Changes to the quantity and processing of starchy foods in a western diet can increase polysaccharides escaping digestion and improve in vitro fermentation variables

Dietary fibres and IBS: translating functional characteristics to clinical value in the era of personalised medicine

Clinical guidelines in the use of fibre supplementation for patients with IBS provide one-size-fits-all advice, which has limited value. This narrative review addresses data and concepts around the functional characteristics of fibre and subsequent physiological responses induced in patients with IBS with a view to exploring the application of such knowledge to the precision use of fibre supplements. The key findings are that first, individual fibres elicit highly distinct physiological responses that are associated with their functional characteristics rather than solubility. Second, the current evidence has focused on the use of fibres as a monotherapy for IBS symptoms overall without attempting to exploit these functional characteristics to elicit specific, symptom-targeted effects, or to use fibre types as adjunctive therapies. Personalisation of fibre therapies can therefore target several therapeutic goals. Proposed goals include achieving normalisation of bowel habit, modulation of gut microbiota function towards health and correction of microbial effects of other dietary therapies. To put into perspective, bulking fibres that are minimally fermented can offer utility in modulating indices of bowel habit; slowly fermented fibres may enhance the activities of the gut microbiota; and the combination of both fibres may potentially offer both benefits while optimising the activities of the microbiota throughout the different regions of the colon. In conclusion, understanding the GI responses to specific fibres, particularly in relation to the physiology of the individual, will be the future for personalising fibre therapy for enhancing the personalised management of patients with IBS.

Health benefits of whole grain: effects on dietary carbohydrate quality, the gut microbiome, and consequences of processing

Grains are important sources of carbohydrates in global dietary patterns. The majority of these carbohydrates, especially in refined-grain products, are digestible. Most carbohydrate digestion takes place in the small intestine where monosaccharides (predominantly glucose) are absorbed, delivering energy to the body. However, a considerable part of the carbohydrates, especially in whole grains, is indigestible dietary fibers. These impact gut motility and transit and are useful substrates for the gut microbiota affecting its composition and quality. For the most part, the profile of digestible and indigestible carbohydrates and their complexity determine the nutritional quality of carbohydrates. Whole grains are more complex than refined grains and are promoted as part of a healthy and sustainable diet mainly because the contribution of indigestible carbohydrates, and their co-passenger nutrients, is significantly higher. Higher consumption of whole grain is recommended because it is associated with lower incidence of, and mortality from, CVD, type 2 diabetes, and some cancers. This may be due in part to effects on the gut microbiota. Although processing of cereals during milling and food manufacturing is necessary to make them edible, it also offers the opportunity to still further improve the nutritional quality of whole-grain flours and foods made from them. Changing the composition and availability of grain carbohydrates and phytochemicals during processing may positively affect the gut microbiota and improve health.

Understanding the interplay between food structure, intestinal bacterial fermentation and appetite control

Epidemiological and clinical evidence highlight the benefit of dietary fibre consumption on body weight. This benefit is partly attributed to the interaction of dietary fibre with the gut microbiota. Dietary fibre possesses a complex food structure which resists digestion in the upper gut and therefore reaches the distal gut where it becomes available for bacterial fermentation. This process yields SCFA which stimulate the release of appetite-suppressing hormones glucagon-like peptide-1 and peptide YY. Food structures can further enhance the delivery of fermentable substrates to the distal gut by protecting the intracellular nutrients during upper gastrointestinal digestion. Domestic and industrial processing can disturb these food structures that act like barriers towards digestive enzymes. This leads to more digestible products that are better absorbed in the upper gut. As a result, less resistant material (fibre) and intracellular nutrients may reach the distal gut, thus reducing substrates for bacterial fermentation and its subsequent benefits on the host metabolism including appetite suppression. Understanding this link is essential for the design of diets and food products that can promote appetite suppression and act as a successful strategy towards obesity management. This article reviews the current evidence in the interplay between food structure, bacterial fermentation and appetite control.

Review article: insights into colonic protein fermentation, its modulation and potential health implications

BACKGROUND

Beneficial effects of carbohydrate fermentation on gastrointestinal health are well established. Conversely, protein fermentation generates harmful metabolites but their relevance to gastrointestinal health is poorly understood.

AIM

To review the effects of increased protein fermentation on biomarkers of colonic health, factors influencing fermentative activity and potential for dietary modulation to minimise detrimental effects.

METHODS

A literature search was performed in PubMed, Medline, EMBASE and Google scholar for clinical and pre-clinical studies using search terms - 'dietary protein', 'fermentation', 'putrefaction', 'phenols', 'sulphide', 'branched-chain fatty acid', 'carbohydrate fermentation', 'gastrointestinal'.

RESULTS

High protein, reduced carbohydrate diets alter the colonic microbiome, favouring a potentially pathogenic and pro-inflammatory microbiota profile, decreased short-chain fatty acid production and increased ammonia, phenols and hydrogen sulphide concentrations. These metabolites largely compromise the colonic epithelium structure, causing mucosal inflammation but may also directly modulate the enteric nervous system and intestinal motility. Increased protein fermentation as a result of a high-protein intake can be attenuated by addition of oligosaccharides, resistant starch and nonstarch polysaccharides and a reduction in total protein or specifically, aromatic and sulphur-containing amino acids. These factors may have clinical importance as novel therapeutic approaches to problems, in which protein fermentation may be implicated, such as malodorous flatus, irritable bowel syndrome, ulcerative colitis and prevention of colorectal cancer.

CONCLUSIONS

The direct clinical relevance of excessive protein fermentation in the pathogenesis of irritable bowel syndrome, malodorous flatus and ulcerative colitis are underexplored. Manipulating dietary carbohydrate and protein intake have potential therapeutic applications in such settings and warrant further clinical studies.

Optimisation of inoculum concentration and incubation duration for an in vitro hindgut dry matter digestibility assay

The aim was to optimise inoculum concentration and incubation duration for a published in vitro hindgut digestibility assay using ileal digesta (sampled from the chicken or rat) pertaining to a mixed human diet as the substrate. The study also sought to investigate the digestibility of the inoculum itself and the importance of correcting for this in the in vitro hindgut digestion assay. For two assays, hindgut dry matter digestibility (DMD) generally increased with inoculum concentration. A sharp increase in DMD observed at high inoculum concentrations may have been related to problems with filtering the inoculum. An inoculum concentration of 160 g/L was considered optimal based on close agreement of observed values with previously published in vivo hindgut dry matter digestibility for similar diets. One of the methods was chosen for optimisation of the duration of incubation. Ileal substrate organic matter digestibility (OMD) increased with increasing time of incubation for all diets. An incubation duration of 18 h using a mean inoculum digestibility value for calculation purposes was considered optimal based on observed in vivo hindgut DMD values in humans, but there was little difference in estimated in vitro hindgut DMD between 18 and 24h incubation durations. Although considerably lower than the OM digestibility of the substrate (no less than 51% after 48 h), the OM digestibility of the inoculum (13% after 48 h) itself was of significance in calculating estimated digestibility. The optimised assay gave realistic hindgut OMD values ranging from 55% to 79% (Wheat Bran Diet and Pectin Diet, respectively) using an 18-h incubation duration.

Influence of resistant starch alone or combined with wheat bran on gastric emptying and protein digestion in healthy volunteers

OBJECTIVE

Resistant starch (RS) is not absorbed in the small intestine, but is partly fermented in the colon and may positively influence putative risk factors for colon cancer. The purpose of the study was to investigate the influence of RS type 3 (retrograded amylose) alone or combined with wheat bran on gastric emptying (GE) and protein assimilation.

MATERIAL AND METHODS

GE and protein assimilation were investigated by means of breath-test technology in 20 healthy volunteers who were randomly divided into two groups, each subject performing two tests. In each test, the volunteers received a labelled test meal either as such or in combination with, respectively, 15 g RS3 or 15 g RS3 combined with 6 g wheat bran (WB). Breath samples were collected during the 6 h after administration of a test meal containing egg proteins, intrinsically labelled with (13)C-leucine, to measure protein digestion and sodium-(14)C-octanoate for measurement of GE.

RESULTS

Intake of RS3 +WB did not influence GE time compared to baseline values, whereas intake of RS3 seemed to hasten GE: from 93 +/- 32 min to 55 +/- 15 min (p = 0.012). The overall protein assimilation parameters at baseline were not significantly different from those obtained after simultaneous intake of RS3 +WB, whereas RS3 significantly shortened the time of maximum excretion compared to baseline, but the extent of protein digestion after RS3 intake was not affected (12.54 +/- 3.60% versus 13.43 +/- 3.40%).

CONCLUSIONS

The presence of RS3 alone or in combination with wheat bran demonstrates that there are no adverse effects on protein digestion and no influence on the nitrogen supply to the colon.

An acute ileal amino acid digestibility assay is a valid procedure for use in human ileostomates

An acute (24-h) feeding/digesta sampling procedure was evaluated in a preliminary study using growing pigs. The validated acute procedure was then applied using human ileostomates to determine apparent and true ileal amino acid digestibilities of 4 dietary protein sources. The acute method involved feeding ileostomized pigs a single meal containing the test protein as part of a purified diet, with no previous dietary adaptation, followed by an 8-h collection of digesta. Apparent ileal N digestibility did not differ between the acute and conventional (14-d study) procedures. Eight adult human ileostomates each received a single meal of protein-free biscuits and a drink containing sodium caseinate, whey protein concentrate, soy protein isolate, or soy protein concentrate; this meal was followed by a 9-h total digesta collection period. Acid insoluble ash was used as an indigestible marker. True ileal amino acid digestibilities (means +/- SE) ranged from 90.5 +/- 2.74% for cysteine in soy protein concentrate to 105.3 +/- 5.66% for cysteine in sodium caseinate and were markedly higher than their apparent counterparts. True ileal digestibilities for total nitrogen were 101.9 +/- 0.70, 98.3 +/- 0.80, 99.5 +/- 0.80, and 98.5 +/- 1.20% for sodium caseinate, whey protein concentrate, soy protein isolate, and soy protein concentrate, respectively. The 4 protein sources were virtually completely digested in humans by the end of the small intestine.