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Effect of a dietary fiber (beet fiber) on Dimethylhydrazine‐induced colon cancer in wistar rats a

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Inger Thorup , Otto Meyer & Eva Kristiansen

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National Food Agency , Institute of Toxicology , 19, Mørkhøj Bygade, Søborg, 2860, Denmark Published online: 04 Aug 2009.

To cite this article: Inger Thorup , Otto Meyer & Eva Kristiansen (1992) Effect of a dietary fiber (beet fiber) on Dimethylhydrazine‐induced colon cancer in wistar rats, Nutrition and Cancer, 17:3, 251-261, DOI: 10.1080/01635589209514194 To link to this article: http://dx.doi.org/10.1080/01635589209514194

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Effect of a Dietary Fiber (Beet Fiber) on Dimethylhydrazine-Induced Colon Cancer in Wistar Rats Inger Thorup, Otto Meyer, and Eva Kristiansen

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Abstract The modifying effect of a dietary fiber, Fibeta (beet fiber), on experimentally induced colorectal cancer was studied in Wistar rats. The rats were fed a powdered semisynthetic casein-based diet in which the carbohydrate pool was substituted with Fibeta as the sole source of fiber. Dimethylhydrazine dihydrochloride (DMH-2HCI) was used as initiator in a dose of 20 mg/kg body wt and given by gavage once a week for 10 weeks. Throughout the experiment the rats were offered the diets with different levels of fiber in a preinitiation period of 8 weeks, during the initiation, or in a 30-week postinitiation period. The study was terminated after one year. A protective effect of the fiber was not found at any stage of the colorectal carcinogenic process. Even though differences (not statistically significant) in tumor incidences were seen, these did not reflect any effect of the high or low fiber intake during the study. Analysis for volatile fatty acids in cecal content showed that continuous feeding with a fiber-rich diet resulted in significant increase in most of the volatile fatty acids. The relative change was highest for butyric acid. These findings do not support the hypothesis that butyric acid has a protective effect on colorectal cancer. The tumor yield in the present study was low compared with that reported in the literature, and possible causes for this are discussed. (Nutr Cancer 17, 251-261, 1992)

Introduction

A large number of investigations on the possible risk-modifying effect of dietary fiber on experimentally induced colorectal cancer have been performed during the last decades. Epidemiological (1) as well as experimental data (2) point toward a protective role of a fiber-rich diet against colon cancer. On the other hand, animal experiments carried out with different kinds of fiber showed inconsistent results (2-4). An explanation for these discrepancies, besides differences in age, sex, species, and animal strains used in the experiments, could be that the term dietary fiber covers a broad range of carbohydrates that differ widely in physicochemical and physiological properties. Therefore it is difficult to predict the risk-modifying properties of dietary fibers in general, and a case-by-case evaluation of implicated dietary fibers may be necessary (5). In the present study the effect of a well-specified dietary fiber (a beet fiber) on colorectal The authors are affiliated with the Institute of Toxicology, National Food Agency, 2860 Søborg, Denmark.

Copyright © 1992, Lawrence Erlbaum Associates, Inc.

cancer has been studied in an animal model using Wistar rats initiated with dimethylhydrazine (DMH). The dietary fiber Fibeta was chosen because it is a well-specified isolate from sugar beet intended for use in foodstuffs. The fiber is rich in both insoluble and soluble nonstarch polysaccharides. Because little information concerning the modulating effects of dietary fiber on the different stages of colon carcinogenesis is available from the literature (6), the present experiment was designed to study the modulating effect of this particular fiber during preinitiation, initiation, promotion, and progression. Materials and Methods

Animals Three hundred three-week-old male outbred rats [Mol: WIST (SPF)] were purchased from MiSllegârd Breeding Centre (Skensved, Denmark).

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Diet The animals were fed a powdered semisynthetic diet based on the formula given by Meyer and co-workers (7). After a two-week accommodation period, the cellulose was replaced by carbohydrates, and the pool of carbohydrates was substituted with two levels of Fibeta to obtain diets with 1% or 20% Fibeta (Table 1). These levels correspond to 0.7% and 14% dietary fiber, respectively, in the rat diet. Dietary Fiber The dietary fiber used was a beet fiber (Fibeta, De Danske Sukkerfabrikker, Nakskov, Denmark). Fibeta consists of 45% (wt/wt) insoluble and 24% (wt/wt) soluble fibers, determined by the method of the Association of Official Analytical Chemists (8). According to the method of Theander and Aman (9), the Fibeta consists of the following (wt/wt) 18% cellulose, 2% lignin, and 43% hemicellulose/pectin. The hemicellulose/pectin was further divided into (wt/wt) 17% pentoses, 7% hexoses, and 19% uronic acid. Chemical 1,2-Dimethylhydrazine dihydrochloride (DMH-2HC1) was obtained from Sigma Chemical (St. Louis, MO). Table 1. Composition of Diets Low-Fiber Diet Na-caseinate, Vo Carbohydrate, 97b Soya bean oil (with vitamins A, D, and E), % Mineral mixture, % Vitamins B and K, choline chloride, and inositol, % Methionine, % Fibeta, % Caloric density," kcal/g

High-Fiber Diet

20

20

70.3

51.3

4 3.3

4 3.3

1 0.4 1 3.9

1 0.4 20 3.7

a: Calculation is as follows: fat = 9 kcal/g, protein = 4 kcal/g, carbohydrate = 4 kcal/g, Fibeta = 3 kcal/g.

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Nutrition and Cancer 1992

Housing The animals were kept in disposable plastic cages with an inserted steel grid floor, two animals per cage, in flexible film isolators (Isotec 12134, Olac, Oxford, UK) during the 1-week predosing period, the 10-week dosing period, and for 2 weeks after termination of the dosing with DMH-2HC1. For the remaining period of the study, the animals were kept in stainless steel wire cages, two animals per cage. During the study the temperature was maintained at 22 ± °1C and relative humidity at 55 ± 5%, air was changed 8-10 times per hour, and fluorescent light was on from 2100 to 0900 hours.

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Experimental Design The experimental protocol included nine groups to which the animals were randomly assigned. DMH-2HC1 was used as initiator, and the rats were given DMH-2HC1 (20 mg/kg body wt) or the solvent (0.9% NaCl solution) by gavage (5 ml/kg body wt) once a week for 10 weeks according to Table 2. From the age of five weeks the rats were offered 19 g of the 1% Fibeta diet per day; the amount of Fibeta diet was increased to 23 g at seven weeks of age and thereafter. For rats fed the 20% Fibeta diet the corresponding amounts were 1 g higher to compensate for the difference in energy of the Fibeta and the substituted carbohydrate in the diet. Throughout the experiment the rats were given diets with different levels of the dietary fiber according to Table 2. Body weight and food and water consumption were measured weekly. The study was terminated after one year. Gross necropsy and histopathological examination were performed on all the animals. The following organs were fixed in 4% neutral buffered formaldehyde: small and large intestine, liver, kidneys, lungs, heart, and tissues showing gross lesions. Before fixation, the large intestine was cut longitudinally, rinsed in 0.9% NaCl solution, cut into four pieces, and pinned on a cork slab. The divisions were made in the light of anatomic conditions based on the principles outlined by Lindström (10): 1) proximal colon (7 cm), 2) major flexure (5 cm), 3) distal colon (6 cm), and 4) residual colorectum, which varied in length. Paraffin-embedded slides, 4-6 ¡xm thick and stained with hematoxylin and eosin, were prepared for microscopic examination. Analysis for volatile fatty acids (VFAs) (9) was performed at the end of the study on samples of the cecal content from 15 animals in Group I (1% Fibeta) and 16 animals in Table 2. Experimental Design

Effect of a dietary fiber (beet fiber) on dimethylhydrazine-induced colon cancer in Wistar rats.

The modifying effect of a dietary fiber, Fibeta (beet fiber), on experimentally induced colorectal cancer was studied in Wistar rats. The rats were fe...
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