BIOCHEM - Health Effects of Fibre

Question 1

What is a monosaccharide?
- give examples of monosaccharides.

Answer 1

Single sugar molecule in ring structure
- glucose
- galactose
- fructose

Question 2

What is a disaccharide?
- give examples of disaccharides and their constituents.

Answer 2

Two monosaccharides joined together in a condensation (dehydration) reaction forming glycosidic bonds.

• sucrose (glucose & fructose)
• lactose (glucose & galactose)
• maltose (glucose & glucose)

Question 3

What is an oligosaccharide?

Answer 3

3-10 monosaccharides bonded together by glycosidic bond

Question 4

What is a polysaccharide?

Give examples of some polysaccharides.

Answer 4

11 + monomers joined by glycosidic bonds - can be a chain of thousands of monomers.

• Starch (amylose & amylopectin)
• Glycogen
• Fibre

Question 5

What is the difference between alpha glucose (α-glucose) and beta glucose (β-glucose)?

Answer 5

Glucose can be present as a straight molecule or in ring structure.

50% of the time it is open and 50% closed( randomly opens/closes in solution)

When it is in ring formation it can be:
α-glucose or β-glucose.
The difference is the position of the hydroxyl groups.

Question 6

Why can’t humans break down cellulose?

Answer 6

Both α-glucose and β-glucose can be utilised in the body when in their single monomer forms.

Depending on which isomer of glucose are joined depends on whether α-glycosidic bonds or β-glycosidic bonds are formed.

Enzymes in the body can only break down α-glycosidic bonds.

Question 7

What is the difference between starch and cellulose?

Answer 7

Starch is α-glucose joined by many
α-1,4 glycosidic bonds.
Can occur as amylose or amylopectin:
amylose - 1,4 glycosidic bonds (straight chain molecule)
amylopectin - 1,4 & 1,6 glycosidic bonds (branched chain molecule)

Cellulose is β-glucose joined by β-1,4 glycosidic bonds.

Question 8

What is hemicellulose?

What are the differences between hemicellulose and cellulose?

Answer 8

Hemicellulose is monosaccharides joined by b-1,4 glycosidic bonds and other glycosidic bonds.

Cellulose has a clear straight structure, is strong and resistant to hydrolysis.
Hemicellulose has a random branched/amorphous structure with little strength.

Question 9

What is pectin?

Answer 9

Pectins are the largest/most complex naturally occurring molecules in existence.
Soluble fibre.

Question 10

What is the definition of dietary fibre?

Answer 10

European Commission (and CODEX alimentarius):
Carbohydrate polymers with three or more monomeric units which are neither digested nor absorbed.

BNF:
Dietary fibre consists of one or more of
– edible carbohydrate polymers naturally occurring in the food as consumed;
– carbohydrate polymers that have been obtained from food raw material by physical, enzymatic or chemical means;
- synthetic carbohydrate polymers

Question 11

What are the key differences between soluble and insoluble fibre?

Answer 11

Soluble fibre:
- Inulin, pectin
- high affinity for water, binds easily and forms gel
- adds bulk to stools
- delays gastric emptying, increases satiety
- used by colonic MO’s to metabolise SCFA’s
- reduces LDL/cholesterol
- reduces glycaemic response

Insoluble fibre:
- Cellulose, lignin
- low affinity for water
- little use by colonic MO’s
- increases gut transit time
- adds bulk to stool

Question 12

What is resistant starch?

Answer 12

Starch that resists digestion in gut/products of starch digestion that are not absorbed by intestine.

There are 4 types of RS:
- Starch that is trapped in other plant material e.g. within cellulose cell wall.
- Starch present in large granular form that digestive enzymes cannot hydrolyse.
- Retrograded starch
- Chemically modified starch to resist digestion.

Commonly found in legumes/pulses.

Question 13

What is the fate of ingested fibre?
- soluble fibre
- insoluble fibre

Answer 13

Most cell wall polysaccharides cannot be hydrolysed in the small intestine. The digestive process does not release monosaccharides which can be absorbed and utilised.

MO’s in the colon metabolise soluble fibre. Soluble fibre is nutrition for gut MO’s.

Colonic MO’s can utilise insoluble fibre to a much lesser extent.

Question 14

Briefly describe the gut microbiome?

Answer 14

Genome of gut MO’s

Healthy adult human gut contains trillions of MO’s - over 1000 different species identified.

One human usually carries 160 different species
but 30-40 species make up 99% of all gut MO’s.

Humans are born with a sterile gut - gut microbiome is colonised through birth, breastmilk, eating, environment.

Question 15

Describe some of the roles of gut microbes.

Answer 15

• Production & secretion of cellulolytic enzymes
• Release of monomers that deliver chemical energy to the MO
• Produce gas: CO2 , H, methane
• Produce short chain fatty acids e.g., butyric acid which are used by cells to make ATP (metabolisable energy is calculated for fibre: 8.4 kJ/g fibre)
• Useful as changes the pH of colon (higher acidity)
• Picked up and absorbed by colonocytes and metabolised for ATP
• Energy source for colonic enterocytes
• Reduction of pH in lumen of colon

Question 16

Describe how carbohydrates are fermented by gut MO’s

Answer 16

Majority of gut MO’s use the glycolytic pathway to derive energy from carbohydrates, which are initially converted to pyruvate and acetyl CoA.

The pentose phosphate pathway also occurs in many gut bacteria, where it is used in the dissimilatory metabolism of pentoses or to produce NADPH and pentoses for biosynthetic purposes.

Question 17

What are the effects of fibre in the GIT - mouth

Answer 17

Foods containing dietary fibre need to be chewed and chewing promotes flow of saliva.

Question 18

What are the effects of fibre in the GIT - stomach

Answer 18

Soluble fibre increase viscosity of stomach contents slowing down gastric emptying.
Swells and contributes to the feeling of fullness (recommended as a weight loss strategy)

Insoluble fibre mixes with enzymes and inhibits their action, slowing digestion (renin, pepsin and trypsin 3 main stomach enzymes).

Question 19

What are the effects of fibre in the GIT - small intestine

Answer 19

Soluble fibre increases viscosity of the materials passing through small intestine.

Insoluble fibre inhibits enzymes trying to digest food, slowing digestion.

– Both these effects slow down the release of nutrients from food and absorption into the blood
– Results in a more gradual release of nutrients and uptake
- Steady rise in blood glucose levels.

Question 20

What are the effects of fibre in the GIT - large intestine.

Answer 20

Soluble fibre is a substrate for fermentation by anaerobic bacteria increases bacterial mass.

Insoluble fibre binds and absorbs water, soluble dissolves in water - both increase the stool weight & volume → stimulates propulsive contractions → better excretion.

Fibre reduces risk of colorectal cancer.

Question 21

How can fibre protect against colorectal cancer?

Answer 21

Reduced exposure of gut lining to carcinogens due to increased transit time through colon (due to increased bulk)

Presence of SCFAs and their effects on colonic enterocytes.

Fermentation reduces the pH of the bowel. This may prevent over growth of pH sensitive pathogenic bacteria.

Question 22

How can fibre protect against diverticulitis?

Answer 22

Big muscular force pushing down can open up circular muscles in the gut wall and allow material (i.e. chymus) to enter and stagnate, ferment, inflame = diverticulitis.

Fibre obstructs diverticular opening, prevents chyme from collecting.

Question 23

How can fibre protect against CVD?

How does dietary fibre reduce cholesterol?

Answer 23

Soluble fibre supports the excretion of cholesterol and reduced cholesterol levels in blood (LDL)

Fibre also has role management of obesity and overweight (reduced glycemic index).

Binding of bile acids (→ reduced re-absorption of bile acids → cholesterol excretion)
some soluble fibers bind bile acids or cholesterol during the formation of micelles . The resulting reduction in the cholesterol content
of liver cells leads to an up-regulation of the LDL receptors and increased clearance of LDL cholesterol.

Question 24

How can dietary fibre reduce LDL cholesterol?

Answer 24

bile acids are produced in the liver from cholesterol.
they are typically reabsorbed back into the ileum and recycled in the liver as cholesterol.
soluble fibre binds to bile acids and prevents them being absorbed back into the liver –> they are excreted.