Short-chain fatty acids and human colonic function: roles of resistant starch and nonstarch polysaccharides

Resistant starch (RS) is starch and products of its small intestinal digestion that enter the large bowel. It occurs for various reasons including chemical structure, cooking of food, chemical modification, and food mastication. Human colonic bacteria ferment RS and nonstarch polysaccharides (NSP; major components of dietary fiber) to short-chain fatty acids (SCFA), mainly acetate, propionate, and butyrate. SCFA stimulate colonic blood flow and fluid and electrolyte uptake. Butyrate is a preferred substrate for colonocytes and appears to promote a normal phenotype in these cells. Fermentation of some RS types favors butyrate production. Measurement of colonic fermentation in humans is difficult, and indirect measures (e.g., fecal samples) or animal models have been used. Of the latter, rodents appear to be of limited value, and pigs or dogs are preferable. RS is less effective than NSP in stool bulking, but epidemiological data suggest that it is more protective against colorectal cancer, possibly via butyrate. RS is a prebiotic, but knowledge of its other interactions with the microflora is limited. The contribution of RS to fermentation and colonic physiology seems to be greater than that of NSP. However, the lack of a generally accepted analytical procedure that accommodates the major influences on RS means this is yet to be established.

Gut microbial metabolites limit the frequency of autoimmune T cells and protect against type 1 diabetes

Gut dysbiosis might underlie the pathogenesis of type 1 diabetes. In mice of the non-obese diabetic (NOD) strain, we found that key features of disease correlated inversely with blood and fecal concentrations of the microbial metabolites acetate and butyrate. We therefore fed NOD mice specialized diets designed to release large amounts of acetate or butyrate after bacterial fermentation in the colon. Each diet provided a high degree of protection from diabetes, even when administered after breakdown of immunotolerance. Feeding mice a combined acetate- and butyrate-yielding diet provided complete protection, which suggested that acetate and butyrate might operate through distinct mechanisms. Acetate markedly decreased the frequency of autoreactive T cells in lymphoid tissues, through effects on B cells and their ability to expand populations of autoreactive T cells. A diet containing butyrate boosted the number and function of regulatory T cells, whereas acetate- and butyrate-yielding diets enhanced gut integrity and decreased serum concentration of diabetogenic cytokines such as IL-21. Medicinal foods or metabolites might represent an effective and natural approach for countering the numerous immunological defects that contribute to T cell-dependent autoimmune diseases.

The adaptive effects of dietary fish and safflower oil on lipid and lipoprotein metabolism in perfused rat liver

To elucidate the mechanisms underlying the plasma triacylglycerol-lowering effects of certain fish oils, livers from male rats fed either a standard commercial diet (controls) or diets supplemented with 15% (w/w) fish or safflower oils were perfused with undiluted rat blood. Rates of hepatic lipogenesis, measured by the incorporation of 3H2O into fatty acids, followed the order: control greater than safflower oil greater than fish oil. Secretion of newly synthesized fatty acids in very-low-density lipoproteins was also inhibited by the feeding of both oil-supplemented diets with the greater suppression being seen in livers from animals fed fish oil. The hepatic release of very-low-density lipoprotein triacylglycerol mass was also significantly depressed in animals fed the fish oil-supplemented diet but not in those fed safflower oil. Ketogenesis did not differ between livers from rats fed the control and safflower oil diets but was significantly raised in the fish oil group. Increased ketogenesis with fish oil was paralleled by a decrease in the sensitivity of carnitine palmitoyl transferase of isolated mitochondria to inhibition by malonyl-CoA. The inhibitory effect of malonyl-CoA in the safflower oil group was intermediate between that in the fish oil and control groups. Activities of glycerophosphate acyltransferase with either palmitoyl-CoA or oleyl-CoA were increased by feeding oil-supplemented diets. Activity with palmitoyl-CoA that was suppressible by N-ethylmaleimide was also considerably diminished in both groups. The results indicate that the lowering of plasma triacylglycerols by fish oil reflects: (a) diminished lipogenesis; (b) increased fatty acid oxidation possibly in peroxisomes; and (c) diminished secretion of triacylglycerols by the liver.

The immediate effects of insulin and fructose on the metabolism of the perfused liver. Changes in lipoprotein secretion, fatty acid oxidation and esterification, lipogenesis and carbohydrate metabolism

1. When livers from fed rats were perfused with blood containing elevated concentrations of rat insulin or blood to which fructose was added, the oxidation of free fatty acids was depressed and their esterification was increased. 2. Raised concentrations of insulin or addition of fructose increased secretion of triglyceride in very-low-density lipoproteins, but only insulin caused more of the free fatty acids taken up by the liver to be incorporated into very-low-density lipoproteins. 3. When insulin and fructose were added together the combined effect on oxidation and esterification of free fatty acids and on secretion of very-low-density lipoproteins was equal to the sum of the effects of either alone. No statistically significant interaction between the effects of fructose and insulin was found for any of the parameters investigated. 4. Bovine insulin had similar effects, in most respects, to comparable studies with raised concentrations of rat insulin. 5. Lipogenesis was increased in the livers treated with fructose plus bovine insulin. 6. A significant proportion of the fatty acids in very-low-density lipoproteins were derived either from the liver triglyceride pool or from lipogenesis. This fraction was increased both by treatment with insulin or fructose, and was augmented further when both insulin and fructose were present together. 7. The uptake of fructose by the perfused liver was similar to that found in vivo. It was unaffected by the presence of insulin. 8. Addition of fructose to the perfused liver caused perfusate lactate concentrations to increase, as a result of diminished hepatic uptake of lactate. 9. The uptake of free fatty acids by the perfused liver was unaffected by the addition of either insulin or fructose. 10. The distribution among the various lipid classes in plasma lipoproteins of label arising from the hepatic uptake of [(14)C]oleate was unaltered by the addition of either fructose or insulin. 11. It is suggested that the effects described are due principally to control of the balance between esterification of fatty acids and lipolysis of the ensuing triglyceride, fructose enhancing esterification and insulin inhibiting lipolysis.

Resistant starch as a prebiotic and synbiotic: state of the art

Non-infectious diseases such as CHD and certain cancers have become major causes of death and disability in affluent countries. Probiotics (principally lactic acid bacteria; LAB) may assist in lowering the risk of these diseases. Experimental studies with probiotics have given generally inconclusive outcomes for infectious disease and for biomarkers for non-infectious disease. In part this situation may reflect their inability to colonise the adult human gut effectively. Prebiotics can assist in promoting colonisation, and resistant starch (RS), as a high-amylose starch, is a prebiotic and synbiotic. This starch exerts its synbiotic action through adhesion of the bacteria to the granule surface. Consumption of RS assists in recovery from infectious diarrhoea in man and animals. A rice porridge, high in RS, appears to modify the autochthonous porcine large-bowel microflora favourably through lowering Escherichia coli and coliform numbers. Many of the beneficial effects of RS on large-bowel function appear to be exerted through short-chain fatty acids (SCFA)formed by bacterial fermentation. In man LAB are found in relatively highest numbers in milk-fed infants where the profile of fermentation products differs quite markedly from that in adults. It appears unlikely that ingestion of current probiotics will alter either total SCFA or the proportions of the major acids. More emphasis needs to be given to the investigation of the effects of complex carbohydrates, including RS, on the autochthonous microflora of the human large bowel.

Fecal butyrate levels vary widely among individuals but are usually increased by a diet high in resistant starch

Butyrate and other SCFA produced by bacterial fermentation of resistant starch (RS) or nonstarch polysaccharides (NSP) promote human colonic health. To examine variation in fecal variables, especially butyrate, among individuals and the response to these fibers, a randomized cross-over study was conducted that compared the effects of foods supplying 25 g of NSP or 25 g of NSP plus 22 g of RS/d over 4 wk in 46 healthy adults (16 males, 30 females; age 31-66 y). Fecal SCFA levels varied widely among participants at entry (butyrate concentrations: 3.5-32.6 mmol/kg; butyrate excretions: 0.3-18.2 mmol/48 h). BMI explained 27% of inter-individual butyrate variation, whereas protein, starch, carbohydrate, fiber, and fat intake explained up to 16, 6, 2, 4, and 2% of butyrate variation, respectively. Overall, acetate, butyrate, and total SCFA concentrations were higher when participants consumed RS compared with entry and NSP diets, but individual responses varied. Individual and total fecal SCFA excretion, weight, and moisture were higher than those for habitual diets when either fiber diet was consumed. SCFA concentrations (except butyrate) and excretions were higher for males than for females. Butyrate levels increased in response to RS in most individuals but often decreased when entry levels were high. Fecal butyrate and ammonia excretions were positively associated ((2) = 0.76; P < 0.001). In conclusion, fecal butyrate levels vary widely among individuals but consuming a diet high in RS usually increases levels and may help maintain colorectal health.

Over-expression of specific HvCslF cellulose synthase-like genes in transgenic barley increases the levels of cell wall (1,3;1,4)-β-d-glucans and alters their fine structure

Cell walls in commercially important cereals and grasses are characterized by the presence of (1,3;1,4)-β-d-glucans. These polysaccharides are beneficial constituents of human diets, where they can reduce the risk of hypercholesterolemia, type II diabetes, obesity and colorectal cancer. The biosynthesis of cell wall (1,3;1,4)-β-d-glucans in the Poaceae is mediated, in part at least, by the cellulose synthase-like CslF family of genes. Over-expression of the barley CslF6 gene under the control of an endosperm-specific oat globulin promoter results in increases of more than 80% in (1,3;1,4)-β-d-glucan content in grain of transgenic barley. Analyses of (1,3;1,4)-β-d-glucan fine structure indicate that individual CslF enzymes might direct the synthesis of (1,3;1,4)-β-d-glucans with different structures. When expression of the CslF6 transgene is driven by the Pro35S promoter, the transgenic lines have up to sixfold higher levels of (1,3;1,4)-β-d-glucan in leaves, but similar levels as controls in the grain. Some transgenic lines of Pro35S:CslF4 also show increased levels of (1,3;1,4)-β-d-glucans in grain, but not in leaves. Thus, the effects of CslF genes on (1,3;1,4)-β-d-glucan levels are dependent not only on the promoter used, but also on the specific member of the CslF gene family that is inserted into the transgenic barley lines. Altering (1,3;1,4)-β-d-glucan levels in grain and vegetative tissues will have potential applications in human health, where (1,3;1,4)-β-d-glucans contribute to dietary fibre, and in tailoring the composition of biomass cell walls for the production of bioethanol from cereal crop residues and grasses.

Resistant starch, large bowel fermentation and a broader perspective of prebiotics and probiotics

The metabolic end products of the large bowel microbiota contribute significantly to human health. After weaning to solid foods, some of the most important of these are the short chain fatty acids (SCFA) produced by the fermentation of undigested dietary components and endogenous secretions. The main SCFA are acetate, propionate and butyrate which have numerous documented effects promoting large bowel function. Of the major acids, butyrate seems especially important. It is a major metabolic fuel for colonocytes and promotes a normal phenotype in these cells, potentially lowering the risk of diseases such as colo-rectal cancer. Imbalances in the microbiota are thought to predispose to large bowel dysfunction and probiotics are being developed to correct this. However, most commercial products contain bacteria (lactobacilli and bifidobacteria) which are dominant species in milk-fed infants but have limited roles in adults. Prebiosis is defined usually by the specific stimulation of these bacteria. However, the end products of most probiotics do not include butyrate or propionate which raises questions about their effectiveness in promoting bowel health in adults. Resistant starch (RS) is a dietary fibre component and its fermentation generally favours butyrate production. Dietary RS intakes and faecal butyrate levels are high in populations at low risk of diet-related large bowel diseases. Conversely, RS intakes and faecal butyrate levels are very low in high risk groups. This raises the possibility that greater RS consumption could be of health benefit. RS is not regarded widely as a prebiotic but (according to the accepted definition) most forms show the requisite features in stimulating specific bacteria, giving raised total SCFA and butyrate levels and a consequent benefit to the host. Current efforts to improve public health through increasing RS consumption could be facilitated by greater recognition of its prebiotic role.

Acetylated, propionylated or butyrylated starches raise large bowel short-chain fatty acids preferentially when fed to rats

Maize starch was acylated with acetic, propionic or butyric anhydride to produce the corresponding acylated starch. In the first experiment, butyrylated starch at a degree of substitution (DS) of 0.25 (i.e., 1 acyl unit per 4 glucosyl units) was fed to rats for 3 d. Cecal and distal colonic SCFA concentrations were 170 and 78% higher, respectively, in rats fed the butyrylated starch. However, the greatest increase was in butyrate with corresponding increases of 460 and 212%. Subsequently, acetylated, propionylated or butyrylated starches with DS of approximately 0.18 were prepared on a larger scale. Body weight gain did not differ between rats fed these acylated starches or a control starch for 14 d. Large bowel pH was significantly lower and digesta mass significantly higher throughout the large bowel in rats fed the acylated starches. Cecal + distal colonic starch averaged 12 mg in rats fed the control starch and 103, 134 and 135 (pooled SEM = 6) mg in rats fed acetylated, propionylated or butyrylated starch, respectively. Large bowel SCFA concentrations and pools were significantly higher in rats fed the three acylated starches and were disproportionately greater in the SCFA that had been esterified to the starch. In the cecum, acetate, propionate and butyrate pools were 280, 690 and 1060% higher, respectively, in rats fed the corresponding acylated starch than in those fed the control diet. In the distal colon, the corresponding increases were 320, 940 and 1370%. These data indicate that acylated starches are resistant starch (RS) and raise large bowel SCFA, apparently through bacterial release of the esterified fatty acid and fermentation of the residual starch.

Degree of polymerization of inulin-type fructans differentially affects number of lactic acid bacteria, intestinal immune functions, and immunoglobulin A secretion in the rat cecum

This study examined the role of degree of polymerization (DP) of inulin-fructans in modulating the interaction between lactic acid bacteria and IgA cecal secretion. Rats were fed a control diet or a diet containing one of the fructans with different DP. Consuming fructans increased the cecal IgA concentrations in the order DP4 > DP8 > DP16. Cecal lactobacilli counts were higher in DP4, DP8, and DP16, whereas bifidobacteria were higher in DP8, DP16, and DP23. Cecal IgA concentrations were correlated with cecal lactobacilli counts (P < 0.01). DP4, DP8, and DP16, but not DP23, significantly increased IgA-producing plasma cells in the cecal mucosa. IFN-γ and IL-10 production in the cecal CD4(+) T cells was enhanced solely in DP4. The results show that fructans with lower DP enhance cecal IgA secretion and increase the plasma cells and suggest that the increased lactobacilli may contribute to the stimulation of cecal IgA secretion.

Two high-amylose maize starches with different amounts of resistant starch vary in their effects on fermentation, tissue and digesta mass accretion, and bacterial populations in the large bowel of pigs

Four groups of young pigs (n 6) were fed a diet containing 50% maize starch as either a highly digestible waxy starch (control; 0% amylose) or one of three resistant starch (RS) diets, namely a high-amylose maize starch (HAMS; 85% amylose), this starch subjected to hydrothermal treatment (HTHAMS; 85% amylose), or a blend of HAMS and HTHAMS included in equal amounts, for 21 d. Food intake and live weight at the end of the study were similar among the four groups. Ileal starch digestibility was lower in pigs fed the three RS diets but was greater for HAMS(88%) than for HTHAMS (70%; P<0.05). Faecal output and large bowel digesta mass, and concentrations and pools of individual and total SCFA were higher (by about two- to threefold; all P<0.05) and digesta pH lower (by about 1 unit, all P<0.001) in pigs fed either HAMS or HTHAMS compared to the controls. These differences in biomarkers were seen along the length of the large bowel. Colon length was 0.5-0.9 m longer (19-35%) in pigs fed the high-RS diets relative to those fed the highly digestible starch diet (P<0.05). Faecal and proximal colonic lactobacilli and bifidobacteria numbers were higher (by 1 and 3 log units; P< 0.05) in pigs fed the HAMS or HTHAMS diets. Although both high-amylose starches promoted fermentation throughout the large bowel, the data suggest that the effects of HTHAMS may be more pronounced in the distal region compared to those of HAMS.

Resistant starch prevents colonic DNA damage induced by high dietary cooked red meat or casein in rats

In a previous study we have shown that high levels of dietary protein (as casein) result in increased levels of colonic DNA damage, measured by the comet assay, and thinning of the colonic mucus layer in rats when dietary resistant starch (RS) is negligible. Feeding RS abolishes these effects. This study aimed to establish whether a diet high in protein as cooked red meat would have similar effects and whether RS was protective. Rats were fed a diet containing 15% or 25% casein or 25% cooked lean red beef, each with or without the addition of 48% high amylose maize starch (a rich source of RS) for four weeks. As expected, high dietary casein caused a 2-fold increase in colonic DNA damage compared with a low casein diet and reduced the thickness of the colonic mucus layer by 41%. High levels of cooked meat caused 26% greater DNA damage than the high casein diet but reduced mucus thickness to a similar degree to casein. Addition of RS to the diet abolished the increase in DNA damage and the loss of colonic mucus thickness induced by either high protein diet. Cecal and fecal short chain fatty acid pools were also increased by inclusion of RS in the diet. Because DNA damage is an early step in the initiation of cancer, these findings suggest that increased DNA damage due to high dietary protein as cooked red meat or casein could increase colorectal cancer risk but inclusion of resistant starch in the diet could significantly reduce that risk.

Fecal numbers of bifidobacteria are higher in pigs fed Bifidobacterium longum with a high amylose cornstarch than with a low amylose cornstarch

Twelve young male pigs consumed a purified diet containing wheat bran as fiber source. Starch provided 50% of total daily energy either as a low amylose cornstarch or as a high amylose (amylomaize) starch. The pigs were given a supplement of a freeze-dried probiotic organism (Bifidobacterium longum CSCC 1941). A block crossover design was used so that at any one time two groups of three pigs consumed either the high or low amylose cornstarch without probiotic and a further two groups of three pigs consumed either high or low amylose cornstarch with probiotic. Neither food intake nor body weight gain was affected by diet. Fecal output was higher when pigs were fed the high amylose cornstarch, but moisture content was unaffected. Fecal concentrations and excretion of total volatile fatty acids were higher when pigs were fed the high amylose cornstarch. Concentrations of acetate were unaffected by dietary starch, but those of propionate and butyrate were higher when the high amylose cornstarch was consumed. Fecal excretion of all three acids was higher during high amylose cornstarch feeding. Bifidobacteria were detected in the feces only when pigs were fed Bifidobacterium longum. Fecal bifidobacteria counts (expressed per gram of wet feces) and their daily fecal excretion were higher when pigs were fed high amylose cornstarch. Feeding the probiotic did not alter fecal starch or volatile fatty acids. None of the variables studied was affected by the order of feeding of starch or probiotic. The data show that a high amylose starch acts as a prebiotic in promoting the fecal excretion of probiotic organisms.

Resistant starches protect against colonic DNA damage and alter microbiota and gene expression in rats fed a Western diet

Resistant starch (RS), fed as high amylose maize starch (HAMS) or butyrylated HAMS (HAMSB), opposes dietary protein-induced colonocyte DNA damage in rats. In this study, rats were fed Western-type diets moderate in fat (19%) and protein (20%) containing digestible starches [low amylose maize starch (LAMS) or low amylose whole wheat (LAW)] or RS [HAMS, HAMSB, or a whole high amylose wheat (HAW) generated by RNA interference] for 11 wk (n = 10/group). A control diet included 7% fat, 13% protein, and LAMS. Colonocyte DNA single-strand breaks (SSB) were significantly higher (by 70%) in rats fed the Western diet containing LAMS relative to controls. Dietary HAW, HAMS, and HAMSB opposed this effect while raising digesta levels of SCFA and lowering ammonia and phenol levels. SSB correlated inversely with total large bowel SCFA, including colonic butyrate concentration (R(2) = 0.40; P = 0.009), and positively with colonic ammonia concentration (R(2) = 0.40; P = 0.014). Analysis of gut microbiota populations using a phylogenetic microarray revealed profiles that fell into 3 distinct groups: control and LAMS; HAMS and HAMSB; and LAW and HAW. The expression of colonic genes associated with the maintenance of genomic integrity (notably Mdm2, Top1, Msh3, Ung, Rere, Cebpa, Gmnn, and Parg) was altered and varied with RS source. HAW is as effective as HAMS and HAMSB in opposing diet-induced colonic DNA damage in rats, but their effects on the large bowel microbiota and colonocyte gene expression differ, possibly due to the presence of other fiber components in HAW.

Resistant Starch Attenuates Colonic DNA Damage Induced by Higher Dietary Protein in Rats

Epidemiologic studies suggest that dietary complex carbohydrates are protective against colorectal cancer but dietary protein may increase risk. However, experimental data to support these relationships are scant. We have shown in rats that consumption of a high-protein (25% casein) diet for 4 wk resulted in a twofold increase in damage to colonocyte DNA compared with a low-protein (15% casein) diet. This was associated with thinning of the colonic mucous barrier and increased levels of fecal p-cresol. Addition of resistant starch as a high-amylose maize starch to the diet increased cecal short-chain fatty acid pools and attenuated DNA damage, suggesting protection against genotoxic agents. In humans, this could translate to altered risk of colonic cancer.

The effects of dietary fish oil on hepatic high density and low density lipoprotein receptor activities in the rat

Rats were fed either a standard ration diet or that diet supplemented with 8% by wt of a marine fish oil or safflower oil. After 10 days, plasma triacylglycerols, total cholesterol, high density lipoprotein (HDL) cholesterol, hepatic cholesterol and fatty acid synthesis and hepatic low density lipoprotein (LDL) receptor activity were significantly depressed while HDL receptor activity was significantly increased in rats fed fish oil. Fish oil-induced effects on cholesterol metabolism in the rat therefore include reciprocal changes in the activities of hepatic LDL and HDL receptors.

Effects of high-amylose maize starch and butyrylated high-amylose maize starch on azoxymethane-induced intestinal cancer in rats

Colorectal cancer (CRC) is a major cause of death worldwide. Studies suggest that dietary fibre offers protection perhaps by increasing colonic fermentative production of butyrate. This study examined the importance of butyrate by investigating the effects of resistant starch (RS) and butyrylated-RS on azoxymethane (AOM)-induced CRC in rats. Four groups (n = 30 per group) of Sprague-Dawley rats were fed AIN-93G-based diets containing a standard low-RS maize starch (LAMS), LAMS + 3% tributyrin (LAMST), 10% high-amylose maize starch (HAMS) and 10% butyrylated HAMS (HAMSB) for 4 weeks. Rats were injected once weekly for 2 weeks with 15 mg/kg AOM, maintained on diets for 25 weeks and then killed. Butyrate concentrations in large bowel digesta were higher in rats fed HAMSB than other groups (P < 0.001); levels were similar in HAMS, LAMS and LAMST groups. The proportion of rats developing tumours were lower in HAMS and HAMSB than LAMS (P < 0.05), and the number of tumours per rat were lower in HAMSB than LAMS (P < 0.05). Caecal digesta butyrate pools and concentrations were negatively correlated with tumour size (P < 0.05). Hepatic portal plasma butyrate concentrations were higher (P < 0.001) in the HAMSB compared with other groups and negatively correlated with tumour number per rat (P < 0.009) and total tumour size for each rat (P = 0.05). HAMSB results in higher luminal butyrate than RS alone or tributyrin. This is associated with reduced tumour incidence, number and size in this rat model of CRC supporting the important protective role of butyrate. Interventional strategies designed to maximize luminal butyrate may be of protective benefit in humans.

Dietary manipulation of oncogenic microRNA expression in human rectal mucosa: a randomized trial

High red meat (HRM) intake is associated with increased colorectal cancer risk, while resistant starch is probably protective. Resistant starch fermentation produces butyrate, which can alter microRNA (miRNA) levels in colorectal cancer cells in vitro; effects of red meat and resistant starch on miRNA expression in vivo were unknown. This study examined whether a HRM diet altered miRNA expression in rectal mucosa tissue of healthy volunteers, and if supplementation with butyrylated resistant starch (HRM+HAMSB) modified this response. In a randomized cross-over design, 23 volunteers undertook four 4-week dietary interventions; an HRM diet (300 g/day lean red meat) and an HRM+HAMSB diet (HRM with 40 g/day butyrylated high amylose maize starch), preceded by an entry diet and separated by a washout. Fecal butyrate increased with the HRM+HAMSB diet. Levels of oncogenic mature miRNAs, including miR17-92 cluster miRNAs and miR21, increased in the rectal mucosa with the HRM diet, whereas the HRM+HAMSB diet restored miR17-92 miRNAs, but not miR21, to baseline levels. Elevated miR17-92 and miR21 in the HRM diet corresponded with increased cell proliferation, and a decrease in miR17-92 target gene transcript levels, including CDKN1A. The oncogenic miR17-92 cluster is differentially regulated by dietary factors that increase or decrease risk for colorectal cancer, and this may explain, at least in part, the respective risk profiles of HRM and resistant starch. These findings support increased resistant starch consumption as a means of reducing risk associated with an HRM diet.

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.

Soluble fiber polysaccharides: effects on plasma cholesterol and colonic fermentation

Many soluble-fiber polysaccharides, used as stabilizers and thickeners by the food industry, lower plasma cholesterol and slow small intestinal transit and nutrient absorption. Although nondigestible by human enzymes, these polysaccharides are fermented by the large-bowel microflora, yielding short-chain fatty acids that are absorbed and contribute to energy. The caloric yield from fiber polysaccharides needs to be quantified. Short-chain fatty acid production from soluble fibers is modified by the presence of insoluble fibers but, in total, is probably less than from other carbohydrates, e.g., resistant starch. Short-chain fatty acids do not seem to mediate effects of fiber on plasma cholesterol, but in the large bowel they exert the trophic and antineoplastic effects of dietary fiber. The mechanism for cholesterol reduction by soluble fibers relates to enhanced steroid excretion and altered fat absorption and may be a function of the viscosity of these fibers in solution. The relationships between the chemical structure of soluble polysaccharides and their documented physiologic effects are not yet clear. By using polysaccharides of defined structure and properties, it should be possible to identify those characteristics that predict physiologic actions.

Butyrate delivered by butyrylated starch increases distal colonic epithelial apoptosis in carcinogen-treated rats

Animal studies show that increasing large bowel butyrate concentration through ingestion of butyrylated or resistant starches opposes carcinogen-induced tumorigenesis, which is consistent with population data linking greater fiber consumption with lowered colorectal cancer (CRC) risk. Butyrate has been shown to regulate the apoptotic response to DNA damage. This study examined the impact of increasing large bowel butyrate concentration by dietary butyrylated starch on the colonic epithelium of rats treated with the genotoxic carcinogen azoxymethane (AOM). Four groups of 10 male rats were fed AIN-93G based-diets containing either low amylose maize starch (LAMS), LAMS with 3% tributyrin, 10% high amylose maize starch (HAMS) or 10% butyrylated HAMS (HAMSB). HAMS and HAMSB starches were cooked by heating in water. After 4 weeks, rats were injected once with AOM and killed 6 h later. Rates of apoptosis and proliferation were measured in colonic epithelium. Short-chain fatty acid concentrations in large bowel digesta and hepatic portal venous plasma were higher in HAMSB than all other groups. Apoptotic rates in the distal colon were increased by HAMSB and correlated with luminal butyrate concentrations but cellular proliferation rates were unaffected by diet. The increase in apoptosis was most marked in the base and proliferative zone of the crypt. Regulation of luminal butyrate using HAMSB increases the rates of apoptotic deletion of DNA-damaged colonocytes. We propose this pro-apoptotic function of butyrate plays a major role reducing tumour formation in the AOM-treated rat and that these data support a potential protective role of butyrate in CRC.

A high amylose (amylomaize) starch raises proximal large bowel starch and increases colon length in pigs

Young male pigs consumed a diet of fatty minced beef, safflower oil, skim milk powder, sucrose, cornstarch and wheat bran. Starch provided 50% of total daily energy either as low amylose cornstarch, high amylose (amylomaize) cornstarch or as a 50/50 mixture of corn and high amylose starch. Neither feed intake nor body weight gain as affected by dietary starch. Final plasma cholesterol concentrations were significantly higher than initial values in pigs fed the 50/50 mixture of corn and high amylose starch. Biliary concentrations of lithocholate and deoxycholate were lower in pigs fed high amylose starch. Large bowel length correlated positively with the dietary content of high amylose starch. Concentrations of butyrate in portal venous plasma were significantly lower in pigs fed high amylose starch than in those fed cornstarch. Neither large bowel digesta mass nor the concentrations of total or individual volatile fatty acids were affected by diet. However, the pool of propionate in the proximal colon and the concentration of propionate in feces were higher in pigs fed amylose starch. Concentrations of starch were uniformly low along the large bowel and were unaffected by starch type. In pigs with cecal cannula, digesta starch concentrations were higher with high amylose starch than with cornstarch. Electron micrographic examination of high amylose starch granules from these animals showed etching patterns similar to those of granules obtained from human ileostomy effluent. It appears that high amylose starch contributes to large bowel bacterial fermentation in the pig but that its utilization may be relatively rapid.