Dietary Fat

1
Lecture 13

• Dietary Fat

2
Dietary Fat

• Lipogenesis is not very active in people on a
  Western diet
• Lipogenic enzyme expression is down-regulated by
  fat consumption
• Most of our fat comes from the diet 100g/day
• Most fat in white adipose tissue will have come
  from dietary fat and not de novo lipogenesis
• Fat is hydrophobic
• Problems for digestion and transport
• Digestive enzymes need fat to be in an emulsion
• Fat needs to be carried around the bloodstream
  within lipoproteins

3
Formation of Emulsions

• Needs molecules which have both hydrophilic and
  hydrophobic characteristics
• amphiphilic
• amphipathic
• Phospolipids that comprise cell membranes are
  ampiphatic
• As is the phosphatidic acid and lysolecithin that
  you use in salad dressing
• Amphiphilic molecules act as detergents which
  emulsify fat into tiny particles
• micelles

4
Bilayers and Micelles

• In both structures, polar heads are facing the
  aqueous environment while the hydrophobic tails
  are buried in the core
• Micelles can also be formed using bile salts

5
Fat Digestion

• Fat is trapped in the core of micelles formed
  with bile salts
• Churning of dietary fat with bile salts in the
  intestine
• Chyme
• Emulsion
• Easy for lipase to interact with
• Pancreatic Lipase
• Hydrolyses fat into FA and glycerol
• Plus mixture of mono- and di-acyl glycerols

lipase
FAT
Bile salts
Chol
6
Digestion - Pancreatic Lipase
7
Bile Salts

• Produced in the liver
• Made from cholesterol
• Cholesterol itself it not amphiphilic enough to
  be a detergent ? needs modification by addition
  of polar groups
• Stored in the gall bladder
• Reabsorbed and taken back to the liver
• Hepatic portal vein

8
Bile Salts

• Polar groups are added to cholesterol to make it
  more amphiphilic
• The only way to get rid of cholesterol is to make
  them into bile salts (chol cannot be oxidised)

9
Undigested Fat

• If gall bladder is blocked by gall stones, no
  bile salts can be secreted into the small
  intestine ? no fat digestion
• Less calorie intake
• But more oily stools
• Inhibitors of fat digestion as weight-loss drugs
• Orlistat (brand name Xenical) is an inhibitor of
  lipase
• Listed side effects include
• oily spotting
• oily or fatty stools
• orange or brown colored oil in your stool
• gas with discharge, an oily discharge
• an urgent need to go to the bathroom
• an inability to control bowel movements,
• an increased number of bowel movements.
• Olestra (Olean) is a fat substitute
• FA attached to sucrose
• Not attacked by lipases
• Olestra will passes through gut undigested
• Strips fat soluble vitamins
• D, E, K need to be added as supplements

10
Lipoproteins

• The phospholipid shell can be up to 1 mm in
  diameter
• Apoproteins
• Enzymes
• Structural
• Docking
• Different types of lipoproteins characterised by
  size and by types of apoproteins
• First lipoproteins made by intestinal cells
• Chylomicrons
• Enter lymphatic system