INTRODUCTION:

Colorectal cancer (CRC) is one of the most commonly occurring cancers in the world.¹As the third most common cancer and the fourth leading cause of cancer death in the world, CRC caused about 694, 000 deaths in 2012, with over 1.4 million new cases diagnosed globally.² It is a leading cause of morbidity and mortality in industrialised nations.³Colorectal cancer is still a major public health burden.⁴ Incidence and mortality rates are also rapidly increasing in many previously low-risk countries.⁵Etiological studies have shown that diet is a very important factor in colorectal carcinogenesis.⁶Risk of CRC decreased by 10% (95% confidence interval [CI], 6%–14%) per 10-g/day increase in dietary fibre intake.⁷

Dietary fibre might be a protective factor for colorectal cancer.⁸ Several dietary components coming primarily from fruits, vegetables, cereals, and legumes in the diet may decrease the risk of colorectal cancer.⁵Fiber-rich foods, such as their low calorie density and their high content of cancer-inhibiting phytochemicals, may explain this beneficial effect. The protective effect of fiber is confirmed, the specific types of fiber that are most effective in humans would remain to be characterized. The various chemically defined components of fiber are known to differ in their biological activities in ways that may have relevance to cancer development.^9,10

Fruit and vegetables inhibit the development of colorectal cancer. Its contain several anticarcinogenic components, such as antioxidant vitamins, folate, phytoestrogens, and protease inhibitors, that protect against DNA damage and mutations.^11-15 Dietary fiber may be most beneficial against colorectal cancer development and at which stages along the adenoma-carcinoma pathway fiber may act.¹⁶ The development of CRC is thought to be the result of an intimate and still poorly understood interplay between environmental and genetic factors. Dietary and lifestyle factors are among the most important environmental factors implicated.^17,18 Dietary modifications, nutritional or drug supplements and exercise were associated with a lower risk of CRC.¹⁹

Dietary fiber:

Dietary fibers (DFs) are often simply described as any non-digestible carbohydrates that are not broken down in the intestinal tract²⁰. Scientific and regulatory bodies around the world define fiber differently. In 2009, the Codex Committee on Nutrition and Foods for Special Dietary Uses (CCNFSDU)²¹ established an internationally accepted legal definition of DF. The definition states, DF means carbohydrate polymers with ten or more monomeric units, which are not hydrolyzed by the endogenous enzymes in the small intestine of humans and belong to the following three categories: (1) Edible carbohydrate polymers naturally occurring in the food as consumed. (2) Carbohydrate polymers, which have been obtained from food raw material by physical, enzymatic or chemical means and which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities. (3) Synthetic carbohydrate polymers which have been shown to have a physiological effect of benefit to health as demonstrated by generally accepted scientific evidence to competent authorities.

DF as the complex carbohydrate in the diet from plant sources that escapes small bowel digestion and thus reaches the colon. The endogenous components of plant materials in the diet that are resistant to digestion by enzymes produced by humans. It is composed predominantly of nonstarch polysaccharides and nonpolysaccharides (mainly lignins). Nonstarch polysaccharides include cellulose and noncellulosic polysaccharides (e.g., hemicelluloses, pectins, gums, and mucilages).²² “Fibre” is a broad term which encompasses a wide range of material. The most abundant organic material on this planet is cellulose but the sugar molecules in cellulose are linked in such a way that mammalian digestion cannot break them down. Symbiotic bacteria in the hindgut of monogastric mammals, such as humans, ferment cellulose. Other mammals have developed complex foregut fermentation chambers and gain over 70% of their daily energy intake from the production of short chain fatty acids by microbial fermentation.

Not all fibre is of plant origin as the second most abundant organic material on earth is chitin, which makes up the exoskeleton of arthropods and also requires similar fermentation for its breakdown. It regarded as a DF and some whales digest crustaceans by a fore-stomach fermentation similar to that seen in ruminants.²³ The shellfish industry generates large quantities of waste chitin and chemically modified chitin is widely used as a DF supplement.DF can be further analytically classified as soluble (some hemicelluloses, pectins, gums, and mucilages) and insoluble (most hemicelluloses, celluloses, and lignins), depending on its solubility in water and buffer solution.²⁴

When the effect of DF on the colon is being considered, the classification of fiber as fermentable (i.e., metabolized by colonic bacteria) and nonfermentable is also useful. Some starch escapes small bowel digestion and reaches the colon. 5%–10% of dietary starch reaches the colon and called this resistant starch.²⁵ The original standard classification of dietary fibre as being composed of remnants of plant cells that are resistant to hydrolysis by human alimentary enzymes.²⁶ This includes all indigestible polysaccharides such as celluloses, hemicelluloses, oligosaccharides, pectins, gums and waxes as well as lignin, a chemical compound most commonly derived from wood that is also present in plants. DF is further classified as soluble (i.e. pectin, agar) or insoluble (cellulose, heteroxylans and lignified cell walls (wheat bran)), with the soluble form being less protective against cancer.^27-29DF as non-digestible carbohydrates plus lignin, including non-starch polysaccharides: cellulose; hemicelluloses; pectins; hydrocolloids (i.e. gums, mucilages, glucans); resistant oligosaccharides, fructo-oligosaccharides, galacto-oligosaccharides, other resistant oligosaccharides; resistant starch, consisting of physically enclosed starch, some types of rawstarch granules, retrograded amylose, chemically or physically modified starches, or both; and lignin associated with the dietary fibre polysaccharides.³⁰

Identification of a specific dietary factor which affords protection against the risk of colorectal cancer would be important for two reasons. First, it directly impinges upon any recommended use of dietary supplements, as opposed to the increased consumption of foods rich in certain nutrients. Second, dietary modification that involves the increased consumption of foods such as fruits and vegetables may be easier to achieve than, for example, reducing total energy intake or reducing the proportion of total energy intake that comes from fat.³¹ Fibre can be classified into soluble fibre and insoluble fibre, according to differences in water solubility. According to foodsources, fibre can be further classified into grain, soy, vegetable, and fruit fibre and so on. One study showed that vegetable fibre can reduce the risk of colorectal cancer.³²

Diet high in fiber, as described above, could on the long run maintain a stable and healthy gut community possessing high microbial diversity and low pathogen abundance, increasing butyrate-producing bacteria, promoting fiber hydrolysis, providing energy for colonocytes and possessing potential anti-inflammatory properties, eventually playing a protective role against CRC.

Potential mechanisms:

The protective effect of fibre on CRC development has been considered to be the fibre’s effect on modulation of colonic transit time, alteration of bile acid metabolism or increase in the production of Short Chain Fatty Acid(SCFA).^33,34 It is under debate whether it is mainly total fibre intake that may be of benefit, or subtypes of fibre in specific fibre-rich food sources such as vegetables, fruits, berries or fibre-rich cereal products. Besides fibre, other bioactive components present in fibre-rich foods may have contributed to previously observed associations between fibre intake and CRC.³⁵

Several biologically plausible mechanisms have been postulated to explain the link between fiber and prevention of colorectal cancer. Increased fiber intake may lead to a dilution of fecal carcinogens, reduced transit time, and bacterial fermentation of fiber to short-chain fatty acids with anticarcinogenic properties.^36,37

Proposed underlying mechanisms include increased stool bulk and dilution of carcinogens in the colonic lumen, reduced transit time, and bacterial fermentation of fiber to shortchain fatty acids.^38-41

Health Implications of Dietary Fiber for colorectal cancer:

Extensive epidemiologic evidence supports the theory that dietary fiber may protect againstlarge-bowel cancer. Epidemiologic studies that compare colorectal cancer incidence or mortality rates among countries with estimates of national dietary fiber consumption suggest that fiber in the diet may protect against colon cancer. Data collected from 20 populations in 12 countries showed that average stool weight varied from 72 to 470 g/day and was inversely related to colon cancer risk.⁴² When results of 13 case-control studies of colorectal cancer rates and dietary practices were pooled, the authors concluded that the results provided substantive evidence that consumption of fiber-rich foods is inversely related to risks of both colon and rectal cancers.⁴³

The authors estimated that the risk of colorectal cancer in the US population could be reduced by about 31% with an average increase in fiber intake from food sources of about 13 g/day.

Three intervention studies do not support the protective properties of dietary fiber against colon cancer.^44-46 The studies found no significant effect of high-fiber intakes on the recurrence of colorectal adenomas. Each article describes a well-planned dietary intervention to determine whether high-fiber food consumption could lower colorectal cancer risk, as measured by a change in colorectal adenomas, a precursor of most largebowel cancers. Several reasons have been given for the failure to demonstrate a benefit. Perhaps the fiber interventions were not long enough, the fiber dose was not high enough, recurrence of adenoma is not an appropriate measure of fiber’s effectiveness in preventing colon cancer, or these individuals had already optimized their diets because the fiber intake by the low-fiber control subjects exceeded that of the american population. Yet increasing dietary fiber consumption during 3 years did not alter recurrence of adenomas. Despite the inconsistency in the results of fiber and colon cancer studies, the scientific consensus is that there is enough evidence on the protectiveness of dietary fiber against colon cancer that health professionals should be promoting increased consumption of dietary fiber.⁴⁷

Recent follow-up of the polyp prevention trial also found no effect of a low-fat, high-fiber, high-fruit and vegetable diet on adenoma recurrence 8 years after randomization.⁴⁸ The polyp prevention Trial was a 4-year trial and there was some thought that differences with dietary intervention would take longer to occur. Even though the trial had ended, the experimental group continued to consume more fiber in their diet. Still they found no effect of the intervention on later polyp recurrence.

The european prospective investigation into cancer and nutrition is a prospective cohort study comparing the dietary habits of more than a halfmillion people in 10 countries with colorectal cancer incidence.⁴⁹ They found that people who ate the most fiber (those with total fiber from food sources averaging 33 g/day) had a 25% lower incidence of colorectal cancer than those who ate the least fiber (12 g/day). The investigators estimated that populations with low average fiber consumption could reduce colorectal cancer incidence by 40% by doubling their fiber intake. Dukas and colleagues⁵⁰ reported that in the Nurses’ Health Study, women in the highest quintile of DF intake (median intake 20 g/day) were less likely to experience constipation than women in the lowest quintile (median intake 7 g/day).

Although dietary fiber intake may not protect against colorectal cancer in prospective studies, some support exists for the protective properties of whole-grain intake. Schatzkin and colleagues⁵¹investigated the relationship between whole-grain intake and invasive colorectal cancer in the prospective National Institutes of Health- American Association of Retried Persons (NIH-AARP) Diet and Health Study. Total dietary fiber intake was not associated with colorectal cancer risk whereas whole grain consumption was associated with a modest reduced risk. The association with whole grain intake was stronger for rectal than for colon cancer.

Other roles for fiber in health:

As a result of fiber serving as a substrate for bacteria in the large bowel, changes in intestinal bacterial populations, especially with the consumption of large amounts of purified, homogenous fibers (eg, fructooligosaccharides and arabinogalactans) have been reported. A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health.⁵² The most data for prebiotic activity have been published on inulin, a fructooligosaccharide, although trans-galactooligosaccharides also meet the criteria needed for prebiotic classification according to Roberfroid.⁵² Accepted methods to document whether a fiber is deemed a prebiotic are still developing; other functional fibers known to alter the intestinal microflora may eventually be deemed prebiotics.

Fibers have also been found to affect mineral absorption, bone mineral content, and bone structure.⁵³ Although we typically think of dietary fibers as decreasing mineral absorption, inulin, oligosaccharides, resistant starch, and other fibers have been found to enhance mineral absorption, particularly for calcium. Most of the supportive trials in human beings have been conducted in adolescents⁵⁴ and postmenopausal women⁵⁵ two groups generally with poor calcium intakes. Whether the prebiotic fibers will enhance calcium absorption in the general population remains to be seen.

Other components in fiber-containing:

Foods There is substantial scientific evidence that vegetables, fruits, and whole grains reduce risk of chronic diseases, including cancer and heart disease.^{56, 57} In epidemiologic studies, it is often easier to count servings of whole foods than translate information on food frequency questionnaires to nutrient intakes. Recent studies suggest that whole foods offer more protection against chronic diseases than dietary fiber, antioxidants, or other biologically active components in foods. Associations between dietary fiber and disease identified through epidemiologic studies may actually be reflections of a synergy among dietary fiber and these associated substances, or of an effect of only the associated materials. This suggests that the addition of purified dietary fiber to foodstuffs is less likely to be beneficial as opposed to changing American diets to include whole foods high in dietary fiber. The concept of synergy among components in whole foods and the attendant overall healthfulness of a varied diet are important aspects of any dietary counselling.

Disease risk reduction and therapeutic uses of fiber:

A lot of what is known about the benefits of a higher-fiber diet comes from epidemiologic studies and Dietary Reference intake (DRI) recommendations for DF intake are based on epidemiologic findings. Sometimes there are disparities between epidemiologic and metabolic studies. One possible source of discrepancy is the time of collection of diet information because the food supply and food habits change continuously. Foods in current databases may not be reflective of what was consumed more than a decade ago; this is particularly true for data for DF in foods that have been gathered largely in the past 15 years. There are now fewer differences among methods of determination of total dietary fiber in US foods so that current fiber databases are improved over those that were available previously and are reasonably useful for epidemiologic diet studies.

In contrast, the division of total fiber between soluble and insoluble remains very method dependent. The proportion of the total fiber that is soluble varies by two- to threefold across major methods of analysis, meaning that there is the same extent of variation among the values for insoluble fiber. The use of databases to differentiate the effects of soluble vs insoluble fiber with disease could produce statistically significant relationships, when in fact there are none. Also, the use of isolated, frequently single, fiber sources in metabolic studies is not representative of a mixed, high-fiber diet.

Clinical uses of dietary fiber:

Diverticulosis is a movement of material through the colon is stimulated in part by the presence of residue in the lumen. When chronic insufficient bulk characteristic of a low-fiber diet occurs in the colon, the colon responds with stronger contractions to propel the smaller mass distally. This chronic increased force leads to the creation of diverticula, which are herniations of the mucosal layer through weak regions in the colon musculature. Adequate intake of dietary fiber may prevent the formation of diverticula by providing bulk in the colon so that less forceful contractions are needed to propel it. Although few clinical studies have been conducted on dietary fiber and diverticular disease, case-control studies and case studies report success with high-fiber intakes.⁵⁸ A high-fiber diet is standard therapy for diverticular disease of the colon⁵⁹. Formed diverticula will not be resolved by a diet adequate in fiber, but the bulk provided by such a diet will prevent the formation of additional diverticula, lower the pressure in the lumen, and reduce the chances that one of the existing diverticula will burst or become inflamed. Generally, small seeds or husks that may not be fully digested in the upper gastrointestinal tract are eliminated from a high-fiber diet for a patient with diverticulosis as a precaution against having these small pieces of residue become lodged within a diverticulum.

Prevention of diverticular disease with DF is still unclear from the limited research. About 10% to 25% of individuals with diverticular disease will develop diverticulitis and it is not clear if dietary fiber could protect against diverticulitis.⁶⁰

CONCLUSION:

Present study suggests that vegetables and fibre (including vegetable fibre, soluble fibre, and insoluble fibre) are associated with a decreased risk of colorectal cancer and colon cancer. Intake of vegetables and soy foods was inversely related to rectal cancer risk. More in-depth studies are needed to confirm the relationship between fruit and colorectal cancer stratified by gender.

Chronic insufficient intake of dietary fiber represents a challenge for food and nutrition professionals that can be met with enthusiastic recommendations for a healthful dietary pattern. Increased consumption of fruits, vegetables, legumes, and whole- and high-fiber grain products as recommended by Mypyramid would bring the majority of the North American adult population close to the recommended range of dietary fiber of 14 g/1, 000 kcal. In addition, a higher fiber intake provided by foods is likely to be less calorically dense and lower in fat and added sugar. The benefits of such a varied dietary plan cannot be overemphasized. Many of the diseases of public health significance obesity, cardiovascular disease, and type 2 diabetes—as well as the less prevalent, but no less significant diseases of colonic diverticulosis and constipation, can be prevented or treated by increasing the amounts and varieties of fiber-containing foods. Promotion of such a food plan by food and nutrition professionals and implementation by the adult population should increase fiber intakes of children.

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Received on 26.01.2019 Modified on 28.02.2019

Asian J. Res. Pharm. Sci. 2019; 9(3):209-214.

DOI: 10.5958/2231-5659.2019.00033.X

Dietary Fiber Intake and Benefit of Colorectal Cancer