Dietary fibre digestion and fermentation of SCFA

Question 1

Dietary fibre

Answer 1

-carbohydrates that are not digested or are poorly digested by enzymes in the small intestine
**non-starch polysaccharides

Question 2

Predominant Non-starch Polysaccharides

Answer 2

-Cellulose
-Hemicellulose

Question 3

Cellulose

Answer 3

-a linear unbranched chain of glucose with beta- (1,4) linkages
-tightly packed

Question 4

Hemicellulose

Answer 4

-branched chain polysaccharide
>hexoses and pentoses

Question 5

Where is hemicellulose commonly found?

Answer 5

-commonly found in grains as Xylan

Question 6

Xylan

Answer 6

-xylose backbone with side chains composed of arabinose, mannose, galactose, and glucose

Question 7

Insoluble plant fibres

Answer 7

-cellulose
-hemicellulose
-Lignin

Question 8

Soluble plant fibres

Answer 8

-beta-glucans
-guar gum
-inulin, oligofructose, fructooligosaccharides
-pectins
-resistant starch

Question 9

Manufactured soluble fibres

Answer 9

-psyllium
-polydextrose
-inulin
-oligosaccharides

Question 10

Beat pulp

Answer 10

-combination of cellulose, hemicellulose, pectin, and lignin

Question 11

Solubility vs. fermentability

Answer 11

-related terms but not equal
-can have low solubility and high fermentability and vice verse
-soluble: dissolve in water
-fermentability: metabolized by gut microbes

Question 12

How does fibre effect the small intestine and stomach?

Answer 12

-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

Question 13

How does fibre effect the colon?

Answer 13

-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

Question 14

Dietary fiber in dogs

Answer 14

-no effect of high or low fermentable diets on insulin, PYY, GLP-1, or ghrelin

Question 15

Fermentation in colon

Answer 15

-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

Question 16

Butyrate

Answer 16

-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

Question 17

Proprionate

Answer 17

-taken up by the liver, metabolized to succinyl-CoA then to TCA intermediates or used for gluconeogenesis

Question 18

Colon Fermentation and pH

Answer 18

-lowers the pH
-affects bacterial metabolism and growth

Question 19

Fermentation of nondigestible CHO in the colon

Answer 19

-variable extent which nondigestible carbohydrates can be used by colonic bacteria
>minimal use of cellulose
>good use of resistant starch, oligosaccharides, undigested disaccharides

Question 20

Branched chain SCFA

Answer 20

-derived from fermentation of the branched chain amino acids by Bacteroides and Clostridium genera

Question 21

Fermentation environment in colon/cecum

Answer 21

-anaerobic
-highest concentration of microbes in proximal large intestines

Question 22

Fermentation pathways for synthesis of SCFA

Answer 22

1.Breakdown of fermentable poly or oligosaccharides to monosaccharides by bacterial glycosidases
2.Mainly use glycolytic pathway, but no ETC
*energy output relatively low

Question 23

Acetate formation

Answer 23

-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

Question 24

Butyrate formation

Answer 24

-2 acetyl coA are condensed to form butyryl CoA which is then converted to butyric acid
-converted to B-hydroxybutyrate (ketone)

Question 25

Propionate formation

Answer 25

-succinate decarboxylated to propionate by bacteria
>selenomnas ruminantium
>Bacteroides succinogens
-lactate to propionate via acryl CoA

Question 26

SCFA-producing bacteria families

Answer 26

-Streptococcaceae
-enterococcaceae
-bifidobacteriaceae
-lactobacillaceae
-clostridiaceae
-prevotellaceae
-enterobacteriaceae
-ruminococcaceae

Question 27

Absorption of volatile fatty acids (VFA)

Answer 27

-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

Question 28

Usage of VFAs

Answer 28

-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

Question 29

Proprionate (and lactate) metabolized in hepatocytes

Answer 29

> proprionate converted to oxaloacetate then glucose
lactate converted to pyruvate then to glucose

Question 30

Butyrate metabolized by colonic mucosa as energy source

Answer 30

> used to make ketone body (beta-hydroxybutyrate in colonocytes for energy
also used as carbon skeleton for fatty acids in milk

Question 31

SCFAs in colon

Answer 31

-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

Question 32

Prebiotics

Answer 32

-increase growth of specific colon bacteria

Question 33

SCFA as signalling molecules

Answer 33

-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