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Lecture 13
Dietary Fat
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
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
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
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
Chol
Bile salts
Digestion - Pancreatic Lipase
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
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)
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
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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
– See anti-Olestra sites at www.cspinet.org/olestra/
Lipoproteins
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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