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.
Introduction: A major component of the digesta reaching the colon from the distal ileum is carbohydrate. This carbohydrate is subject to microbial fermentation and can radically change bacterial populations in the colon and the metabolites they produce, particularly short-chain fatty acids (SCFA). However, very little is currently known about the forms and levels of carbohydrate in the ileum and the composition of the ileal microbiota in humans. Most of our current understanding of carbohydrate that is not absorbed by the small intestine comes from ileostomy models, which may not reflect the physiology of an intact gastrointestinal tract. Methods: We will investigate how ileal content changes depending on diet using a randomised crossover study in healthy humans. Participants will be inpatients at the research facility for three separate 4-day visits. During each visit, participants will consume one of three diets, which differ in carbohydrate quality: 1) low-fibre refined diet; 2) high-fibre diet with intact cellular structures; 3) high-fibre diet where the cellular structures have been disrupted (e.g. milling, blending). On day 1, a nasoenteric tube will be placed into the distal ileum and its position confirmed under fluoroscopy. Ileal samples will be collected via the nasoenteric tube and metabolically profiled, which will determine the amount and type of carbohydrate present, and the composition of the ileal microbiota will be measured. Blood samples will be collected to assess circulating hormones and metabolites. Stool samples will be collected to assess faecal microbiota composition. Subjective appetite measures will be collected using visual analogue scales. Breath hydrogen will be measured in real-time as a marker of intestinal fermentation. Finally, an in vitro continuous fermentation model will be inoculated with ileal fluid in order to understand the shift in microbial composition and SCFA produced in the colon following the different diets. Registration: ISRCTN11327221.
Introduction: A major component of the digesta reaching the colon from the distal ileum is carbohydrate. This carbohydrate is subject to microbial fermentation and can radically change bacterial populations in the colon and the metabolites they produce, particularly short-chain fatty acids (SCFA). However, very little is currently known about the forms and levels of carbohydrate in the ileum and the composition of the ileal microbiota in humans. Most of our current understanding of carbohydrate that is not absorbed by the small intestine comes from ileostomy models, which may not reflect the physiology of an intact gastrointestinal tract. Methods: We will investigate how ileal content changes depending on diet using a randomised crossover study in healthy humans. Participants will be inpatients at the research facility for three separate 4-day visits. During each visit, participants will consume one of three diets, which differ in carbohydrate quality: 1) low-fibre refined diet; 2) high-fibre diet with intact cellular structures; 3) high-fibre diet where the cellular structures have been disrupted (e.g. milling, blending). On day 1, a nasoenteric tube will be placed into the distal ileum and its position confirmed under fluoroscopy. Ileal samples will be collected via the nasoenteric tube and metabolically profiled, which will determine the amount and type of carbohydrate present, and the composition of the ileal microbiota will be measured. Blood samples will be collected to assess circulating hormones and metabolites. Stool samples will be collected to assess faecal microbiota composition. Subjective appetite measures will be collected using visual analogue scales. Breath hydrogen will be measured in real-time as a marker of intestinal fermentation. Finally, an in vitro continuous fermentation model will be inoculated with ileal fluid in order to understand the shift in microbial composition and SCFA produced in the colon following the different diets. Registration: ISRCTN11327221.
Objectives Complex food structures can act like barriers towards digestive enzymes and reduce nutrient bioavailability. Nutrients that escape digestion in the upper gut (mainly fibre), becomes available for bacterial fermentation in the distal gut and generates short chain fatty acids (SCFA) which stimulate the release of appetite suppressing hormones Glucagon like peptide 1 (GLP-1) and Peptide YY (PYY). Processing can alter food structures, leading to more digestible products, but reducing nutrients reaching the distal gut, fermentation and appetite suppression. This study aimed to investigate the impact of food structures on the level of carbohydrate reaching the distal ileum and its subsequent effect on SCFA production and appetite hormone release. Methods Healthy volunteers (n = 10, 18–65 years) attended three separate 4-day inpatient visits. A nasoenteric tube was inserted into their distal ileum. During each visit, participants had one of three dietary interventions: Low fibre, processed diet (LF); High fibre, unprocessed diet (HF); same as HF but domestically processed (HFP). On day 4, ileal, blood and breath hydrogen (BH) samples were collected at baseline and hourly following food intake for 8 h. Blood samples were analyzed for appetite hormones and SCFAs using radioimmunoassay and GC-MS. Ileal samples were analyzed for metabolic profiling using 1H-NMR spectroscopy, partial least squares discriminant analysis with Monte-Carlo cross-validation and repeated-measures design. Results HFP but not HF had higher PYY levels than the LF (P = 0.004). BH and serum total SCFAs were higher in HF and HFP than LF indicating a higher bacterial fermentation (P = 0.021, 0.015 and P = 0.000, 0.003). HFP group had higher total SCFA than HF (P = 0.015). Distinct metabolic differences were identified in ileal samples from groups (e.g., time = 2 h, HF vs HFP R2Y: 0.99, Q2Y: 0.65). Ileal samples will be analysed for microbial profile (16 s rRNA, Illumina MiSeq), glucose and starch (spectroscopy) and SCFAs (GC-MS). Conclusions Current data suggests a potential benefit of domestically processed food structures on appetite suppression as measured by postprandial PYY levels. This may be related to the different metabolic profiles generated in the ileum. Better understanding of this link can aid the design of diets that enhance appetite suppression and reduce food intake. Funding Sources BBSRC.
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