Effects of dietary fiber in vervet monkeys fed "Western" diets

The interactions of diet and colonic microflora in regulating colonic mucosal growth

The colonic mucosa can adapt its growth to alterations in diet. Metabolites from colonic microflora are frequently implicated as the primary factor in mediating the colonic mucosal response to diet; however, there is also evidence indicating that diet may have a direct effect in mediating this response. The aim of this study was to determine the role of diet, microflora, and microflora metabolites in altering the growth of the colonic mucosa. Two 28-day feeding studies were conducted using Sprague-Dawley rats. The first study compared the growth of the colonic mucosa in germ-free and conventional rats fed 6 different diets. The second study compared the growth of the colonic mucosa to the concentration of bacterial-derived short-chain fatty acids (SCFs), bile acids, and ammonia. The diets that were fed consisted of (1) AIN-76a diet without dietary fiber; (2) standard AIN-76a diet, which contained 5% cellulose; (3) AIN-76a diet with 5% guar gum; (4) a "Western" human diet with 20% fat and 10% cellulose; (5) AIN-76a diet formulated to mimic Diet 4 in fat content but with 2.5% cellulose; and (6) Purina Rodent Chow. Quantitative volumetric and stereologic analysis was used to assess changes in total colonic mucosal volume as a measure of mucosal growth. In germ-free rats, Diets 2-4 and 6 induced a significant increase (18-38%) in mucosal volume compared to Diet 1. In conventional animals, only Diets 4 and 6 induced a significant increase (up to 63%) in mucosal volume compared to Diet 1. Relative to the germ-free animals, only conventional animals on Diets 4 and 6 had an increase in mucosal volume. The increases in mucosal volume in Diets 4 and 6 were not consistently associated with increased SCFAs, ammonia, or bile acids. There was a wide range in the colonic concentrations of SCFAs (2-fold), ammonia (6-fold), and bile acids (10-fold). The presence of colonic microflora in and of itself does not lead to enhanced colonic mucosal growth. Rather, there are unique interactions between specific types of diet and microflora that lead to a growth-promoting effect. This effect could not be explained by alterations in the concentration of SCFAs, ammonia, or bile acids in colonic contents.

A new dietary model to study colorectal carcinogenesis: experimental design, food preparation, and experimental findings

Experimental dietary studies of human colorectal carcinogenesis are usually based on the AIN-76A diet, which is dissimilar to human food in source, preparation, and content. The aims of this study were to examine the feasibility of preparing and feeding rats the diet of a specific human population at risk for colorectal neoplasia and to determine whether changes in the colonic morphology and metabolic contents would differ from those resulting from a standard rat diet. The mean daily food intake composition of a previously evaluated adenoma patient case-control study was used for the "human adenoma" (HA) experimental diet. Foods were prepared as for usual human consumption and processed by dehydration to the physical characteristics of an animal diet. Sixty-four female Sprague-Dawley rats were randomized and fed ad libitum the HA or the AIN-76A diet. Every eight weeks, eight rats from each group were sacrificed, and the colons and contents were examined. Analysis of the prepared food showed no significant deleterious changes; food intake and weight gain were similar in both groups. Compared with the controls, the colonic contents of rats fed the HA diet contained significantly less calcium, concentrations of neutral sterols, total lipids, and cholic and deoxycholic acids were increased, and there were no colonic histological changes other than significant epithelial hyperproliferation. This initial study demonstrated that the HA diet can be successfully processed for feeding to experimental animals and is acceptable and adequate for growth but induces significant metabolic and hyperproliferative changes in the rat colon. This dietary model may be useful for studies of human food, narrowing the gap between animal experimentation and human nutritional research.

Dietary fiber, inulin, and oligofructose: a review comparing their physiological effects

Dietary fiber is a general term. It covers a wide variety of substances that belong to the family of carbohydrates that resist hydrolysis by human alimentary enzymes but are fermented by colonic microflora. The main physiological effects of dietary fiber are primarily on gastric emptying and small intestinal transit time, resulting in an improved glucose tolerance and a decreased digestion of starch: second, on colonic transit time and large bowel functions due to fermentation by ceco-colonic microbial flora or bulking action. The so-called soluble dietary fibers are fermented to a large extent by a wide variety of anaerobic bacteria that result in an increase in bacterial biomass, an increase in fecal mass, a change in intracolonic pH, and production of short chain fatty acids and various gases as metabolic end products. The insoluble fibers are only marginally fermented: they serve almost exclusively as bulking agents that result in shorter transit time and increased fecal mass. The short chain fatty acids resulting from the colonic fermentation of dietary fiber are largely absorbed via the portal blood and reach both the liver and the peripheral tissues. They induce changes in glucose and fat metabolism leading to post-prandial hypoglycemia and long-term hypolipidemia. Inulin and oligofructose are fructans with a degree of polymerization of 2 to 60 and 2 to 20, respectively. Due to the structural conformation of their osidic bridge (beta 2-1), they both resist the hydrolysis by human alimentary enzymes. Moreover, when reaching the colon, both inulin and oligofructose are almost quantitatively fermented almost exclusively by colonic bifidobacteria and bacteroides. Such an extensive fermentation causes an increase in fecal bacterial biomass, a decrease in ceco-colonic pH, and produces a large amount of fermentation products among which the short chain fatty acids that exert systemic effects on lipid metabolism. Thus, both inulin and oligofructose have most of the characteristics of a dietary fiber and the proposal is made to classify them as such. Moreover, they are bifidogenic factors, because, due to still unknown reasons, they are primarily fermented by bifidobacteria. It is concluded from this review that "nondigestible fructo-oligosaccharides," even though they are not included in the carbohydrate fraction that is quantified as dietary fiber by classic analytical methods, have most of the physiological effects of a dietary fiber.(ABSTRACT TRUNCATED AT 400 WORDS)