Lecture 12: Fat nutrition, digestion and absorption

Question 1

What are the lipids in the diet?

Answer 1

• Triglycerides
• Phospholipids
• Cholesterol
• Cholesterol esters
• Fat soluble vitamins

Question 2

How much energy does fat supply?

Answer 2

37Kj/g i.e double rest

Question 3

What does a triglyceride consist of?

Answer 3

3 FA bonded to glycerol

Question 4

Where are long, medium and short chain FA found?

Answer 4

Long = Found primarily in meat, fish and vegetable oils.

Medium and short chain FA found in diary products

Question 5

What influences the characteristics of fats?

Answer 5

• Short or long chain

- Saturated, mono-unsaturated, polyunsaturated, CIS/TRANS

Question 6

What are sterols?

Answer 6

Multiple ring shaped structures

Question 7

What is cholesterol found in?

Answer 7

Animal foods only. (Is a sterol)

Plant based sterols can interfere with cholesterol absorption i.e logical and flora proactiv.

Question 8

What are ring sterols used for?

Answer 8

• Starting material for bile, sex hormones, adrenal hormones and Vit D
• Structural component of cell membranes

Question 9

How much cholesterol is produced daily?

Answer 9

Liver produces 800-1500mg of cholesterol daily

Question 10

What can cholesterol cause?

Answer 10

Atherosclerosis when Cholesterol forms deposits on the vascular walls.

Question 11

Describe fat digestion;

Answer 11

1) Lingual lipase (acid stable)
2) Gastric lipase
3) Stomach muscle contractions disperse fat into smaller droplets.
4) CCK stimulates bile release and fat emulsification
5) Pancreas releases pancreatic lipases
6) SI absorbs fats after they are digested / emulsified.

During this process fats form mixed micelles which are important in the absorption process.

Question 12

What are the 3 pancreatic enzymes?

Answer 12

Pancreatic lipase
- (TAG->Glycerol and FA), (require pancreatic enzyme colipase to be active.)

Pancreatic cholesterase,
- remove FA from; cholesterol esters, 2-MAGS, TAG containing short / medium chain FA, fat soluble vitamins (A, D3 and E).

Phospholipase A2;
- Removes FA from 2 pos. on phospholipids.

Question 13

What happens to bile salts during digestion?

Answer 13

Stored in the liver between meals, Absorbed mainly in the terminal ilium and re-used via the enterohepatic circulation

Question 14

What are the four roles of bile during fat digestion?

Answer 14

1) Emulsification of fat droplets
2) Form mixed micelles (Removing the products of fat digestion from lipid droplets i.e 2-MAG, cholesterol etc)
3) Cofactors for lipase activity
4) Aid absorption (Mixed miscelles increase the delivery rate of FA digestion products across the enterocyte membrane)

Question 15

Describe fat absorption;

Answer 15

Fat digestion results in the formation of glycerol, FAs, cholesterol which with the emulsification effect of bile form micelles, micelles get near the apical membrane and allow the diffusion of the digestion products into the cells.

Once in the cell the FAs and glycerol are resynthesised into TAG which are packaged into chylimicrons and delivered into the lymphatic system.

i.e in the gut lumen fats must be broken down into their basic units with the help of lipases and bile salts. These can freely diffuse into the cells, but are repackaged into TAG / cholesterol and then chylimicrons for delivery into lacteals

Question 16

What is packages into chylomicrons?

Answer 16

Cholesterol, fat soluble vitamins (A,D,E and K), Apolipoproteins

Question 17

What are micelles?

Answer 17

Monoglycerides and long chain fatty acids combine with bile forming micells that are sufficiently water soluble to penetrate the watery solution that bathes the absorptive cells. There the lipid contents diffuse into the cells.

Question 18

How much of cholesterol is made by the liver?

Answer 18

80%

Question 19

How are fats transported in the blood?

Answer 19

Lipoproteins, of which the largest are chylomicrons.

• Very low density lipoproteins (VLDL) (primarily of triglycerides)
• Low density lipoproteins (LDL) (Compose primarily of cholesterol)
• High density lipoproteins (HDL) (Composed primarily of proteins)

Question 20

What are the general recommendations for fat?

Answer 20

• Limit saturated and trans fats (<10% total)
• Choose lean healthy foods
• Low fat, trim visible fat and cook without oils
• Avoid low fat high sugar foods
• Mediterranean diet

Question 21

What are the essential fats?

Answer 21

Alpha-linolenic acid (Omega 3)
Linoleic acid (Omega 6)

Question 22

Why are fats important?

Answer 22

• Major energy source and fuel store
• Required for transporting fat soluble vitamins
• Provide essential fatty acids - Linoleic acid and alpha linolenic acid
• Important for insulating and protecting the body

Question 23

Why are linoleic acid and alpha linolenic acid important?

Answer 23

• They share enzyme pathways to eventually produce eicosanoids which have roles in pro-inflammation and anti-inflammation.

The essential FA are polymerised. The intermediates can be consumed and gained in diets.

Question 24

Whats the implication of sharing an enzyme system?

Answer 24

Over consumption of one FA will prevent the other essential FA from accessing the enzymes and forming its products.

Omega 6 = Proinflam
Omega 3 = Anti-inflam

Thus long term will shape these profiles if diet imbalance and change risk for conditions.

Question 25

Whats the ideal balance for omega 3:6

Answer 25

1:2-4`

Question 26

Whats the point of chylomicrons?

Answer 26

They are formed with phospholipid bilayer on the outside, with tails inwards thus are water soluble and enter lacteals / lymphatics / thoracic duct

Question 27

What happens to the chylomicrons as it travels to the liver?

Answer 27

Fats are taken up from the chylomicrons by the cells as it travels and its remnants end up at the liver where it is recycled.