Lipid metabolism
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98% of total dietary lipids are
triacylglycerol
TAG are insoluble in water
– Salivary enzymes has no effect
Digestion occurs in the stomach
– Physical digestion
– Breaks up into small globules
– Resulting material is called
Chyme
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Chyme
– Thick semi-liquid
material made up of
small TAG globules,
partially digested
food, gastric secretion
• Gastric lipase
– High fat foods stays
longer in the stomach
– High fat meal causes
a person to feel full
for a longer period of
time
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Fats in the stomach triggers the release
of the hormone CHOLECYSTOKININ
– Stimulates gall bladder to contract
– Release bile
– Fat emulsification
– Fatty acid micelles
• FA and small
monoacylglycerol and bile
Pancreatic lipases
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Chylomicron
– Newly assembled TAG
(from FA and
monoacylglycerol in the
intestines) are combined
with membrane
phospholipids and
cholesterol and water
soluble proteins
– Transports TAG from
intestinal cells via
lymphatic system to
bloodstream
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After heavy meal
– Chylomicron concentration
increases dramatically
– Starts to rise 2 hrs after a
meal
– Peak in 4-6 hrs
– Drops to normal level as
chylomicrons move into
adipose cells and liver
Triacylglycerol
mobilization
Hydrolysis of TAG stored in
the adipose tissue followed
by release into the
bloodstream of the FA and
glycerol
TAG energy reserves are the
human body’s major source
of stored energy
Glycerol
metabolism
TAG mobilization produces 1 molecule of glycerol
After entering the bloodstream, glycerol travels to
kidneys or liver
Converted to
dihydroxyacetone
then to pyruvate
Then acetyl CoA
Then CO2 or can be used to form glucose
Produced when the
amount of acetyl CoA
is excessive
Ketone
bodies
Can serve as sources
of energy in the
heart, muscle and
renal cortex
Brain derives energy
from ketone bodies in
dieting situations
Ketogenesis
Metabolic pathway by which ketone bodies
are synthesized from acetyl CoA
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Primary site for ketogenesis
is the Liver mitochondria
1st ketone body produced is
acetoacetate
Acetoacetate is converted to
hydroxybutyrate (2nd ketone
body)
Acetoacetate and
hydroxybutyrate synthesized
in the liver are released to
the bloodstream where
Acetone (3rd ketone body) is
produced
Acetone in the bloodstream,
is a volatile substance that is
mainly excreted by
exhalation
– Its sweet odor is
detectable in the breath
of diabetics
Body condition in which high
levels of ketone bodies are
present in both blood and urine
Ketosis
Ketosis is often detectable by the
smell of acetone on a person’s
breath
Symptoms of mild ketosis
• Headache, dry mouth, acetone-smelling
breath
Fates of Fatty Acid
Generated Acetyl CoA
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Converted to ATP
Conversion to ketone bodies
Can be stored in the form of triacylglycerol
Can be used as starting materials for the
production of lipids other than FA
– Cholesterol- precursor of bile salts, sex hormones,
adrenal hormone