Clinical trial: the microbiological and immunological effects of synbiotic consumption - a randomized double-blind placebo-controlled study in active Crohn's disease.

BACKGROUND: Crohn's disease is an inflammatory illness in which the immune response against gut microorganisms is believed to drive an abnormal immune response. Consequently, modification of mucosal bacterial communities, and the immune effects they elicit, might be used to modify the disease state. AIM: To investigate the effects of synbiotic consumption on disease processes in patients with Crohn's disease. METHODS: A randomized, double-blind placebo-controlled trial was conducted involving 35 patients with active Crohn's disease, using a synbiotic comprising Bifidobacterium longum and Synergy 1. Clinical status was scored and rectal biopsies were collected at the start, and at 3- and 6-month intervals. Transcription levels of immune markers and mucosal bacterial 16S rRNA gene copy numbers were quantified using real-time PCR. RESULTS: Significant improvements in clinical outcomes occurred with synbiotic consumption, with reductions in both Crohn's disease activity indices (P = 0.020) and histological scores (P = 0.018). The synbiotic had little effect on mucosal IL-18, INF-gamma and IL-1beta; however, significant reductions occurred in TNF-alpha expression in synbiotic patients at 3 months (P = 0.041), although not at 6 months. Mucosal bifidobacteria proliferated in synbiotic patients. CONCLUSION: Synbiotic consumption was effective in improving clinical symptoms in patients with active Crohn's disease.

Possible implications for health of the different definitions of dietary fibre.

There is impressive evidence from epidemiological and experimental studies that dietary fibre derived form vegetables, fruit and wholegrain cereals protects against and may be useful in the treatment of a wide range of diseases. However, while there is some evidence of benefit of extracted and synthetic fibres in terms of lowering levels of cardiovascular risk factors, improving measures of glycaemic control and gastrointestinal function, epidemiological confirmation of clinical benefit and long term safety are lacking. An appropriate definition of dietary fibre is essential, given that claiming a food is high in dietary fibre is in effect making a health claim, without formally doing so. The new Codex definition acknowledges the difference between naturally occurring carbohydrate polymers which are neither digested nor absorbed in the human small intestine and synthetic or extracted polymers. However the latter two groups may also be defined as dietary fibre provided "beneficial physiological effect has been demonstrated by generally accepted scientific evidence". Given the need for a definition of dietary fibre which can be used for food labelling, setting nutrient reference values and decisions relating to health claims it is important to achieve agreement as to what constitutes a meaningful physiological effect and the level of evidence required to be certain of such effect.

Studies of a urinary biomarker of dietary inorganic sulphur in subjects on diets containing 1-38 mmol sulphur/day and of the half-life of ingested 34SO4(2-).

OBJECTIVE: Sulphites are widely used food additives that may damage health, hence limits are set on their use. They are excreted in urine as sulphate, along with sulphate derived from sulphur amino acids. Dietary intakes of sulphites are hard to determine, so we have tested the utility of urinary nitrogen:sulphate ratio as a biomarker of inorganic sulphur (IS) intake. Additionally we determined the half-life of ingested (34)SO(4)(2-) from its urinary excretion. SUBJECTS: Twenty healthy adult subjects were recruited by poster advertisement, for a 24-h study where they ate specified foods, which were high in IS, in addition to their normal diet. The half-life of ingested (34)SO(4)(2-) was assessed in five healthy volunteers, given 5.9 mmols of Na(2)(34)SO(4) as a single dose and collecting all urine specimens for 72-96 h. Urine and duplicate diets from three previously conducted studies were analysed for nitrogen and sulphate content, thus expanding the range of IS intakes for evaluation. METHODS: Duplicate diets were analysed for IS content by ion exchange chromatography, while IS intake was predicted from urinary sulphate (g/day S)-(urinary nitrogen (g/day)/18.89). (32)S:(34)S ratios were determined by liquid chromatography mass spectrometry/mass spectrometry. RESULTS: The range of IS intake was 1.3-37.5 mmol S/day. Actual and predicted IS intakes were mmol/day+/-s.e. 9.2+/-0.65 and 7.0+/-0.45, respectively, and were correlated r=0.60 (n=108). The mean half-life of ingested (34)SO(4)(2-) was 8.2 h. CONCLUSIONS: From a 24-h urine collection, IS intake from the habitual diet can be determined for groups of individuals. To predict individual intakes of IS, which may include high sporadic amounts from beer and wine, at least 48 h of urine collection would be required.

Physiological aspects of energy metabolism and gastrointestinal effects of carbohydrates.

The energy values of carbohydrates continue to be debated. This is because of the use of different energy systems, for example, combustible, digestible, metabolizable, and so on. Furthermore, ingested macronutrients may not be fully available to tissues, and the tissues themselves may not be able fully to oxidize substrates made available to them. Therefore, for certain carbohydrates, the discrepancies between combustible energy (cEI), digestible energy (DE), metabolizable energy (ME) and net metabolizable energy (NME) may be considerable. Three food energy systems are in use in food tables and for food labelling in different world regions based on selective interpretation of the digestive physiology and metabolism of food carbohydrates. This is clearly unsatisfactory and confusing to the consumer. While it has been suggested that an enormous amount of work would have to be undertaken to change the current ME system into an NME system, the additional changes may not be as great as anticipated. In experimental work, carbohydrate is high in the macronutrient hierarchy of satiation. However, studies of eating behaviour indicate that it does not unconditionally depend on the oxidation of one nutrient, and argue against the operation of a simple carbohydrate oxidation or storage model of feeding behaviour to the exclusion of other macronutrients. The site, rate and extent of carbohydrate digestion in, and absorption from the gut are key to understanding the many roles of carbohydrate, although the concept of digestibility has different meanings. Within the nutrition community, the characteristic patterns of digestion that occur in the small (upper) vs large (lower) bowel are known to impact in contrasting ways on metabolism, while in the discussion of the energy value of foods, digestibility is defined as the proportion of combustible energy that is absorbed over the entire length of the gastrointestinal tract. Carbohydrates that reach the large bowel are fermented to short-chain fatty acids. The exact amounts and types of carbohydrate that reach the caecum are unknown, but are probably between 20 and 40 g/day in countries with 'westernized' diets, whereas they may reach 50 g/day where traditional staples are largely cereal or diets are high in fruit and vegetables. Non-starch polysaccharides clearly affect bowel habit and so, to a lesser extent, does resistant starch. However, the short-chain carbohydrates, which are also found in breast milk, have little if any laxative role, although do effect the balance of the flora. This latter property has led to the term 'prebiotic', which is defined as the capacity to increase selectively the numbers of bifidobacteria and lactobacilli without growth of other genera. This now well-established physiological property has not so far led through to clear health benefits, but current studies are focused on their potential to prevent diarrhoeal illnesses, improve well-being and immunomodulation, particularly in atopic children and on increased calcium absorption.

Carbohydrate terminology and classification.

Dietary carbohydrates are a group of chemically defined substances with a range of physical and physiological properties and health benefits. As with other macronutrients, the primary classification of dietary carbohydrate is based on chemistry, that is character of individual monomers, degree of polymerization (DP) and type of linkage (alpha or beta), as agreed at the Food and Agriculture Organization/World Health Organization Expert Consultation in 1997. This divides carbohydrates into three main groups, sugars (DP 1-2), oligosaccharides (short-chain carbohydrates) (DP 3-9) and polysaccharides (DP> or =10). Within this classification, a number of terms are used such as mono- and disaccharides, polyols, oligosaccharides, starch, modified starch, non-starch polysaccharides, total carbohydrate, sugars, etc. While effects of carbohydrates are ultimately related to their primary chemistry, they are modified by their physical properties. These include water solubility, hydration, gel formation, crystalline state, association with other molecules such as protein, lipid and divalent cations and aggregation into complex structures in cell walls and other specialized plant tissues. A classification based on chemistry is essential for a system of measurement, predication of properties and estimation of intakes, but does not allow a simple translation into nutritional effects since each class of carbohydrate has overlapping physiological properties and effects on health. This dichotomy has led to the use of a number of terms to describe carbohydrate in foods, for example intrinsic and extrinsic sugars, prebiotic, resistant starch, dietary fibre, available and unavailable carbohydrate, complex carbohydrate, glycaemic and whole grain. This paper reviews these terms and suggests that some are more useful than others. A clearer understanding of what is meant by any particular word used to describe carbohydrate is essential to progress in translating the growing knowledge of the physiological properties of carbohydrate into public health messages.

Review article: prebiotics in the gastrointestinal tract.

BACKGROUND: Prebiotics are short-chain carbohydrates that alter the composition, or metabolism, of the gut microbiota in a beneficial manner. It is therefore expected that prebiotics will improve health in a way similar to probiotics, whilst at the same time being cheaper, and carrying less risk and being easier to incorporate into the diet than probiotics. AIM: To review published evidence for prebiotic effects on gut function and human health. METHODS: We searched the Science Citation Index with the terms prebiotic, microbiota, gut bacteria, large intestine, mucosa, bowel habit, constipation, diarrhoea, inflammatory bowel disease, Crohn's disease, ulcerative colitis, pouchitis, calcium and cancer, focussing principally on studies in humans and reports in the English language. Search of the Cochrane Library did not identify any clinical study or meta-analysis on this topic. RESULTS: Three prebiotics, oligofructose, galacto-oligosaccharides and lactulose, clearly alter the balance of the large bowel microbiota by increasing bifidobacteria and Lactobacillus numbers. These carbohydrates are fermented and give rise to short-chain fatty acid and intestinal gas; however, effects on bowel habit are relatively small. Randomized-controlled trials of their effect in a clinical context are few, although animal studies show anti-inflammatory effects in inflammatory bowel disease, while calcium absorption is increased. CONCLUSIONS: It is still early days for prebiotics, but they offer the potential to modify the gut microbial balance in such a way as to bring direct health benefits cheaply and safely.

Mucosal bacteria in ulcerative colitis.

Ulcerative colitis (UC) is an acute and chronic inflammatory bowel disease of unknown aetiology, although bacterial species belonging to the normal colonic microbiota are known to be involved in its initiation and maintenance. Several organisms have been linked to the disease; however, mucosa-associated bacteria are more likely to be involved than their luminal counterparts, due to their close proximity to the host epithelium. Comparative bacteriological analyses were done on rectal biopsies to investigate differences in mucosal bacteria in patients with UC and healthy controls. Complex bacterial communities were found in both groups, with significant reductions in bifidobacterial numbers in UC, which suggested that they might have a protective role in the disease. Accordingly, a therapy for treating UC was designed, with the aim of modifying the mucosal microbiota to increase bifidobacterial colonisation and reduce inflammation. Ranges of mucosal and faecal bifidobacteria were tested for their substrate preferences and their abilities to survive under a variety of environmental conditions. A synbiotic comprising a probiotic (Bifidobacterium longum) isolated from healthy rectal mucosa combined with a prebiotic (oligofructose-enriched inulin - Synergy 1) was developed. The treatment was used in a randomised controlled trial involving eighteen patients with active UC, for a period of 1 month. Rectal biopsies were collected at the beginning and end of the study. Bacteriological analysis and transcription levels of epithelium-related immune markers were assessed. Results demonstrated that short-term synbiotic treatment resulted in increased bifidobacterial colonisation of the rectal mucosa and induced significant reductions in the expression of molecules that control inflammation in active UC.

Synbiotic therapy (Bifidobacterium longum/Synergy 1) initiates resolution of inflammation in patients with active ulcerative colitis: a randomised controlled pilot trial.

BACKGROUND AND AIMS: Ulcerative colitis (UC) is an acute and chronic inflammatory disease of the large bowel with unknown aetiology. The immune response against normal commensal microorganisms is believed to drive inflammatory processes associated with UC. Therefore, modulation of bacterial communities on the gut mucosa, through the use of probiotics and prebiotics, may be used to modify the disease state. METHODS: A synbiotic was developed for use in UC patients combining a probiotic, Bifidobacterium longum, isolated from healthy rectal epithelium, and a prebiotic (Synergy 1), a preferential inulin-oligofructose growth substrate for the probiotic strain. Treatment was employed in a double blinded randomised controlled trial using 18 patients with active UC for a period of one month. Clinical status was scored and rectal biopsies were collected before and after treatment, and transcription levels of epithelium related immune markers were measured. RESULTS: Sigmoidoscopy scores (scale 0-6) were reduced in the test group (start 4.5 (1.4), end 3.1 (2.5)) compared with placebo (start 2.6 (2.1), end 3.2 (2.2)) (p=0.06). mRNA levels for human beta defensins 2, 3, and 4, which are strongly upregulated in active UC, were significantly reduced in the test group after treatment (p=0.016, 0.038, and 0.008, respectively). Tumour necrosis factor alpha and interleukin 1alpha, which are inflammatory cytokines that drive inflammation and induce defensin expression, were also significantly reduced after treatment (p=0.018 and 0.023, respectively). Biopsies in the test group had reduced inflammation and regeneration of epithelial tissue. CONCLUSIONS: Short term synbiotic treatment of active UC resulted in improvement of the full clinical appearance of chronic inflammation in patients receiving this therapy.

Prebiotic carbohydrates modify the mucosa associated microflora of the human large bowel.

BACKGROUND AND AIMS: The mucosa associated flora of the large intestine is important in determining mucosal function although what controls its composition is unknown. This study has determined the effect of the prebiotic carbohydrates oligofructose and inulin on the mucosal flora. METHODS: An in vitro chemostat model of both planktonic and surface associated bacteria was used followed by an intervention study in 29 subjects undergoing colonoscopy. SUBJECTS: Fourteen subjects, recruited from colonoscopy waiting lists, supplemented their diet for two weeks with a mix of 7.5 g of oligofructose and 7.5 g inulin. Fifteen subjects were recruited at the time of colonoscopy and given no supplement. Multiple endoscopic biopsies were taken from the caecum, transverse and descending colon, and rectum. The mucosal flora was characterised by culture and to species level by cellular fatty acid profiles. Cell proliferation was assessed by immunohistochemical staining for minichromosome maintenance protein 2, Ki67, and proliferating cell nuclear antigen. RESULTS: In vitro prebiotics increased surface counts of bifidobacteria from 6.6 to 7.3 log(10) colony forming units (CFU) per slide (p<0.0006) with no significant changes in planktonic bacteria. In the feeding study, prebiotics increased mucosal bifidobacteria (log CFU/g mucosa (SEM)) in both the proximal (control 5.3 (0.4) v prebiotic 6.3 (0.3)) (p = 0.059) and distal (control 5.2 (0.3) v prebiotic 6.4 (0.3)) colon (p = 0.01). Lactobacilli were also increased (3.0 (0.1) v 3.7 (0.2) (p = 0.02) in the proximal and 3.1 (0.1) v 3.6 (0.2) (p = 0.04) in the distal colon, respectively). There were significantly more eubacteria in fed subjects but no changes in total anaerobes clostridia, bacteroides, or coliforms, nor in proliferation indices. CONCLUSION: Prebiotic carbohydrates can change the composition of the mucosa associated flora significantly.

Investigation of the reliability of 24 h urine excretion as a biomarker of isoflavone exposure over time and over a wide range of isoflavone intakes.

OBJECTIVE: To study the variation in genistein + daidzein intake over a 6-month period and test the reliability of 24 h urinary isoflavones as a biomarker of exposure over time. DESIGN: Dietary genistein + daidzein intake was assessed at various time points throughout six months in 15 healthy subjects. Group 1 (n=8) followed nonsupplemented diets and Group 2 (n=7) took a 35 mg/d isoflavone supplement for 3 months and each subject provided a 24 h urine collection, validated with para-aminobenzoic acid, during weeks 7, 15 and 19. Urine was analysed for genistein and daidzein using LC-MS. RESULTS: Isoflavone intake in Groups 1 and 2 ranged from 0.00 to 1.1 mg/d and 0.1 to 53.1 mg/d, respectively. Urine excretion for both groups ranged from 0.20 to 9.56 mg/d. The relationship between 24 h excretion and isoflavone intake is y=0.44 x +/- 0.03(standard deviation) + 1.57; r=0.89, P<0.001. CONCLUSION: The 24 h urinary isoflavones can be used as biomarkers of isoflavone exposure over time.

Identification and quantitation of mucosal and faecal desulfovibrios using real time polymerase chain reaction.

BACKGROUND: Desulfovibrios produce sulphide, which is toxic to colonic epithelial cells. These bacteria have previously been linked to ulcerative colitis. Traditional methods of culturing these organisms are slow, and often unreliable, while molecular approaches are either non-quantitative or lack sensitivity. AIMS: To develop a sensitive method for quantitating desulfovibrios in stools and biopsy tissue, and to investigate the effects of age and disease on these bacteria. METHODS: Rectal biopsies were taken from 10 colitis patients and 10 healthy controls. Stool samples were obtained from 10 healthy infants (mean age 1.01 (0.18) years), 10 healthy young adults (26.7 (1.2) years), and 10 healthy elderly people (71.7 (1.2) years). Primers were designed and developed for analysing Desulfovibrio populations in the bowel using real time polymerase chain reaction (PCR). RESULTS: The PCR primers were highly specific for desulfovibrios. Large numbers (approximately 10(6)-10(7)/g) occurred in biopsies in colitis patients and healthy subjects, and no disease related differences were observed. Measurements of mucosal desulfovibrios over 12 months showed marked changes in some patients. Infants (10(6)-10(7)/g) and elderly people (10(7)-10(8)/g) had significantly higher numbers of desulfovibrios in stools compared with young adults (10(5)/g). CONCLUSIONS: Real time PCR analysis of desulfovibrios was an efficient and accurate method for studying these potentially harmful microorganisms. Desulfovibrios were ubiquitous in the bowel, irrespective of age. As rectal mucosae were heavily colonised in health and disease, if these bacteria play a role in colitis, some host defect, possibly in sulphide detoxication pathways or in bacterial antigen handling, is required for manifestations of pathogenicity.

Systemic antibodies towards mucosal bacteria in ulcerative colitis and Crohn's disease differentially activate the innate immune response.

BACKGROUND AND AIMS: The mucosa in ulcerative colitis (UC) is replete with antibody producing plasma B cells and polymorphonuclear leucocytes (PMN). This combination of effector cells requires a crosslinking antigen to evoke an antibody driven PMN inflammatory response via their Fc receptors. The stimulus for activation is thought to be commensal bacteria colonising the gut mucosa. The aim of this investigation was to compare the principal culturable bacterial populations on the rectal mucosa of UC patients, and to determine whether specific antibodies towards these bacteria can activate infiltrating PMN through opsonisation. This would provide an explanation for this chronic inflammatory condition. METHODS: Bacteria colonising rectal tissue were characterised using chemotaxonomic techniques. Systemic antibody responses were measured against total antigens and surface antigens of these organisms in UC and Crohn's disease (CD) patients, together with healthy controls. Antibody enhancement of the respiratory burst in PMN was also investigated, against a range of mucosal isolates. RESULTS: Distinct differences were observed in some bacterial populations in UC biopsies, which were generally reflected in antibody responses towards these organisms. UC patients had higher IgG responses to surface antigens, primarily IgG1, whereas the response in CD was mainly IgG2. Antibodies from UC patients greatly enhanced the respiratory burst in PMN, in response to individual bacterial species. CONCLUSIONS: Changes in mucosal bacteria, and a switch from internal to surface antigen/antibody reactivity of a predominantly IgG1 type, leads to greater opsonisation of the respiratory burst in PMN, providing a mechanism for maintaining the inflammatory state in UC.

Intestinal bacteria and ulcerative colitis.

Convincing evidence from both animal models and the study of patients with ulcerative colitis (UC) implicates the intestinal microflora in the initiation and maintenance of the inflammatory processes in this condition. Despite this, no specific pathogen has been identified as causal and the disease is widely believed to occur as the result of a genetically determined, but abnormal immune response to commensal bacteria. When compared with healthy people, UC patients have increased levels of mucosal IgG directed against the normal microflora. Studies of mucosal bacterial populations in UC indicate that there may be increased numbers of organisms, but reduced counts of "protective" bacteria such as lactobacilli and bifidobacteria. In animal models of colitis, antibiotics, particularly metronidazole, clindamycin, ciprofloxacin and the combination of vancomycin/impinemem protect against UC, especially if given before the onset of inflammation. These antibiotics target anaerobes and some Gram-positive organisms such as enterococci. However, antibiotic use in more than a dozen randomised control trials has been very disappointing, probably because we do not know which species to target, when to give the antibiotics, for how long and in what combinations. Surprisingly, therefore, there is a consistent benefit in the small number of studies reported of probiotics to manage UC and pouchitis. There is scope for more work in this area focussing on the mucosal microflora, its interactions with the gut immune system, its metabolic properties and the potential ways of modifying it.

The formation of colonic digestible films of amylose and ethylcellulose from aqueous dispersions at temperatures below 37 degrees C.

The film forming properties of a commercial aqueous ethylcellulose dispersion (Surelease) mixed with a range of ratios of an amylose/butanol complex in the presence of a range of concentrations of a plasticiser has been studied by measuring the minimum film forming temperature (MFFT). Contrary to what was to be anticipated from the literature, it was found that an additional 4% of the plasticiser (dibutyl sebacate), normally present in the standard formulation of the ethyl cellulose dispersion, was sufficient to lower the MFFT to allow the formation of films at 35 degrees C. This was confirmed by assessment of the glass transition temperature of free films prepared by casting and drying at 35 degrees C by the application of dynamic mechanical analysis. This technique also demonstrated that the ethylycellulose and the amylose were not miscible. The ability of faecal slurry to digest the films formed at low temperatures was confirmed by the use of a batch fermenter. The extent of digestion was directly related to the amylose content of the films, ensuring the potential to provide films, which could function as colon specific coatings.

Gastrointestinal effects of prebiotics.

The defining effect of prebiotics is to stimulate selectively the growth of bifidobacteria and lactobacilli in the gut and, thereby, increase the body's natural resistance to invading pathogens. Prebiotic carbohydrates may also have additional, less specific, benefits because they are fermented in the large intestine. The prebiotic carbohydrates that have been evaluated in humans at the present time largely consist of fructans or galactans. There is consistent evidence from in vitro and in vivo studies that these are not digested by normal human enzymes, but are readily fermented by anaerobic bacteria in the large intestine. There are no reports of faecal recovery of measurable quantities of prebiotic carbohydrates. Through fermentation in the large intestine, prebiotic carbohydrates yield short-chain fatty acids, stimulate the growth of many bacterial species in addition to the selective effects on lactobacilli and bifidobacteria, they can also produce gas. Along with other fermented carbohydrates, prebiotics have mild laxative effects, although this has proved difficult to demonstrate in human studies because the magnitude of laxation is small. Potentially, the most important effect of prebiotic carbohydrates is to strengthen the body's resistance to invading pathogens and, thereby, prevent episodes of diarrhoea. At the present time, this effect has not been convincingly demonstrated in either adults or children, although there have been attempts to ameliorate the diarrhoea associated with antibiotics and travel, but without success. However, prebiotic carbohydrates clearly have significant and distinctive physiological effects in the human large intestine, and on the basis of this it is likely that they will ultimately be shown to be beneficial to health.

A study of fructo oligosaccharides in the prevention of travellers' diarrhoea.

BACKGROUND: Prebiotic carbohydrates selectively stimulate the growth of bifidobacteria and lactobacilli in the human colon. These bacteria form part of the gut's inherent defence against invading pathogens. AIM: To test the effectiveness of fructo oligosaccharides in preventing travellers' diarrhoea. METHODS: A total of 244 healthy subjects, travelling to high and medium risk destinations for travellers' diarrhoea, took part in a randomized, double-blind, placebo-controlled study. The protocol comprised a preliminary week for recording bowel habit by diary, a 2-week pre-holiday period with the diary and consumption of 10 g of fructo oligosaccharides or placebo daily, followed by a 2-week holiday with continuation of treatment and diary. A post-study questionnaire was completed by all subjects on their return to the UK. RESULTS: The consumption of fructo oligosaccharides led to a small (6%; P < 0.02) increase in stool frequency in the pre-holiday period and gave a significantly better sense of 'well-being' during the holiday, although subjects reported more flatulence. There were non-significant decreases in episodes of diarrhoea with 20% on placebo and 11% on fructo oligosaccharides recording episodes in the post-study questionnaire (P=0.08) and 46% placebo, 38% fructo oligosaccharides recording episodes in the diary (P > 0.1). No change in bowel frequency, consistency or stool size was recorded. CONCLUSION: Travel to high risk areas increases diarrhoea. Fructo oligosaccharides alone are not sufficient to prevent this, although do have some benefits for the subjects.

Prebiotic digestion and fermentation.

Prebiotics, as currently conceived of, are all carbohydrates of relatively short chain length. To be effective they must reach the cecum. Present evidence concerning the 2 most studied prebiotics, fructooligosaccharides and inulin, is consistent with their resisting digestion by gastric acid and pancreatic enzymes in vivo. However, the wide variety of new candidate prebiotics becoming available for human use requires that a manageable set of in vitro tests be agreed on so that their nondigestibility and fermentability can be established without recourse to human studies in every case. In the large intestine, prebiotics, in addition to their selective effects on bifidobacteria and lactobacilli, influence many aspects of bowel function through fermentation. Short-chain fatty acids are a major product of prebiotic breakdown, but as yet, no characteristic pattern of fermentation acids has been identified. Through stimulation of bacterial growth and fermentation, prebiotics affect bowel habit and are mildly laxative. Perhaps more importantly, some are a potent source of hydrogen in the gut. Mild flatulence is frequently observed by subjects being fed prebiotics; in a significant number of subjects it is severe enough to be unacceptable and to discourage consumption. Prebiotics are like other carbohydrates that reach the cecum, such as nonstarch polysaccharides, sugar alcohols, and resistant starch, in being substrates for fermentation. They are, however, distinctive in their selective effect on the microflora and their propensity to produce flatulence.

Contribution of dietary protein to sulfide production in the large intestine: an in vitro and a controlled feeding study in humans.

BACKGROUND: Hydrogen sulfide is a luminally acting, bacterially derived cell poison that has been implicated in ulcerative colitis. Sulfide generation in the colon is probably driven by dietary components such as sulfur-containing amino acids (SAAs) and inorganic sulfur (eg, sulfite). OBJECTIVE: We assessed the contribution of SAAs from meat to sulfide production by intestinal bacteria with use of both a model culture system in vitro and an in vivo human feeding study. DESIGN: Five healthy men were housed in a metabolic suite and fed a sequence of 5 diets for 10 d each. Meat intake ranged from 0 g/d with a vegetarian diet to 600 g/d with a high-meat diet. Fecal sulfide and urinary sulfate were measured in samples collected on days 9 and 10 of each diet period. Additionally, 5 or 10 g bovine serum albumin or casein/L was added to batch cultures inoculated with feces from 4 healthy volunteers. Concentrations of sulfide, ammonia, and Lowry-reactive substances were measured over 48 h. RESULTS: Mean (+/-SEM) fecal sulfide concentrations ranged from 0.22 +/- 0.02 mmol/kg with the 0-g/d diet to 3.38 +/- 0.31 mmol/kg with the 600-g/d diet and were significantly related to meat intake (P: < 0.001). Sulfide formation in fecal batch cultures supplemented with both bovine serum albumin and casein correlated with protein digestion, as measured by the disappearance of Lowry-reactive substances and the appearance of ammonia. CONCLUSION: Dietary protein from meat is an important substrate for sulfide generation by bacteria in the human large intestine.