Dietary fibre digestion and fermentation of SCFA Flashcards
Dietary fibre
-carbohydrates that are not digested or are poorly digested by enzymes in the small intestine
**non-starch polysaccharides
Predominant Non-starch Polysaccharides
-Cellulose
-Hemicellulose
Cellulose
-a linear unbranched chain of glucose with beta- (1,4) linkages
-tightly packed
Hemicellulose
-branched chain polysaccharide
>hexoses and pentoses
Where is hemicellulose commonly found?
-commonly found in grains as Xylan
Xylan
-xylose backbone with side chains composed of arabinose, mannose, galactose, and glucose
Insoluble plant fibres
-cellulose
-hemicellulose
-Lignin
Soluble plant fibres
-beta-glucans
-guar gum
-inulin, oligofructose, fructooligosaccharides
-pectins
-resistant starch
Manufactured soluble fibres
-psyllium
-polydextrose
-inulin
-oligosaccharides
Beat pulp
-combination of cellulose, hemicellulose, pectin, and lignin
Solubility vs. fermentability
-related terms but not equal
-can have low solubility and high fermentability and vice verse
-soluble: dissolve in water
-fermentability: metabolized by gut microbes
How does fibre effect the small intestine and stomach?
-delay gastric emptying (increased viscosity)
-promote satiety
>distension
>delayed gastric emptying
>release of satiety related hormones (GLP-1, PYY, CCK)
-reduced rate of nutrient absorption, reduced postprandial glucose and cholesterol absorption
How does fibre effect the colon?
-increased bacterial mass = increased stool mass
-increased rate of microbial fermentation =alters gut microbial composition and SCFA production
-excess can cause gastrointestinal intolerance (high intakes of rapidly fermentable)
-fermentation
>alter microbial composition
>production of SCFA
Dietary fiber in dogs
-no effect of high or low fermentable diets on insulin, PYY, GLP-1, or ghrelin
Fermentation in colon
-fermentation of fibre is a way to obtain energy
>fibre broken down into SCFAs (acetate,butyrate,proprionate)
-leads to proliferation of colonic bacteria which increases stool mass
Butyrate
-important energy source for colonocytes (60-70% of energy required)
-important for barrier function and gut integrity
-some is taken up by the liver and oxidized to acetyl-CoA
Proprionate
-taken up by the liver, metabolized to succinyl-CoA then to TCA intermediates or used for gluconeogenesis
Colon Fermentation and pH
-lowers the pH
-affects bacterial metabolism and growth
Fermentation of nondigestible CHO in the colon
-variable extent which nondigestible carbohydrates can be used by colonic bacteria
>minimal use of cellulose
>good use of resistant starch, oligosaccharides, undigested disaccharides
Branched chain SCFA
-derived from fermentation of the branched chain amino acids by Bacteroides and Clostridium genera
Fermentation environment in colon/cecum
-anaerobic
-highest concentration of microbes in proximal large intestines
Fermentation pathways for synthesis of SCFA
1.Breakdown of fermentable poly or oligosaccharides to monosaccharides by bacterial glycosidases
2.Mainly use glycolytic pathway, but no ETC
*energy output relatively low
Acetate formation
-produced directly from acetyl-coA or via Wood Ljundahl pathway using CO2 (released from pyruvate) as an electron acceptor
-methanogens use the H2 to produce methane
Butyrate formation
-2 acetyl coA are condensed to form butyryl CoA which is then converted to butyric acid
-converted to B-hydroxybutyrate (ketone)
Propionate formation
-succinate decarboxylated to propionate by bacteria
>selenomnas ruminantium
>Bacteroides succinogens
-lactate to propionate via acryl CoA
SCFA-producing bacteria families
-Streptococcaceae
-enterococcaceae
-bifidobacteriaceae
-lactobacillaceae
-clostridiaceae
-prevotellaceae
-enterobacteriaceae
-ruminococcaceae
Absorption of volatile fatty acids (VFA)
-absorbed through SCFA/HCO3- exchanger active transport, driven by H+, Na+, or just facilitative diffusion
-non-ionized SCFAs are freely absorbed BUT the ionization depends on the VFA pKa and the colon pH
Usage of VFAs
-Acetate transported through portal vein and goes to liver and periphery
>forms acetyl-coA which will eventually be used in CAC and fatty acid synthesis
Proprionate (and lactate) metabolized in hepatocytes
> proprionate converted to oxaloacetate then glucose
lactate converted to pyruvate then to glucose
Butyrate metabolized by colonic mucosa as energy source
> used to make ketone body (beta-hydroxybutyrate in colonocytes for energy
also used as carbon skeleton for fatty acids in milk
SCFAs in colon
-leads to activation of SCFA G-protein coupled receptors (FFAR2 and FFAR3) which are expressed by L-cells in various tissues
>L-cells secrete GLP-1 and PYY, leads to reduced inflammation and improved barrier function
-Also alter gut microbial composition and activity which may improve gut barrier/immune function
Prebiotics
-increase growth of specific colon bacteria
SCFA as signalling molecules
-influence gut-brain communication and brain directly or indirectly
*can cross blood brain barrier
-intestinal barrier function and immunity
-hormone production
-activates brown adipose tissue, regulation of mitochondrial function, increased insulin secretion, whole body energy homeostasis
-systemic inflammation
