fats)

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Van Soest (1984). Some characteristics of dietary fibre and their influence on the microbial ecology of the human colon. PROC. NUTR. SOC., 43:25-33.

Anderson, J.(1986). Fibre and health: An overview. Nutr. Today, 21(6):27-30.

Health & Welfare Canada. (1990). NUTRITION RECOMMENDATIONS: THE REPORT OF THE SCIENTIFIC REVIEW COMMITTEE.

III. KINDS OF FIBRE:

It is important to recognize that various kinds of fibre perform different function and therefore a variety of fibre containing foods should be contained in one's diet. Eating oat bran alone is simply a bad way to get fibre. Indeed, there is some folly to the careless practice of adding large amounts of a single source of purified fibre to the diet. Varied whole plant foods is still the best course to take.

Dietary fibre is derived from solely plant cells, mostly from the plant cell wall. It is NOT to be found in any animal product. Some "fibre" substances include:

cellulose - consists of a polymer chain of glucose units This is the only fibre component with a truly fibrous structure. A major component in vegetable and legume fibres. Also found in most fruits. Fermentability: low in cereals and moderate in legumes hemicellulose - these sugar containing substances are most accessible bacterial enzymes than is cellulose. A major constituent of cereal fibre. Wheat bran in particular largely hemicellulose. Fermentability: moderate-high, very low in raw corn bran. pectin - these polysaccharides are water soluble and gel forming. Found in fruits and to a lesser extent in vegetables. Fermentability: high lignin - this is the primary noncarbohydrate component of fibre and is very inert. Highest in mature root vegetables like carrots or fruits with edible seeds like strawberries. gums - these are hydrocolloids secreded by the plant at injury sites. They are composed of various sugars and sugar derivatives. They also can be highly soluble and gel forming. E.g., guar gum. Fermentability: high mucilages & algal polysaccharides - agar and carrageenan are examples of algal polysaccharides. Agar is a seaweed extract. Because of their "hydrophylic" (literally, water-guarding) properties they are used as stablizers. "Guar", which is a mucilage, are in fact secreted by plant cells to protect the seed endosperm from desiccation. Fermentability: high

Sorry about introducing so many new terms, but it is important to understand that there are DIFFERENT kinds of fibres and they do not all play the same physiologic and nutritional role. Just as not all fats are equal (or even saturated fats for that matter), so too with fibre.

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Health & Welfare Canada. (1990). NUTRITION RECOMMENDATIONS: THE REPORT OF THE SCIENTIFIC REVIEW COMMITTEE.

IV. DEFINITION OF FIBRE

If there is confusion about fibre, it may largely be due to the fact that dietary fibre does not constitute a single entity. Indeed, there is "no universally accepted definition for this food component yet evolved" (Hunt & Goff, 1990).

Dietary fibre has been conventionally defined as those foods which enter the cecum (beginning of the large intestine) unchanged. But which "foods" and what kind of changes? Maybe the most widely accepted definition was proposed by Trowell et al (1976):

"plant polysaccharides and lignin which are resistant to hydrolysis by the digestive enzymes of man"

One problems even with this definition is that it doesn't include all the indigestible residues from food that may reach the colon. Another is that it is predicated on the idea of "undigestability" as a criterion, but some so-called "undigestible" foods (e.g., nonstarch polysaccharides) can undergo fermentation by colonic bacteria thereby producing short-chain fatty acids that can be used for energy by the host. On the other hand, potentially digestible starches in varying amounts will reach the colon in an unaltered state. Most researchers believe that materials such as resistant starch and man-made ingredients should not be considered components of dietary fibre.

It is interesting to note that no longer can the potential energy in fibre be considered totally unavailable to the human body.

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

V. WHAT FIBRE DOES:

Fibre has an effect on the throughout the gastrointestinal tact, beginning in the mouth. Insoluble fibre components (lignin, cellulose and most of the hemicellulose) necessitate greater chewing which in turn stimulates saliva secretion, together which serves as a tooth cleaner. Eat some fruit if you forgot your toothbrush :-)

Some of the more important gastrointestinal responses to the ingestion of fibre include:

- increased fecal bulk - decreased intraluminal pressure - greater frequency of defecation - reduced intestinal transit time - delayed gastric emptying - increased postprandial satiety - reduced glucose absorption - changes in pancreatic and intestinal enzyme activity - increased bile-acid excretion - possible alteration in mineral balances

Different fibre components will, of course, produce these effects in different degrees.

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

VI. HOW DOES COOKING AFFECT FIBRE?

While cooking and kitchen processing is not going to decrease or increase the total amount of major fibres, heat from cooking can make certain "indigestible starches" more digestible. Conversely, what is called "Maillard products can occur (enzyme-resistant linkages between amino acids of proteins and the carboxyl groups of reducing sugars), particularly from baking and frying. Of course, there is debate as to whether or not include such Maillard compounds as components of dietary fibre. Most researchers prefer not to consider as components of fibre either the resistant starch or Maillard compounds.

It is also the case that the size of the particles and/or degree of processing of the foods providing fibre appear to influence the GI response to ingested fibre. For example, coarsely ground bran has a higher hydration capacity than that which is finely ground. Hence, coarsely ground bran increases fecal volume by its water-holding capacity, and it also speeds up fecal passage time through the colon. With respect to emptying food from the stomach, these larger particles slow it down rather than speed it up.

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

VII HOW THE GASTROINTESTINAL TRACT (GI) IS AFFECT BY FIBRE

Important characteristics of dietary fibre with respect to its physiologic role in the GI includes:

hydration capacity absorptive attraction for organic molecules cation exchange capacity fermentability

A. THE UPPER GI

The upper GI is affected more by the gellation effect of pectins and hydocolloids (i.e. the gums, mucilages and algal polysaccharides) than by the hydration effects of cellulose and hemicellulose, irrespective of particle size.

Hydocolloids and pectin reduce the rate of glucose absorption, and also decrease the rate of absorption and/or availability of fats and proteins. The reduction in "apparent protein digestibility" is likely nutritionally insignificant. While some hydrocollids are natural components in beans and certain cereals (e.g., oats and barley), most enter the food supply as additives used in processed food.

This decrease on lipid absorption by fibre is not well understood. Some general effects of fibre on nutrient absorption that have been proposed that could in part account for this decreased absorption (e.g.., blunting of villi in the small intestine, decreased secretion of GI and pancreatic hormones, direct reduction of pancreatic enzyme activity, decreased diffusion rate in the proximal intestine due to an increased thickness of the unstirred water layer, and decreased solute movement within the lumen of the intestine). More specific mechanisms include the lowering of bile acid concentration by their absorption into the fibre.

Pectin and guar gum (12-30 g/daily) have been shown to lower serum cholesterol by 6-15% in normal volunteers. A number of mechanism have been proposed for the blood cholesterol lowering effects of fibre. For instance, when fibre absorbs bile acids it thereby removes some bile from circulation. A decrease in bile acids returned to the liver would cause diversion of some cholesterol from lipoprotein synthesis to the synthesis of bile acids, thereby lowering serum cholesterol. Another proposed mechanism involves the fibre stimulated shift of bile acid pools toward chenodexoycholic acid -- which inhibits cholesterol synthesis. It is thought that the chenodeoxycholate alerts the liver through inhibition of a key enzyme that no more cholesterol is needed for bile acid synthesis. Still, neither of these proposed mechanisms fully explains the degree to which fibre can lower serum cholesterol.

Another effect of fibre is its influence on cation aDsorption, particularly calcium, zinc and iron. Not only do the cationic bridges formed by fibre serve as a mechanism for the aDsorption of bile acid and fats, but also of minerals. This can ultimately help or hinder mineral absorption, depending upon the fermentability (or its accessibility to bacterial enzymes) of the fibre when it enters the lower GI.

B. THE LOWER GI

It is here where most of the signification action of dietary fibre occurs. Fermentation of food by colonic anaerobes make available to the body much of the energy of undigested foods reaching the cecum. This has indeed been an overlooked source of energy. For instance, as much as 10% to 15% of the carbohydrates we eat in the West may be fermented in the colon. In general, from 40-95% of dietary fibre is fermented by intestinal flora.

Certain fibres, like the plant gums (and any starch that has passed undigested into the cecum), are rapidly fermented by various anaerobic bacteria residing in the colon. The main metabolites produced by this rapidly fermentable fibre are some short-chain fatty acids (acetic, butyric & propionic acids). By- products of this fermentation are hydrogen, carbon dioxide and methane. Keep matches away! These gases are excreted as flatus or are expired by the lungs.

These fatty acids produced by fermentation are rapidly absorbed or are used by the epithelial cells of the colon for energy. The propionic acid produced from fibre may also contribute to the cholesterol lowering effect of certain fibres by acting to inhibit a rate-limiting enzyme (HGG CoA reductase) in the synthesis of cholesterol in the liver.

The more slowly fermentable or non-fermentable fibres than the gums are particularly helpful for overcoming constipation by increasing fecal bulk (1) water absorption and/or (2) promotion of microbe proliferation. Slowly fermentable fibres, like cereal fibres, are particularly valuable in causing microbial proliferation. Bacterial cells form part of the fecal mass and provide moisture. The volatile fatty acids produced by the bacteria acidify the colonic content, act on the musosa and, following absorption, modify the lipid metabolism. Due to these two factors, it has been shown that for every extra gram of cereal fibre stool weight gains an extra 2 to 9 grams!

Wheat bran, for instance, can absorb 3 times its weight in water thereby producing a much softer, bulkier stool. The large wheat bran particles take a curly shape on fermentation, constituting microenvironments in the distal colon, and providing a physical resistance against the removal of interstitial water and dispersed gases, thus counterbalancing the absorptive capacity of the colon. The resulting decrease in fecal density prevents impaction and constipation. The threshold volume is rapidly attained in the rectum triggering defecation, thus limiting the opportunity for reabsorption and hardening of the intestinal contents.

It should be noted that reducing particle size eliminates this effect since small particles retain non-solid components less effectively. Coarse bran will reduce colon segmenting activity and intraluminal pressure, normalizes slow transit time (40-150 hours) to about 20 hours, increase fecal weight (4 times more than fine bran and 7 times more than oat bran).

Interestingly, rice bran has been found to be even more effective in increasing fecal bulk, frequency of defecation and reduced intestinal transit time. Now only are these responses are particularly important in the prevention of constipation, but they may be advantageous in the management of irritable colon and diverticular disease).

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Dreher, M. (1987). HANDBOOK OF DIETARY FIBER: AN APPLIED APPROACH.

Spiller, G. (1986). CRC HAND BOOK OF DIETARY FIBER IN HUMAN NUTRITION. CRC Pr.

Kay & Truswell (1977). Effect of citrus pectin on blood lipids and fecal steroid excretion in man. AM. J. CLIN. NUTR., 30:171-5.

VIII. DETOXIFICATION

Microbial proliferation and excretion is not only important for increasing fecal volume but is thought to play an important role as a "DETOXIFICATION MECHANISM".

In works as follows. Increased microbial cell synthesis would scavenge degradable nitrogenous substances and thereby sequester those substances into the microbes themselves, which in turn are eventually excreted.

The downside to this function is that excessive microbial proliferation may decrease mineral absorption. What is thought to happen is that certain essential elements may become bound in the microbial cells themselves, to then be excreted rather than absorbed. In contrast, the more rapidly fermentable fibre components release their calcium, zinc and iron for absorption by the colon as fermentation occurs.

Fibre from fruit and vegetables is less effective in increasing fecal bulk since much of their fibre consists of rapidly fermentable pectin and the less microbial promoting cellulose. Hence, for every 1 gram extra of vegetable fibre consumed, only about a 1.9 gram increase in fecal weight occurs. In contrast to cereal fibre, fruit and vegetable fibre which contain considerable amounts of pectin, can delay gastric emptying and reduce glucose absorption because of its gellation quality.

It would seem that both fast and slow fermentable fibres should be consumed. Again, it is not simply the amount of fibre that should be important, but also that fibre from VARIOUS sources be ingested so that a varied selection of fibre components are part of one's diet.

A comparison of the levels of mutagens in the faeces of 12 omnivores, 6 vegetarians and 6 vegans showed even with this small sample significant lower loves in the vegans and vegetarians. volunteers showed. Another study with volunteers on 20-day experimental diets showed that vegan diets produced the lowest concentration of bile acids, and of course cholesterol, in their faeces. Apparently a high concentration of bile acids or cholesterol in faeces is associated with risk of colorectal cancer.

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Van Soest (1984). Some characteristics of dietary fibre and their influence on the microbial ecology of the human colon. PROC. NUTR. SOC., 43:25-33.

Kuhnlein et al. (1981). Mutagens in feces from vegetarians and non-vegetarians. MUTATION RES., 85:1-12.

Van Faasen et al. (1987). Bile acids, neutral steroids, and bacteria in feces as affected by a mixed, a lacto- vegetarian, and a vegan diet. AM. J. CLIN. NUTR., 46:962-67.

IX. POTENTIAL ADVERSE EFFECTS

There are few reports of adverse effects on the gastrointestinal tract directly related to fibre. Excessive intakes of particulate fibres (e.g., cereal fibres), for instance, have been reported to produce intestinal obstruction in susceptible individuals. In general, more finely ground fibre (even from wheat bran) may cause difficult or uncomfortable defecation. The mean particle size of fibre in ready-to-eat breakfast cereals varies from 350um to above 1mm. The number of particles less than 150um appears to be negligible.

Excessive fibre consumption may cause a transient fluid imbalance when the fibre consumed absorbs a lot of water.

An excessive intake of nonfermentible fibre could make for a negative mineral balance, particularly among infants, children, adolescents, and pregnant women whose mineral needs are of course relatively greater than for adult men or nonpregnant woman. If the intake of calcium, zinc and iron is marginal, then excessive fibre could exacerbate the already low intake of these minerals. The nutrition recommendations from the 1990 Canadian scientific review committee concluded that "evidence of mineral binding is unequivocal but it is doubtful whether such effects are of any nutritional importance in the context of an adequate diet".

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Health & Welfare Canada. (1990). NUTRITION RECOMMENDATIONS: THE REPORT OF THE SCIENTIFIC REVIEW COMMITTEE.

Southgate, D. (1987). Minerals, trace elements and potential hazards. AM. J. CLIN. NUTR., 45:1256-66.

IX. POTENTIAL ADVERSE EFFECTS

There are few reports of adverse effects on the gastrointestinal tract directly related to fibre. Excessive intakes of particulate fibres (e.g., cereal fibres), for instance, have been reported to produce intestinal obstruction in susceptible individuals. In general, more finely ground fibre (even from wheat bran) may cause difficult or uncomfortable defecation. The mean particle size of fibre in ready-to-eat breakfast cereals varies from 350um to above 1mm. The number of particles less than 150um appears to be negligible.

Excessive fibre consumption may cause a transient fluid imbalance when the fibre consumed absorbs a lot of water.

An excessive intake of nonfermentible fibre could make for a negative mineral balance, particularly among infants, children, adolescents, and pregnant women whose mineral needs are of course relatively greater than for adult men or nonpregnant woman. If the intake of calcium, zinc and iron is marginal, then excessive fibre could exacerbate the already low intake of these minerals. The nutrition recommendations from the 1990 Canadian scientific review committee concluded that "evidence of mineral binding is unequivocal but it is doubtful whether such effects are of any nutritional importance in the context of an adequate diet".

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Health & Welfare Canada. (1990). NUTRITION RECOMMENDATIONS: THE REPORT OF THE SCIENTIFIC REVIEW COMMITTEE.

Southgate, D. (1987). Minerals, trace elements and potential hazards. AM. J. CLIN. NUTR., 45:1256-66.

X. FIBRE INTAKE AND VEGETARIANS

We've seen that a varied selection of fibre should be ingested, now the question is how much?

The recommendation for the general population has ranged from 20 to 40 grams/day, and may up to 50 gram/day for hypercholesterolemic individuals. The National Health and Nutrition Examination Survey (1976-1980) showed that the consumption of fibre was lower than expected. Young white males (19 to 29 years) had the highest intake of 13 g/d, while older black males (55 to 74 years) and middle-aged black females (30 to 54 years) had the lowest intake averaging 7.4 g/d.

Presumably people are consuming more fibre since that survey was taking, but it is likely that the greater majority of people are still not consuming enough.

A recent survey (Carlson, 1985) of vegetarians have shown:

vegans 45 g/d vegetarians in general 38 g/d omnivores 22

Rather than simply "adding" refined fibre to one's current diet, the better approach is to thinks in terms of a dietary change of foodstuffs that simply include foods with more fibre and excludes foods (like meat or dairy products) that have none. Vegetarians naturally do well in this respect :-)

If you think that you need more fibre in your diet, then consider a dietary change that includes:

1. a greater consumption of fibre-rich legumes 2. increased consumption of fresh fruits and vegetables 3. replacement of refined cereals and flour products to ones made by whole grains.

Vegetarians have no problem in getting enough fibre, but some may not be getting a great enough VARIETY of fibre due to an omission or shortage one or two of the above three areas. A bad practice is to simply consume large amounts of a single source of purified fibre. Better to simply eat a variety of fibre by simply eating a variety of whole foods. By ensuring that at least 60% of energy is in the form of whole, complex carbohydrates the resulting dietary patter will perforce increase present intakes of dietary fibre. Vegetarians, as we have seen, do well in this regard. :-)

~References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM, 1990.

Carlson et al. (1985). A comparative evaluation of vegan, vegetarian and omnivore diets. J. PLANT FOODS, 6:89-100 Lines: 106

XI. DIETARY FIBRE OF SOME COMMON FOODS

Because definitional problems there are different ways to measure fibre depending own what is being tested. Earlier tables only measure for crude fibre (cellulose & lignin) and did not measure for the "noncellulosic polysaccharides like pectin, hemicellulose, and other polysaccharides (e.g., gums, mucilages and algal polysaccharides). The figures for total dietary fibre in the following table may be larger than some other tables you may have, but that may be due simply the following table being more inclusive in what is being measured as "fibre".)

Dietary Fiber Content of Some Common Foods =========================================

Total Cellulose Noncellulose Lignin dietary poly- fiber saccharides (g/100g) (g/100g) (g/100g) (g/100g)

bread white 2.72 .71 2.01 trace whole meal 8.5 1.31 5.95 1.24

Vegetables broccoli 4.10 .85 2.92 .03 beans, baked 7.27 1.41 5.67 .19 cabbage (boiled) 2.83 .69 1.76 .38 corn (canned) 5.68 .64 4.97 .08 lettuce 1.53 1.06 .47 trace onions (raw) 2.10 .55 1.55 trace peas (raw, frozen) 7.75 2.09 5.48 .18 carrots (boiled) 3.70 1.48 2.22 trace tomato (fresh) 1.40 .45 .65 .30

Fruits apple (flesh) 1.42 .48 .94 .01 apples (peels 3.71 1.01 2.21 .49 banana 1.75 .37 1.12 .26 peach (flesh & skin) 2.28 .2 1.46 .62 pear (flesh) 2.44 .67 1.32 .45 pear (peels) 8.59 2.18 3.72 2.67 strawberries 2.12 .33 .98 .81

Preserves strawberry jam 1.12 .11 .85 .15

Peanuts 9.30 1.69 6.40 1.21 peanut butter 7.55 1.91 5.64 trace (adapted from Southgate et al., A guide to calculating intakes of dietary fiber. J. HUM. NUTR., 1976, 30:303-13)

To put things in more practical terms, consider again the above foods but this time in terms of the kinds of actual servings on is more likely to consume at any one meal.

Dietary Fiber Content of Some Common Foods =========================================

Serving Serving Total dietary size weight fiber/serving (g) (g)

bread white 1 slice 23 .63 whole meal 1 slice 23 1.96

Vegetables broccoli 1/2 cup 73 2.99 beans, baked 1/3 cup 85 6.18 cabbage (boiled) 1/2 cup 73 2.07 corn (canned) 1/2 cup 83 4.72 lettuce 1/2 cup 55 .84 onions (raw) 9/4" onion 100 2.10 peas (raw, frozen) 1/2 cup 73 5.66 carrots (boiled) 1/2 cup 75 2.78 tomato (fresh) small tomato 100 1.40

Fruits apple (flesh) 1 medium apple 141 2.00 apples (peels 1 medium apple 11 .41 banana 6" banana 100 1.75 peach (flesh & skin) 1 medium peach 100 2.28 pear (flesh) 1/2 medium pear 87 1.12 pear (peels) 1/2 medium pear 11 .95 strawberries 10 large berries 100 2.12

Preserves strawberry jam 1 Tbsp 20 .22

Peanuts 1 Tbsp 9 .84 peanut butter 1 Tbsp 15 1.13 (adapted from Southgate et al., A guide to calculating intakes of dietary fiber. J. HUM. NUTR., 1976, 30:303-13)

Cheers, ted