The role of Dietary Fiber in Human Disease
Brad A. Pasternak, MD
Cincinnati Children's Hospital
Cincinnati, OH
Robert Karp, MD
Department of Pediatrics
SUNY-Downstate Medical Center, Brooklyn, NY
PRETEST
Q1. Best Answer. Which of the following food provides the most dietary fiber per serving?
A. Wheat Crackers
B. Corn Bran Cereal
C. Peanuts
D. Dried Apricots
Q2. Best Answer. A mom asks, "what is the recommended amount of fiber per day for her 10-year-old son?"
A. 25g/day
B. 20g/day
C. 15g/day
D. 10g/day
Q3. Best Answer. Which of the following diseases has not been shown to be associated with decreased fiber intake?
A. Colorectal Cancer
B. Diabetes Mellitus
C. Coronary Artery Disease
D. Rheumatoid Arthritis
Q4. Best Answer. What is a possible adverse effect of excess fiber intake?
A. Zinc Deficiency
B. Night Blindness
C. Decreased Seizure Threshold
D. Psychosis
OBJECTIVES:
On completion of this module residents will:
1. Become aware of the structure and classification of dietary fiber
2. Understand the functional properties of dietary fiber
3. Recognize the deficiency of Western diets in dietary fiber and age-specific requirements
4. Appreciate the health benefits of adding fiber to a fiber-deficient diet
a. Realize possible negative effects of excess fiber
b. Appreciate the relationship of fiber to so-called "western diseases"
FACILITATOR PREPARATION:
The facilitator should review the section in Pediatric Nutrition Notes on fiber (under the macronutrient section), glycemic index, and prevention of nutritional related disorders later in life. Denis Burkitt's Western Diseases: Their Dietary Prevention and Reversibility (Humana Press Inc, 1994) and his essay titled Dietary Fiber: The Medical Profession Demurs (Nutrition-Today, 1976) provide the initial groundwork for the implications of dietary fiber on human health. Burkitt's original emphasis on lack of fiber in the diet as a single cause for deterioration in health with age in economically developed societies was overstated; Nevertheless, his thesis is worthy of review in the context of an interdependent, multi-factorial construct for this phenomenon.
Read also:
Kay WM. Dietary Fiber. J Lipid Research, 1982;23: 221-242.
A supplement dedicated exclusively to dietary fiber in children Pediatrics, 1995;96S: 985-1029.
American Academy of Pediatrics. (YEAR) Pediatric Nutrition Handbook, 5th Edition RE Kleinman ed. AAP Washington DC.
These should be appreciated with an emphasis on the importance in preventing diseases of adulthood through diet modification during childhood, with special focus on the epidemiological basis for these arguments brought up by Burkett and others.
INTRODUCTION:
Dietary fiber is a significant determinant of developing human disease as a result of its functional properties, and as observed epidemiological,. In fact, its effect could be traced back to the Book of Daniel in the Old Testament. Daniel asked of King Nebuchadnezzar's prince not to feed him meat and wine, but a diet of pulses (cereals and legumes). A feeding study was than initiated which illustrated the beneficial effects of fiber over meat. Hippocrates also recognized the need for fiber in maintaining bowel regularity.
Dietary fiber was initially defined by its chemical properties. Burkett and Trowell redefined fiber as the complex carbohydrate in diet from plant sources that escapes small bowel digestion and, thus, reaches the colon in tact. Carbohydrate left behind after digestion has become the accepted definition.
TEACHING CAPTION: Many forms of dietary fiber are also composed of minor components serving functional roles (phytates, cutins, saponins, lectins, waxes, silicon). In addition, there are related compounds that fit the accepted definition, but differ in structure. (from: The Pediatric Nutrition Handbook, p254)
Level of solubility refers to fermentability of these compounds, which ultimately impacts their functional capabilities. Thus, pectic substances are highly soluble and completely fermented by colonic bacteria. Fruit and seeds serve as a main source of pectic substances. The endogenous colonic bacteria also metabolize gums and mucilages, however they are of limited palatability. These consist of oat bran, guar, and psyllium seed. Lignins are the main component of the insoluble fiber group. In fact, they are the most resistant of all to enzymatic degradation. Whole cereals and vegetable peels serve as main sources of lignins. Most fiber-rich foods contain a mixture of soluble and insoluble fibers.
Whole grains are a well-known source of dietary fiber worth discussing. These substances are also composed of resistant starches, trace minerals, vitamins, phytoestrogens and antioxidants. Major cereal grains are wheat, rice, and corn while minor grains are oats, rye, barley, sorghum, and millet. Poor palatability of these substances has resulted in the development of a refining process to make these products more desirable and marketable. With this process, however, dietary fiber, vitamins and all beneficial components are lost.
E the participant assumes appropriate for the individual being portrayed.
CASE STUDY
AJ is a 6 year old boy, born in Manhattan and living on the Upper West Side. He is currently in 1st grade and developmentally appropriate. His diet consists of lots of "junk food"- potato chips, cheese doodles, and cookies. His favorite food is pizza and he also enjoys hamburgers. He, unfortunately, despises fruits, vegetables and wheat. He also distastes pasta. AJ weighs 75lbs (>95%) and suffers from being bullied in school secondary to his obesity. In addition, he complains of hard, painful, and infrequent stooling causing him to withhold even further. His family history is significant for coronary heart disease. His father suffers from Type II diabetes, hypercholesterolemia and hypertension. His grandfather also suffered from these illnesses, as well as diverticulitis. His grandmother passed away from cancer. Mrs. J has come to your office for some counseling about his weight, his constipation, and his chances (at this rate) of "turning into his father" from a health-perspective.
Resident activity. Include AJ in groups with a resident, one or both parents, a teacher, a teasing other child. Rotate roles through the case. Engage with each question on group before proceeding. Answer question within your group with the knowledge
Q1. What are the best sources of dietary fiber?
A1. Dietary fiber's effect on health and human disease is the result of various functional properties, especially its fermentability, its water holding capacity, and its adsorption of organic materials. This table shows the fiber content of serving portions of food
Table 2. Fiber Content of Various Foods
TEACHING CAPTION: Bran from the surface coating of cereal grains provides substantially more fiber than any other source. This tends to be insoluble and effective in increasing stool weight. Fibers from fruits and vegetables are also important as they provide both soluble and insoluble fiber. [See Pediatric Nutrition Handbook 5th Ed pgg 1128-1130]
The Case Continues:
AJ's diet is significantly deficient in fiber. He did enjoy Bran cereal in the morning, which provided him with some strength and energy to fight off the bullies in school! "
Q2. What factors influence the extent of degradation of dietary fiber in the colon?
A2. Extent of degradation in the colon is dependant on the nature of the colonic flora, the transit time through the colon, and the fiber composition of the diet. Colonic transit time exerts its effect by determining the duration of contact with endogenous microflora. Insoluble fibers, though non-fermentable, serve to increase stool bulk and decrease colonic transit time. This indirectly functions in determining the fermentability of soluble fibers. Yes, high fiber diets do make children gassier!
Figure 2.
TEACHING CAPTION: The products of fermentation are Short Chain Fatty Acids (SCFA) such as butyrate, propionate, and acetate, hydrogen and methane. The methane and hydrogen production is responsible for the increased flatulence noticed with an increase in dietary fiber.
Part of AJ's problems he experienced in school was his constipation, which could be improved by increasing his fiber intake. This, however, will increase the amount of flatulence and possibly lead to more problems with the bullies.
Q3. What determines the saturation capacity of dietary fiber (its ability to hold water)?
A3. There is an interaction between the structural and chemical characteristics of the fiber with the pH and electrolyte concentration of the surrounding environment. It is the physical structure of fiber that is the most important in determining how much water is held for transport into the large colon and the stool.
Dietary fiber also has the ability to bind various compounds throughout the digestive tract. The advantage of this is seen in fiber's affect on hypercholesterolemia. The most potent bile acid sequesterant is actually lignin, whose function is dependent on the pH of the surrounding medium (low pH increases absorption). Mucilaginous fibers also show a high affinity to bile acids. Bile acids may become modified through direct interaction with the fiber or indirectly through bacterial modification influenced by the fiber content.
Q4. What are some possible criticisms against the effect of fiber on human disease?
A4. When comparing developing countries to the developed society, Denis Burkett proposed a hypothesis relating the lack of fiber and refining of grains to increased morbidity and mortality in the Western world. This theory has been met with much criticism in the contemporary literature founded upon the notion that fiber is necessary, though not sufficient to prevent degenerative diseases of adult life.
Studies on this subject have been fraught with numerous inconsistencies stemming from a lack of internationally accepted definition from determining the fiber content of foods. As discussed, fiber products contain many other factors, such as antioxidants, which may affect disease more than the fiber itself and were not controlled for. In addition, many studies do not separate consumption into refined versus whole grain intake; however, those studies that have taken this into account have shown positive effects of dietary fiber. Finally, many studies were performed retrospectively requiring recall of dietary history that is a likely source of bias.
Q5. Define the appropriate role for fiber
CAPTION: This is a list of the conditions studied most with regards to fiber playing a significant role. There are other conditions, not listed, that have not shown convincing evidence or have not had sufficient investigation.
Q6. What is the physiology behind constipation?
Figure 2. Pathophysiology of Constipation leading to Stool Retention and Encopresis.
TEACHING CAPTION: The mechanism by which fiber increases stool bulk is dependant on fermentability. Water Holding effect is exhibited by fiber itself and by microorganisms
The average stool weight in Western society is about 100 grams a day. This is about 200grams a day less than cultures ingesting high fiber diets. Fiber increases stool bulk and decreases lower GI motility due to its hydration capacity, production of osmotically active metabolites and induction of bacterial growth. Contrary to popular belief, both soluble and insoluble fiber also maintains soft stools and establishes regular defecation patterns.
Insoluble fibers are more effective at laxation than soluble fibers and result in increased stool bulk. There is a 4.1g increase in stool weight for every gram of wheat bran fiber, as compared to 2.9g increase in stool weight for every gram of fruit and vegetable fiber.
The Hydrated Sponge Theory describes a mechanism by which insoluble fibers trap water within the gastrointestinal tract and alters the interaction between bacteria and solute, thus increasing stool bulk by increased water content mainly due to increased particle size. The soluble fiber effect on stool bulk is through bacterial proliferation and growth.
Q8. What are the direct consequences of lack of dietary fiber?
While Burkitt extended his list of consequences to include almost all of the degenerative diseases of Western Civilization, there are at least four that are a direct result of deficient fiber content in the diet.
As Burkitt once said, " Never diagnose appendicitis in an African who does not speak a Western Language."
Burkitt suggests that lack of fiber affects bowel function by requiring substantially more force from the colonic wall to propel hard, small stool through the colon. In diverticular disease, there is resultant thickening of the circular muscle of the colon and contraction of the teniae coli leading to redundant mucosal folds partially obstructing the lumen. Diverticulae are mucosal hernias where the bowel wall was weakened secondary to prolonged exposure to increased pressure. Burkitt recognized the decreased prevalence in populations ingesting more fiber and described this theory relating to intraluminal pressure.
TEACHING CAPTION: As Burkitt said, "Your typical constipated Englishman has a stool weight of 100 grams and a Transit Time of 3 days. Your "unconstiptated" English vegetarian has a stool weight of 200 grams and a TT of 1 day. Give our constipated Englishman two heaping teaspoons of wheat bran and sufficient water and you will decrease TT to 1 day and increase stool weight to the vegetarian's level." Excess fiber as found in African's with too much fiber in their diet for adequate energy intake has a stool weight of 500 grams and a TT of 12 hours. [The comments are repeated from RK's memory of a delightful lecture by Professor Burkitt (a proud Scotsman). It was heard about thirty-five years ago. see Slavin JLNutrition Today. 2006;41 (4):180-184]
FIGURE 3. Pathophysiology of Diverticulosis
TEACHING CAPTION: Studies supported his hypothesis of abnormally e]elevated intraluminal pressure in patients with this condition; however, not always associated with muscle hypertrophy and some asymptomatic patients found to have increased intraluminal pressure.
The treatment of choice for this condition is a high fiber diet, which results in soft, bulky stool resulting in low intraluminal pressure and relief.
Dr. Burkett also recognized an association between a decreased incidence of colorectal carcinoma in high fiber consuming populations. His initial hypothesis was that increased stool bulk dilutes potential carcinogens and decreases transit time through the gut, thus permitting less contact between carcinogens in the lumen and the gut mucosa. This challenge has been challenged and may be a concomitant, rather than causative, finding.
Q9. What are some other possible mechanisms explaining the effect of fiber on Colon Cancer?
TEACHING CAPTION: Some explanations involve the reduced solubility of bile acids by decreasing fecal pH (bile acids, especially secondary, have tumor promoting activity). Decreased glycemic index and insulin response to dietary fiber (hyperinsulinism found to stimulate growth of colonic tumors through stimulation of IGF and insulin receptors on colonic cancer tissue) and increased stool bulk have also been postulated as possible mechanisms.
Q10. What are dietary fiber's effects on insulin response and cholesterol metabolism?
The water-holding capacity, increased bulk and increased viscosity all interfere with the mixing of intestinal contents and nutrient diffusion ultimately slowing the upper GI transit rate. This, in turn, is associated with a blunting of the insulin response to carbohydrates. Mucilaginous fibers have the most potent affects on glucose metabolism. Guar decreases the release of gastric inhibitory peptide and flattens the postprandial glucose and insulin curves further.
Based on this physiology, it is understood how fiber plays a role in both prevention and control of Diabetes Mellitus. Improvements in diabetic control and reduction in both insulin and sulfonylurea requirements are seen in mild and moderate diabetics on high fiber diets. In fact, even the risk for developing Diabetes Mellitus type II is lower with higher fiber intake. It is a food source with low glycemic index, characterized by its effect on glucose after ingestion, and has become a staple in diabetic diets.
The prevention and modification of hypercholesterolemia has been more extensively studied.
Cholesterol-lowering effects are most pronounced with soluble and viscous fibers. It is limited to LDL with minimal changes observed in HDL and triglyceride concentrations. The greatest effect on LDL cholesterol is in subjects with initially higher lipid concentrations than those with normal baseline values. Guar has been used in clinical settings to reduce cholesterol in patients with type II hyperlipidemia. The question still remains whether the effect is due to fiber itself, or the biological constituents of these fiber sources.
Decreasing the incidence of diabetes mellitus, hypertension and hypercholesterolemia undoubtedly has an effect on the incidence of coronary artery disease. These are all modifiable risk factors affected by high fiber diet. In fact, epidemiological evidence exists examining the decreased incidence of CAD in African, Indian and Mediterranean cultures that largely consume high fiber diets. Even after adjusting for obvious confounding variables such as BMI, age, smoking, and vitamin supplementation, the inverse relationship of high fiber and myocardial infarction continues to persist. The strongest association with decreased risk seems to be with cereal fibers, which intuitively is surprising considering this particular source lack of effect on cholesterol reduction
Q11. Does lack of fiber have an impact on obesity?
The greatest epidemic affecting Western society today is obesity. This can be physiologically defined as an imbalance between energy intake and expenditure. Again, the prevalence of obesity is much less in developing countries where dietary fiber intake is higher.
TEACHING CAPTION: Overall, energy intake is suppressed and feeling of fullness is prolonged once fiber is introduced.
In fact, it has been shown that as little as 5.2g of crude fiber added to a meal reduces overall food intake and increases satiety. It has also been shown that weight loss diets were easier to adhere to once fiber was included in the regimen.
The case continues
Mrs. J says, " This is all very interesting doctor, but just what is it you want me to do?
Question ?? What should we recommend to parents and children with respect to dietary fiber intake?
Let the resident teams come up with recommendations and report them 1 by one without repeats until they run out of ideas. They repeat below.
Many investigations have evaluated fiber consumption across
Western culture. The unanimous conclusion is clearly a state of deficiency and inadequacy for promotion of health and prevention of disease. The National Health and Nutrition Examination Survey (NHANES II) studied dietary intake over 1976-1980 and indicated that dietary fiber intake for 2- to 19- year-olds averaged 12.4g/d with other surveys showing similar results. The Bogalusa Heart Study spanned a 14-year period that also indicated no increase in dietary fiber over those years with a mean of 12g/ day. The American Health Foundation has recommended the age + 5 criteria for dietary fiber intake. This value was determined based on optimal consumption to receive all the health benefits of dietary fiber without the possibility of possible negative effects.
TEACING CAPTION: "KISS" [keep it simple, stupid] is best. An intake of 5 grams dietary fiber plus age in years up to the age 20-25 seems to be the most appropriate as a functional definition of need [hyperlink to nutritional assessment]. Constipation before two years of age requires a careful assessment of the individual child.
Q12. What are the POTENTIAL negative effects of dietary fiber consumption?
Prasad, in his studies of growth retarded, sexually immature children in Egypt and Iran, showed the dangers of an excessively high fiber diet. The caloric density of this diet studied was simply too low and the fiber absorbed zinc and iron resulting in severe deficiencies.
Water-soluble fiber contains numerous "antinutrients" (phytates, phytoagglutinins, saponins, and tannins) that interfere with the absorption of minerals and trace elements.
Phytates form insoluble compounds with calcium, iron, copper, magnesium, phosphorous, and zinc. It is destroyed during the leavening process, so its affect on mineral absorption only remains a problem in countries consuming large amounts of unleavened bread (Middle East).
In Western society, where mineral and nutrient intake is sufficient, the population is able to compensate for decreased absorption with increased dietary fiber. In fact, vegetarian children have the highest amount of dietary fiber consumption, yet do not experience mineral deficiencies in western culture. Studies have shown that the body is able to adapt to continued intake of high fiber by a physiologic mechanism restoring nutrient balance within weeks. Caution, however, is indicated in preschool children, growing adolescents and poverty-stricken individuals where need is greater and resources may be scarce.
Watch the age restriction: Infants and toddlers who are constipated are quite likely to have a metabolic or anatomic problem. A consideration of Cystic Fibrosis or Hurschprungs must be made. The
SUMMARY
Dietary fiber is defined by its ability to escape upper gastrointestinal enzymatic digestion and present unchanged to the small intestine, where it is susceptible to bacterial degradation. It is classified by its solubility and this further affects its functional properties. Fibers that are insoluble play a greater role in increasing stool bulk and laxation [what is 'laxation`], while soluble fibers are involved in cholesterol lowering. Most fiber sources, contain both insoluble and soluble fiber.
Through observations of developing societies, it was postulated that the high fiber intake is protective against various disease states. Western society does not consume an adequate amount of fiber and these diseases are more widespread among this population. Lack of fiber has been directly implicated in appendicitis, hemorrhoids, varicose veins and diverticulitis. Evidence also suggests high fiber diets may be protective against colorectal cancer, diabetes mellitus, hypercholesterolemia, coronary artery disease, and obesity.
Ideal fiber intake has been suggested as 1 gram for each year of age plus five up to 20 grams for a woman and 25 for a man. These recommendations begin at two years of age and include sufficient water for the fiber to absorb.
There is concern over the possibility of adverse effects secondary to dietary fiber over consumption. Excess fiber intake results in an overall decreased caloric intake that may lead to sexual immaturity. In addition, it may interfere with nutrient absorption, thus leading to deficiencies. This is not a concern in Western society, however, where there is adequate intake of all nutrients affected.
POST TEST
Q1: Best Answer. Which of the following food provides the most dietary fiber per serving?
A. Wheat Crackers
B. Corn Bran Cereal
C. Peanuts
D. Dried Apricots
Answer = B
Q2: Best Answer. A mom asks, what is the recommended amount of fiber per day for her 10-year-old son?"
A. 25g/day
B. 20g/day
C. 15g/day
D. 10g/day
Answer = C
Q3: Best Answer. Which of the following diseases has not been shown to be prevented by increasing fiber intake?
A. Colorectal Cancer
B. Diabetes Mellitus
C. Coronary Artery Disease
D. Rheumatoid Arthritis
Answer = D
Q4: Best Answer. What is a possible adverse effect of excess fiber intake?
A. Zinc Deficiency
B. Night Blindness
C. Decreased Seizure Threshold
D. Psychosis
Answer = A
Q5: Best Answer. T or F. The accepted definition of dietary fiber is Carbohydrate left behind after digestion.
Answer = True
Q6: Best Answer. How does dietary fiber impact obesity?
A. Increases Metabolism
B. Promotes Satiety
C. Decreases work of chewing
D. Plays no role
Answer = B
Q7: Best Answer.
A. Appendicitis is a disease with proven correlation to amount of fiber in diet
B. Soluble fibers are the main class increasing stool bulk
C. The increased incidence of diseases in Western Society is a result of increased dietary fiber intake
D. Most fruit and vegetables are composed of only soluble fiber
Answer = A
Q8: Best Answer. True or False. Short Chain Fatty Acids play a role in preventing colorectal cancer by stimulating the conversion of primary to secondary bile acids.
Answer = False
REFERENCES:
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Anderson JW. Whole grains protect against atherosclerotic cardiovascular disease. Proc Nutr Soc 2003; 62: 135-42.
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Brown L, Rosner B, Willett WW, Sacks FM. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr 1999; 69: 30-42.
Burton-Freeman, B. Dietary Fiber and Energy Regulation. J Nutr 2000; 130: 272S-75S.
Dwyer JT. Dietary Fiber for Children: How Much? Pediatrics 1995; 96: 1019-22.
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Greger JL. Nondigestible Carbohydrates and Mineral Bioavailabiltiy. J Nutr 1999; 129: 1434S- 35S.
Hillemeier C. An Overview of the Effects of Dietary Fiber on Gastrointestinal Transit. Pediatrics 1995; 96: 997-99.
Kay, RM. Dietary Fiber. Journal of Lipid Research 1982; 23: 221- 41.
Kimm SY. The Role of Dietary Fiber in the Development and Treatment of Childhood Obesity. Pediatrics 1995; 96: 1010-14.
Kleinman, RE. Ed. Pediatric Nutrition Handbook- fifth edition. American Academy of Pediatrics 2004; 1128-30.
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Lupton JR. Microbial Degradation products influence colon cancer risk: the butyrate controversy. J Nutr 2004; 134 (2): 470-82.
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ANNOTATED PRETEST ANSWERS:
A1. The Answer is B; When considering fiber content, remember that the branny coating of grains is, per gram, by far the most fibrous. All the other foods listed are of moderate fiber content. There's something to be said for reading labels to answer a question like this.
A2. The Answer is C; A formulae for calculating fiber need is "age in years plus 5 = daily fiber need up to 20grams/day for a woman and 25 grams/day." Answers "A" and "B" levels are too high as a diet that is too fibrous will inhibit growth of a child. The 10 gm/day for a 10 year old may be typical, but it is too little.
A3. The Answer is D; The associations between fiber content and cancer, heart disease and diabetes may be indirect, but whether life style or diet are the critical factors affecting onset of these diseases, they exist. No associations between immunologic diseases and fiber content of the diet have been shown. Were the question "Which of the following been shown as a cause for…, the answer could have been appendicitis, varicose veins, hemorrhoids, or diverticulosis." With these conditions the relationship is direct.
A4. The Answer is A; Diets with excess fiber limit growth because they contain too little energy per gram of food, and fiber at high levels binds mineral nutrients. A syndrome of high fiber diet, dwarfism, iron and zinc deficiency, and lack of sexual development has been described in both Egypt and Iran. It is said to occur because the diet contains too much unleavened, unrefined bread.
