Fermentation of dietary fibre components in the rat intestinal tract

Effect of cooking, pH and polyphenol level on carbohydrate composition and nutritional quality of a sorghum (Sorghum bicolor (L.) Moench) food, ugali

1. The present work was undertaken to study the effects of cooking, pH and polyphenol level on carbohydrate composition and nutritional quality of sorghum (Sorghum bicolor (L.) Moench). Three different sorghum varieties; Dabar, Feterita and Argentine containing zero, intermediate to low and high levels of polyphenols respectively were used in the study. From these varieties uncooked, uncooked acidified, cooked, and cooked acidified diets were prepared. Diets were characterized with regard to resistant starch (RS), dietary fibre (DF), acid-detergent fibre (ADF) and amino acid content. Raw materials were further analysed for content and composition of non-starch polysaccharides and Klason lignin. The nutritional properties were studied in balance trials with rats. True protein digestibility (TD), biological value (BV), net protein utilization, digestible amino acids, digestible energy (DE) and digestible DF were used as criteria in the nutritional study. 2. Cooking at neutral and acid pH resulted in significantly higher assayed values for DF. Increase in DF could be accounted for by formation of RS. Approximately 50% of RS was recovered in the faeces. 3. In vitro values for protein associated with ADF and in vivo balance values using rats suggest that an endosperm protein fraction, kafirins, was made unavailable during cooking. This resulted in reduced TD and increased BV. It is assumed that unavailable kafirins serve as a nitrogen source for microflora in the hind-gut. 4. Dietary polyphenols changed the excretory route for N from urine to faeces. This resulted in lower TD and higher BV in Argentine (high in polyphenols) than in Dabar and Feterita (low in polyphenols), although dietary lysine (first limiting amino acid) was the same in the three varieties. 5. Variation in DE of the diets was attributed to DF, RS and the amount of faecal protein, which in turn were influenced by undigested kafirins and polyphenols.

Effect of dietary fiber on colonic cell proliferation and its relationship to colon carcinogenesis

The addition of specific fiber supplements to semipurified diets has been shown to stimulate large bowel cell proliferation in laboratory rodents. Relatively insoluble fibers such as cellulose, which is poorly fermented, the more-soluble oat bran, and inert bulking agents such as kaolin produce little or no effect on cell growth. On the other hand, wheat bran, pectin, guar gum, and degraded carageenan all stimulate large bowel cell proliferation, the greatest growth response tending to occur in the cecum or proximal colon. The proximal large bowel is also the major site for the intestinal fermentation of dietary fiber and any other nonabsorbed carbohydrates. The fermentation of fiber by colonic microorganisms results in the production of short-chain fatty acids and a lower pH of large bowel contents, metabolic events known to be associated with increased epithelial cell growth. In general, factors that stimulate cell growth also enhance tumor development, a concept that holds true in the colon even for dietary fibers such as pectin and guar gum. Wheat bran can also stimulate colon carcinogenesis when fed only during carcinogen exposure. Oat bran and corn bran may stimulate colon carcinogenesis by increasing fecal bile acid excretion, a feature of many soluble fibers, while the acidification of large bowel contents is associated with an increased frequency of chemically induced colonic cancers. A greater understanding of colonic metabolism and cell physiology is needed to define fully the mechanisms by which dietary fibers modify colon cancer development.

Comparative effects of dietary wheat bran and its morphological components (aleurone and pericarp-seed coat) on volatile fatty acid concentrations in the rat

Adult male rats were fed on diets containing 100 g dietary fibre/kg either as alpha-cellulose or wheat bran or the pericarp-seed coat or aleurone layers prepared from that bran by sequential milling and air elutriation and electrostatic separation. After 10 d, concentrations of total volatile fatty acids (VFA) in caecal fluid were significantly different between groups and fell in the order: aleurone greater than wheat bran greater than pericarp-seed coat greater than cellulose. This ranking probably reflected the ease of fermentation of fibre polysaccharides by colonic bacteria which also resulted in a considerably higher faecal bacterial mass in the aleurone group. Because of the differences in the volume of caecal digesta, the mass of caecal VFA was considerably the highest in the aleurone group, intermediate with wheat bran and equally low in the pericarp-seed coat and cellulose groups. The diet based on aleurone gave a relatively higher proportion of propionate but with both pericarp-seed coat and wheat bran the contribution of butyrate was raised. VFA concentrations in hepatic portal venous plasma were proportional to caecal concentrations with very high (greater than 3 mM) values being recorded in the aleurone group. The findings are discussed in relation to the apparent susceptibility of the morphological components of wheat bran to fermentation by large bowel bacteria.

Harry M. Vars award. The effect of a pectin-supplemented elemental diet on intestinal adaptation to massive small bowel resection

The effect of a pectin-supplemented elemental diet on intestinal adaptation to massive small bowel resection in the rat was investigated in this study. Sixty adult Sprague-Dawley rats underwent placement of a feeding gastrostomy and swivel apparatus. Control animals (N = 16) were then returned to their cages while the remaining animals underwent an 80% small bowel resection and anastomosis (resected, N = 44). Postoperatively, animals were randomly assigned to receive either a fat- and fiber-free elemental diet (no pectin) or the same diet supplemented with 2% pectin (pectin). After 8 days of full strength diet, samples of jejunum, ileum, and colon were obtained for analysis. The weights per unit length of the ileum and colon were significantly greater in the resected pectin group than either the resected no pectin or pectin control groups. Mucosal parameters (unit weight, DNA, RNA, and protein content) were significantly increased in the jejunum and ileum of both the resected pectin and resected no pectin groups relative to their respective control groups. However, jejunal and ileal mucosal parameters were significantly greater (1.3- to 2-fold) for the resected pectin group than for the resected no pectin group. All the mucosal parameters of the colons were significantly greater for the pectin resected group than for the pectin control group, while only colonic mucosal RNA was significantly greater for the no pectin resected group than for the no pectin control group. These data indicate that the addition of pectin to an elemental diet significantly enhanced intestinal adaptation to massive small bowel resection in the rat.

Fermentation of dietary fibre in the intestinal tract: comparison between man and rat

1. The breakdown and faecal bulking capacity of dietary fibre preparations from wheat bran, apple, cabbage, carrot, and guar gum were compared in man and rat. 2. The degradation of the fibre showed good correlation between man and rat (r 0.99, regression coefficient 0.86). Wheat bran was the least well-digested, 66 and 59% of the neutral sugars being excreted in faeces of man and rat respectively. The breakdown of the fibre in apple, cabbage, carrot and guar gum was more complete and 4-29% of the neutral sugars were recovered in faeces. 3. The main dietary fibre constituents in each preparation were degraded to a similar extent in man and rat. The main dietary fibre constituents of apple, carrot, cabbage and guar gum were almost completely degraded. Of the xylose in wheat bran 45% (man) and 48% (rat) were recovered in faeces. However, the percentage excretion of glucose and arabinose from bran was higher in man. 4. A faecal glucan other than cellulose was identified in human faeces after guar gum, and has been provisionally identified as starch. No such glucan occurred in rat faeces. 5. A good correlation between the faecal bulking capacity in man and rat was seen (r 0.97, regression coefficient 0.56). Wheat bran had the best bulking capacity, while that of apple, cabbage, carrot and guar gum was less pronounced. Faecal bulking was inversely related to the amount of fibre which was water-soluble in each preparation. 6. It is concluded that this rat experimental model is useful for the prediction of fermentative breakdown and bulking capacity of dietary fibre in man. However, more comparative studies are needed to evaluate animal experiments regarding other physiological effects of dietary fibre.

An experimental design to study colonic fibre fermentation in the rat: the duration of feeding

1. The time-course of metabolic adaptation by rats to diets containing either wheat bran or gum arabic was studied during a 12-week period. 2. Stool weight was increased with wheat bran but not with gum arabic, and had stabilized after 4 weeks. 3. Bacterial mass as measured by diaminopimelic acid content had stabilized by week 4. Bacterial mass increased on feeding gum arabic but was unchanged with wheat bran. 4. There was increased caecal short-chain fatty acid, hydrogen and methane production with gum arabic but not with wheat bran. The change in caecal metabolic activity was slow to stabilize (8-12 weeks at least). 5. The faecal excretion of bile acids increased twofold with the wheat-bran-supplemented diet compared with the gum-arabic-supplemented and unsupplemented diets. Relatively greater amounts of muricholic acids were present in the caeca and faeces of gum-arabic-fed rats compared with the other groups.

Effect of mixtures of dietary fibres on the enzyme activity of the rat caecal microflora

The enzyme activity of the caecal microflora from weanling rats was determined after feeding 1 of 3 basal diets (purified fibre-free; purified plus cellulose; and stock), with or without additional dietary fibre (pectin, i-carrageenan or carboxymethylcellulose 5% w/w). The wet weight of caecal contents and total bacterial numbers were similar for the purified fibre-free and purified plus cellulose diets, yet were significantly higher in animals fed the stock diet. Pectin supplementation of the basal diets had no effect of caecal bacterial numbers, but significantly increased total nitrate reductase activity per caecum except when added to stock diet. Carrageenan decreased caecal bacterial numbers and most enzyme activities with both purified diets, and to a lesser extent with the stock diet. Carboxymethylcellulose increased bacterial numbers and enzyme activities, particularly beta-glucosidase and nitrate reductase when added to the purified diet but not when added to either the purified diet plus cellulose or the stock diet. The results demonstrate that the effects of dietary fibre components on the rat caecal microflora are dependent upon the initial fibre content of the diet base.

Bulk laxatives: their dietary fibre composition, degradation, and faecal bulking capacity in the rat

The intestinal dietary fibre degradation and faecal bulking capacity of various bulk laxatives were investigated by means of balance experiments on rats. Nitrogen, fat, and mineral excretion in faeces was also studied. The dietary fibre content of the various bulk laxatives was quite different (in g/kg dry matter): ACO fibre tablets (barley and citrus pulp), 451; Fiberform (wheat bran-based), 817; Inolaxol (sterkulia gum), 696; and Vi-Siblin (ispaghula husk), 533. The increase in faecal dry matter per 1 g dietary fibre was similar with ACO fibre tablets, Fiberform, and Vi-Siblin. Inolaxol gave a significantly (p less than 0.001) higher faecal dry-weight increment, mainly due to an increased mineral excretion. Of the dry-weight increment, 59-82% constituted undegraded dietary fibre. Thus, 68-97% of the fibre passed through the gastrointestinal tract without being degraded. All the bulk laxatives caused a similar increase in the faecal N content, whereas the increase in faecal lipids was most pronounced with Vi-Siblin. The water-holding capacity of faeces was more pronounced with Inolaxol and Vi-Siblin than with ACO fibre tablets and Fiberform.

Myoelectrical activity and propulsion in the large intestine of fed and fasted rats

Electrical spiking activity of different parts of the colonic wall was studied in relation to the mechanical events in conscious rats fitted with chronically implanted nichrome wire electrodes and miniaturized strain-gauge transducers. The progress of barium sulphate introduced into the caecum and measured radiographically at fixed intervals was used as an index of transit rate of colonic contents in both the fasted and fed state. The basic pattern of colonic myoelectrical activity was characterized by randomly occurring spike bursts at a higher frequency in the proximal (0.9/min) than the distal colon (0.5/min). Their duration in the fasted state, which was shorter in the proximal (5.5 +/- 1.7 s) than the distal colon (12.7 +/- 2.9 s), was similar following a meal. In the fasted state, integrated records showed cyclical periods of more intense electrical activity lasting about 20 and 40 min in the proximal and the distal colon, respectively. The cyclical pattern following a meal occurred at shorter intervals in the different parts of the colon. Conversely, the propulsion of the marker over the whole colon, which lasted 180-200 min, was accelerated by 30% after feeding. Laxatives disrupted these cyclical motor events on the colon, by inducing mass movements which impeded the pellet formation and increased the rate of transit. The cyclical motor activity was also disrupted following the administration of opiate agonists, the rate of transit being decreased and propulsive activity inhibited. The results are consistent with the concept of cyclical motor pattern playing an important part in the control of pellet formation and movement of digestive contents within the colon of the rat.

Differential effects of dietary fibers on rat intestinal circular muscle cell size

The relationship between dietary fiber and intestinal circular muscle cell size was investigated in rats by feeding defined diets supplemented with four different sources of fiber. In the first study, a 20% wheat bran supplement was fed to 10 rats for nine weeks. This resulted in larger muscle cell size, with a 22.5% increase in the proximal (P less than 0.02) and 77.9% increase in the distal colon (P less than 0.01) when compared with a control group of 10 rats fed a fiber-free diet. In the second study, which lasted four weeks, a control group of 10 rats was fed a fiber-free diet, while similar sized experimental groups were fed the same basal diet plus either 20% oat bran, 10% pectin, or 10% guar. Muscle cell size was decreased by 20.6% in the proximal jejunum of the oat bran- and pectin-fed groups (P less than 0.05) and by 43% in the proximal colon of the oat bran-fed group, when compared with the controls (P less than 0.05). These results show that the effects of high fiber diets on intestinal muscle cell size depend on the type of fiber consumed.

The effect of diet on the mammalian gut flora and its metabolic activities

The review will encompass the following points: A brief introduction to the role of the gut flora in the toxicology of ingested food components, contaminants, and additives, including known pathways of activation and detoxication of foreign compounds and the implication of the flora in enterohepatic circulation of xenobiotics. The advantages and disadvantages of the various methods of studying the gut flora (classical bacteriological techniques, metabolic and enzymological methods) will be critically discussed with special reference to their relevance to dietary, toxicological, and biochemical studies. Sources of nutrients available to the gut flora will be described including host products (mucus, sloughed mucosal cells, hormones, proteins) and exogenous nutrients derived from diet. An account of the problems involved in studies of dietary modification with special reference to the use of stock laboratory animal diets, purified diets, and human dietary studies. The influence of dietary modification on the flora will be assessed on the basis of changes in numbers and types of bacteria and their metabolic activity, drawing on data from human and animal studies. The effects of manipulation of the quantity and quality of protein, fat, and indigestible residues (fiber) of the diet will be described together with their possible implications for toxicity of ingested compounds.

Dietary fibre in bread and corresponding flours--formation of resistant starch during baking

Dietary fibre, assayed with an enzymatic/gravimetric method, was higher in wheat/rye bread than in the corresponding flours. The increase was most pronounced in crumbs from bread baked with mainly low-extraction-rate flour, and could be accounted for to a large extent as "resistant starch", i.e. a starch fraction available to amyloglucosidase only after solubilization with 2 m-KOH. The resistant starch was formed at dough-making and/or baking and did not increase further during freezing or storage at room temperature. The chemical modifications leading to resistant starch formation remain to be investigated. Starch-lipid complexes are probably not involved, since these are hydrolyzed by the heat-stable amylase used in the dietary fibre assay.

Hydrocolloid food additives and rat caecal microbial enzyme activities

Agar, carboxymethylcellulose, carrageenan, guar gum, gum acacia, locust-beam gum or pectin (50 g/kg diet), given to weanling rats for 4 wk, increased the weight of the caecal wall and the caecal contents. Feeding carboxymethylcellulose, guar gum or pectin significantly increased, and feeding carrageenan decreased, the total bacterial population of the caecum. Feeding carboxymethylcellulose significantly increased in vitro activity of bacterial azoreductase, beta-glucosidase, beta-glucuronidase, nitrate reductase, nitroreductase and urease. Guar gum, gum acacia and locust-bean gum each increased at least three of these activities. In contrast, feeding carrageenan greatly decreased all microbial enzyme activities, while agar decreased beta-glucosidase, beta-glucuronidase and nitroreductase activities.

Effects of addition of kaolin or cellulose to an elemental diet on intestinal cell proliferation in the mouse

1. Various methods of estimating intestinal mass and cell proliferation were used to compare the effects of a pelleted laboratory diet (diet A), an elemental diet (Flexical; diet B), Flexical plus kaolin (diet C) and Flexical plus cellulose (diet D) on the mucosa and other tissues of the mouse small intestine and colon.

2. The weight of the distal third of the small intestine was significantly decreased in mice given diets B, C and D (P< 0·001). The length of the colon was significantly decreased in the mice given diets B (P< 0·001), C (P< 0·01) and D (P< 0·05); however, the weight of the colon was only decreased in the mice given diet B or diet C (P< 0·001).

Similar changes were noted in the weights of the intestinal muscle and serosa layers; however, no such changes were noted in the weight or DNA content of the mucosa.

3. No changes were observed in the crypt cell production rate in the small intestine, apart from an increase in the mid-region (P< 0·01) of mice given diet C, which was associated with a similar increase in the mucosal content of DNA.

Crypt-cell production was significantly decreased (P< 0·01, 0·001) in the two sites of the colon studied in the mice given diets B or C, but the crypt-cell production rates were not significantly different from control levels at either site in the mice given diet D.

4. The length of the colonic crypts was significantly decreased (P< 0·05, 0·02) in mice given diets B and C, but not in those given diet D; total cell number showed a similar change (P< 0·01, 0·05).

The correlation coefficient between the length of the colonic crypts and absolute cell number was 0·513.

5. The conclusion of the present study was that dietary fibre (cellulose), but not inert bulk (kaolin), prevents mucosal atrophy of the colon of mice given a fibre-free diet, and that many of the observed changes in intestinal weight are due to reduction in the bulk of the muscle layer.