The VFA, also known as short-chain fatty acids, are produced in the gastrointestinal tract by microbial fermentation of carbohydrates and endogenous substrates, such as mucus. This can be of great advantage to the animal, since no digestive enzymes exist for breaking down cellulose or other complex carbohydrates. The VFA are produced in the largest amounts in herbivorous animal species and especially in the forestomach of ruminants. The VFA, however, also are produced in the lower digestive tract of humans and all animal species, and intestinal fermentation resembles that occurring in the rumen. The principal VFA in either the rumen or large intestine are acetate, propionate, and butyrate and are produced in a ratio varying from approximately 75:15:10 to 40:40:20. Absorption of VFA at their site of production is rapid, and large quantities are metabolized by the ruminal or large intestinal epithelium before reaching the portal blood. Most of the butyrate is converted to ketone bodies or CO2 by the epithelial cells, and nearly all of the remainder is removed by the liver. Propionate is similarly removed by the liver but is largely converted to glucose. Although species differences exist, acetate is used principally by peripheral tissues, especially fat and muscle. Considerable energy is obtained from VFA in herbivorous species, and far more research has been conducted on ruminants than on other species. Significant VFA, however, are now known to be produced in omnivorous species, such as pigs and humans. Current estimates are that VFA contribute approximately 70% to the caloric requirements of ruminants, such as sheep and cattle, approximately 10% for humans, and approximately 20-30% for several other omnivorous or herbivorous animals. The amount of fiber in the diet undoubtedly affects the amount of VFA produced, and thus the contribution of VFA to the energy needs of the body could become considerably greater as the dietary fiber increases. Pigs and some species of monkey most closely resemble humans, and current research should be directed toward examining the fermentation processes and VFA metabolism in those species. In addition to the energetic or nutritional contributions of VFA to the body, the VFA may indirectly influence cholesterol synthesis and even help regulate insulin or glucagon secretion. In addition, VFA production and absorption have a very significant effect on epithelial cell growth, blood flow, and the normal secretory and absorptive functions of the large intestine, cecum, and rumen. The absorption of VFA and sodium, for example, seem to be interdependent, and release of bicarbonate usually occurs during VFA absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Energy contributions of volatile fatty acids from the gastrointestinal tract in various species

The concept of resistant starch (RS) has evoked new interest in the bioavailability of starch and in its use as a source of dietary fiber, particularly in adults. RS is now considered to provide functional properties and find applications in a variety of foods. Types of RS, factors influencing their formation, consequence of such formation, their methods of preparation, their methods of estimation, and health benefits have been briefly discussed in this review.

Resistant Starch–A Review

The release of energy from particulate substrates such as dietary fiber and resistant starch (RS) in the human colon may depend on the presence of specialist primary degraders (or 'keystone species') within the microbial community. We have explored the roles of four dominant amylolytic bacteria found in the human colon in the degradation and utilization of resistant starches. Eubacterium rectale and Bacteroides thetaiotaomicron showed limited ability to utilize RS2- and RS3-resistant starches by comparison with Bifidobacterium adolescentis and Ruminococcus bromii. In co-culture, however, R. bromii proved unique in stimulating RS2 and RS3 utilization by the other three bacterial species, even in a medium that does not permit growth of R. bromii itself. Having previously demonstrated low RS3 fermentation in vivo in two individuals with undetectable populations of R. bromii-related bacteria, we show here that supplementation of mixed fecal bacteria from one of these volunteers with R. bromii, but not with the other three species, greatly enhanced the extent of RS3 fermentation in vitro. This argues strongly that R. bromii has a pivotal role in fermentation of RS3 in the human large intestine, and that variation in the occurrence of this species and its close relatives may be a primary cause of variable energy recovery from this important component of the diet. This work also indicates that R. bromii possesses an exceptional ability to colonize and degrade starch particles when compared with previously studied amylolytic bacteria from the human colon.

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Ruminococcus bromii is a keystone species for the degradation of resistant starch in the human colon.

Resistant starch as functional ingredient: A review

Helicobacter pylori is a newly discovered gram-negative bacterium that lives in the human stomach and duodenum. Infection with this organism is strongly associated with type B antral gastritis and with peptic ulcer disease. Recent evidence from human volunteer studies, therapeutic trials with antimicrobial agents, and experiments with animal models indicates that H. pylori plays an etiologic role in the pathogenesis of type B gastritis. Gastric metaplasia is observed in virtually all patients with duodenal ulceration and may be the target tissue for these bacteria in the duodenum. Patients in whom H. pylori can no longer be identified after ulcer therapy remain in remission for significantly longer than do patients in whom the organism can be found. The data concerning an etiologic role of H. pylori in duodenal ulcer are suggestive but not yet conclusive. Present antimicrobial therapy can suppress but usually cannot eliminate H. pylori.

Helicobacter pylori and the Pathogenesis of Gastroduodenal Inflammation

The aim of the study was to determine if there is a difference between ileal and faecal assays for determining amino acid and N digestibilities in adult human subjects. Comparison of true ileal amino acid and N digestibilities was also made between adult human subjects and growing pigs to establish the usefulness of the pig as a model animal. Five subjects with established ileostomies and six subjects with intact large bowels consumed a constant diet consisting of meat, vegetables, fruit, bread and dairy products for 7 d with collection of ileostomy contents or faeces respectively over the last 4 d. The study was repeated using 25 kg body weight ileostomized and intact pigs. Apparent amino acid and N digestibility coefficients were determined. For human subjects the faecal digestibility values were significantly higher (P < 0.05) than the ileal values for Arg, Asp, Gly, Phe, Pro, Ser, Thr and Trp. The faecal digestibility of Met was significantly lower than the ileal value. Determination of DNA, diaminopimelic acid (DAPA) and the digestibilities of pectin, hemicellulose and cellulose in human subjects indicated that some microbial colonization had occurred at the terminal ileum after formation of an ileostomy; however, this was not as extensive as in the large intestine. True ileum amino acid and N digestibilities were calculated after correcting for the endogenous contribution of amino acids at the terminal ileum determined using a protein-free diet. There were no significant differences between adult human subjects and pigs for true ileal dietary amino acid digestibility except for Thr, Phe, Cys and Met. There were no significant differences between adult humans and pigs for the ileal digestibility of dry matter and the faecal digestibility of gross energy.

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Comparison of the ileal and faecal digestibility of dietary amino acids in adult humans and evaluation of the pig as a model animal for digestion studies in man.

A high-fiber diet may protect against colon cancer because of the butyrate generated in the colon by bacterial fermentation of nonstarch polysaccharides. Butryrate can reverse neoplastic changes, at least in vitro, and resistant starch (RS) represents a source of butyrate in vivo. We examined the effects of replacing normal maize starch in the diet of rats with three preparations of RS on the amounts of starch, butyrate, and other short-chain fatty acids in the cecum. We examined the effects on fecal bulking and transit time, which have been suggested to protect against colon cancer. The RS preparations that we tested were potato starch, high-amylose maize starch, and an alpha-amylase-treated high-amylose maize starch. All had major effects on fecal weight and on the weight of the cecum but only slightly shortened transit times. All increased the amount of starch reaching the cecum and increased short-chain fatty acid production in the cecum; potato starch had the greatest effect and high-amylose maize starch the least. Potato starch, unlike high-amylose maize starch, enhanced the proportion of butyrate. Thus there were marked differences among sources of RS, even though these were all classified as RS2. The significance for colon cancer is discussed.

Comparative effects of three resistant starch preparations on transit time and short-chain fatty acid production in rats.

We aimed to show that Campylobacter pylori infection increases the concentration of ammonia in the gastric mucus and alters the movement of H+ through the mucus. Mucus from uninfected and C. pylori-infected stomachs was collected at postmortem. Ammonia was measured enzymatically. The ammonia concentration in C. pylori-infected mucus was fourfold greater than in uninfected mucus. H+ movement experiments were carried out using an in vitro chamber, in which a layer of mucus separated a 0.1 M HCl solution from distilled H2O. The change in pH of the distilled H2O was measured over 30 min. A drop in pH was measured for uninfected mucus. A slight rise in pH was measured for C. pylori-infected mucus. We conclude that C. pylori infection is associated with reduced H+ movement through mucus and that this may be due to increased ammonia concentration within mucus.

Ammonia Produced by Campylobacter pylori Neutralizes H+ Moving through Gastric Mucus

Mucus, a hydrated complex consisting mainly of glycoproteins, forms a layer over the epithelial surface of the gastrointestinal tract. The usual preparative procedures for histological and scanning electron microscopic examination of the gut result in the loss or distortion of this mucus layer. Careful evaluation of two new methods reported to stabilize the mucus layer showed that acrolein vapor did not provide adequate fixation, but application of heat-inactivated antiserum raised in rabbits against rat colon mucus reliably preserved a continuous layer closely adherent to the epithelium. This stabilized layer is continuous with the mucus in the colonic crypts.

Preservation of mucus in situ in rat colon.

The distribution of Campylobacter pylori, its prevalence, and its relationship to gastritis and urease activity have been studied in 54 postmortem stomachs Infection was confirmed by finding C pylori in a Gram-stained smear of gastric mucus harvested from the entire stomach Eight tissue specimens were obtained from predetermined sites from each stomach and examined for histologic gastritis and urease activity Thirty-seven per cent of stomachs were infected, and of these 80% had widespread histologic gastritis The detection of urease activity provided information on the distribution of the organism and had a high correlation with histologic gastritis The organism is capable of infecting any area of the stomach Infection is common and is more prevalent in Polynesian subjects (60%) than in Caucasians (19%)

C. Tasman-Jones

Papers

Comparison of the ileal and faecal digestibility of dietary amino acids in adult humans and evaluation of the pig as a model animal for digestion studies in man.

Comparative effects of three resistant starch preparations on transit time and short-chain fatty acid production in rats.

Ammonia Produced by Campylobacter pylori Neutralizes H+ Moving through Gastric Mucus

Preservation of mucus in situ in rat colon.

Distribution of Campylobacter pylori in the human stomach obtained at postmortem.