Lipid Digestion Lect4

LIPIDS
Lipids (fats & oils)

– most feeds contain 1-5% fat or oil
– composed of 3 fatty acids & glycerol
Lipid Characteristics
• Lipid = a compound that is insoluble in water,
but soluble in an organic solvent (e.g., ether,
benzene, acetone, chloroform)
• “lipid” is synonymous with “fat”, but also
includes phospholipids, sterols, etc.
Classification
Bloor’s Classification
A. Simple lipid - ester of fatty acids with various
alcohols
1. Natural fats and oils (triglycerides)
CH3 CH3 CH3
E E E E
OH
2. Waxes CH3
CH3
(a) True waxes: cetyl alcohol esters of fatty

acids
(b) Cholesterol esters
(c) Vitamin A esters
(d) Vitamin D esters
B. Compound lipid
esters of fatty acids with alcohol plus other groups
1. Phospholipids and spingomyelin:

contains phosphoric acid and often a nitrogenous base
2. Spingolipids
(also include glycolipids and cerebrosides): contains aminoalcohol
spingosine, carbohydrate, N-base; glycolipids contains no phosphate
3. Sulfolipids : contains sulfate group
4. Lipoproteins : lipids attached to plasma/other proteins
5. Lipopolysaccharides: lipids attached to polysaccharides

Classification cont.
C. Derived lipids – hydrolytic products of A & B with lipid characters
1. Saturated & unsaturated fatty acids
2. Monoglycerides and diglycerides
3. Alcohols (b-carotenoid ring, e.g., vitamin A, certain carotenoids)
D. Miscellaneous lipids
1. Aliphatic hydrocarbons: found in liver fat and certain
hydrocarbon found in beeswax and plant waxes
2. Carotenoids
3. Squalene : found in shark and mammalian liver and in
human sebam; an important intermediate in biosynthesis of
cholesterol
4. Vitamin E and K
Classification of Lipids
Based on groups of two’s
• Saponifiable lipids have two subclasses
– Simple saponifable has two sub-categories
• Waxes
• Triglycerides
– Complex saponifable has two sub-categories
17.1 Biological Functions of Lipids
• Phosphoglycerides
• Sphingolipids
• Nonsaponifiable have two subclasses
– Steroids
a chemical reaction in which an ester is
– Prostaglandins heated with an alkali (especially the alkaline
hydrolysis of a fat or oil to make soap)
http://wordnetweb.princeton.edu/perl/web
wn?s=saponification
Lipid Classification
Lipids
Nonsaponifiable
Saponifiable
17.1 Biological Functions of Lipids
Steroids Prostaglandins
Simple Complex
Sphingolipids Phosphoglycerides
Waxes Triglycerides
Nomenclature and Structure
Fats and oils:

Vegetable oils are triglycerides that are liquid at room temp
due to their higher unsaturated or shorter-chain fatty acids
Triglycerides are most abundant natural lipids
Natural fats have D-configuration

Usually R1 and R3 are saturated and R2 is unsaturated
Natural fats are mixture of two or more simple triglycerides
Saturated vs. Unsaturated Fatty Acids
• saturated: the SFA’s of a lipid have no double bonds

between carbons in chain
• polyunsaturated: more than one double bond in the
chain
• most common polyunsaturated fats contain the
polyunsaturated fatty acids (PUFAs) oleic, linoleic and
linolenic acid
• unsaturated fats have lower melting points
• stearic (SFA) melts at 70oC, oleic (PUFA) at 26oC
Fatty Acids Commonly Found in Lipids
Sat. Fatty Acids Formula Melting Point (oC)

Butyric C4H8O2 Liquid
Palmitic C16H22O2 63
Stearic C18H36O2 70
Unsat. Fatty Acids Formula Melting Point (oC)
Oleic C18H34O2 Liquid
Linoleic C18H32O2 Liquid
Linolenic C18H30O2 Liquid
Saturated vs. Unsaturated Fats
• saturated fats tightly packed, clog arteries as
atherosclerosis
• because of double bonds, polyunsaturated
fats do not pack well -- like building a wall with
bricks vs. irregular-shaped objects
• plant fats are much higher in PUFA’s than
animal fats
Saturated vs. Unsaturated FA’s Plant vs.
Animal Fat
corn soy tallow lard
Sat. FA’s
Myristic 3
Palmitic 7.0 8.5 27 32.2
Stearic 2.4 3.5 21 7.8
Unsat. FA’s
Oleic 45.6 17 40 48
Linoleic 45.0 54.4 2 11
Linolenic 7.1 0.5 0.6
Arachid.
Nutritional Uses of Lipids
Functions:
energy source
• 2.25 x more energy than carbohydrates
We already know that lipids are concentrated sources of
energy (9.45 kcal/g)
• other functions include:
• 1) provide means whereby fat-soluble nutrients (e.g.,
sterols, vitamins) can be absorbed by the body
• 2) structural element of cell, subcellular components
• 3) components of hormones and precursors for
prostaglandin synthesis
Lipids digestion and absorption
• The major portion of fat digestion takes place
in the small intestine as a result of pancreatic
lipase
The Liver
• Produces bile (fat

digestion)
• Enterohepatic
circulation
Stores bile
The Gallbladder
• Concentrates it
• Releases to small
intestine when needed
What about Bile???
• These are biological detergents synthesized by the liver and
secreted into the intestine
• they form the spherical structures (micelles) assisting in
absorption
• hydrophobic portion (tails of FA) are located to the inside of the
micelle, with heads (hydrophillic portion) to the outside
• they move lipids from the intestinal lumen to the cell surface
• absorption is by diffusion (complete for FA and monoglycerides,
less for others)
Sites of Secretion, Digestion, and Absorption
Lipid Digestion/Absorption
Five different phases:
hydrolysis of triglycerides (TG) to free fatty acids (FFA) and
monoacylglycerols
 solubilization of FFA and monoacylglycerols by detergents
(bile acids) and transportation from the intestinal lumen
toward the cell surface
 uptake of FFA and monoacylglycerols into the cell and
resynthesis to triglyceride
 packaging of TG’s into chylomicrons
 exocytosis of chylomicrons into lymph
Enzymes Involved in Digestion of Lipids
• lingual lipase: provides a stable interface with

aqueous environment of stomach
• pancreatic lipase: major enzyme affecting
triglyceride hydrolysis
• colipase: protein anchoring lipase to the lipid
• lipid esterase: secreted by pancreas, acts on
cholestrol esters, activated by bile
• phospholipases: cleave phospholipids, activated by
trypsin
Factors Affecting Absorption of Lipids
• amount of fat consumed ( fat =  digestion = 

absorption)
• age of subject ( age =  digestion)
• emulsifying agents
• chain length of FA’s (> 18C =  digestibility)
• degree of saturation of FA ( sat =  digestibility)
• overheating and autooxidation (rancidification at double
bond)
• optimal dietary calcium = optimal FA absorption (high Ca =
 absorption)
Lipid Metabolism/Absorption
• short chain FA’s are absorbed and enter the portal vein
to the liver
• those FA’s with more than 10 carbons are resynthesized
by the liver to triglycerides
• they are then converted into chylomicrons and pass to
the lymphatic system
• some FA’s entering the liver are oxidized for energy,
others stored
• blood lipids: 45% P-lipids, 35% triglycerides, 15%
cholestrol esters, 5% free FA’s
diacylglycerol acyltransferase (DGAT)

Energy driven mechanism

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LIPIDS

Lipids (fats & oils)

(a) True waxes: cetyl alcohol esters of fatty

1. Phospholipids and spingomyelin:

4. Lipoproteins : lipids attached to plasma/other proteins

5. Lipopolysaccharides: lipids attached to polysaccharides

Fats and oils:

Natural fats have D-configuration

• saturated: the SFA’s of a lipid have no double bonds

Sat. Fatty Acids Formula Melting Point (oC)

• Produces bile (fat

• lingual lipase: provides a stable interface with

• amount of fat consumed ( fat =  digestion = 

diacylglycerol acyltransferase (DGAT)

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