1. Nursing - 8
Metabolism of dietary
lipids

2. Definition of lipids
• Lipids may be defined as compounds
which are relatively insoluble in water,
but freely soluble in nonpolar organic
solvents like benzene, chloroform,
ether, hot alcohol, acetone, etc.

3. Function of lipids
1. Storage form of energy (triglycerides).
2. Structural components of bio-membranes
(phospholipids and cholesterol).
3. Metabolic regulators (steroid hormones and
prostaglandins).
4. Help in absorption of fat soluble vitamins (A, D, E
and K).
5. Improve taste and palatability to food.

4. Classification of lipids
Based on the chemical nature, lipids are
classified into:
I. Simple lipids
They are esters of fatty acids with glycerol or
other higher alcohols. They are sub-classified
as:
a. Triacylglycerol or triglycerides or neutral fat.
b. Waxes.

5. II. Compound lipids
• They are fatty acids esterified with
alcohol; but in addition they contain
other groups.
• Depending on these extra groups, they
are sub-classified as:
a. Phospholipids containing phosphoric
acid.
b. Nonphosphorylated lipids.

6. III. Derived lipids:
• They are compounds which are derived from
lipids or precursors of lipids.
• E.g. fatty acids, steroids, prostaglandins, …etc.
IV. Lipids complexes to other
compounds:
Proteolipids and lipoproteins.

7. Fatty acids
• Fatty acids, are included in the group of derived
lipids.
• It is the most common component of lipids in the
body.
• Fatty acids are aliphatic carboxylic acids and have
the general structural formula, R-COOH, where
COOH (carboxylic group) represents the functional
group.
• Depending on the R group (the hydrocarbon chain),
the fatty acids may vary.

8. • Each animal species will have characteristic pattern
of fatty acid composition. Thus, human body fat
contains 50% oleic acid, 25% palmitic acid, 10%
linoleic and 5% stearic acid.
• The carbon atoms of fatty acids are numbered as
C1, C2 etc. starting from the COOH group. Or,
starting from the methyl end, the carbon atoms
may be numbered as omega (ῳ)-1,2,3, etc.
• 6 5 4 3 2 1
CH3 – CH2 – CH2 – CH2 – CH2 – COOH
ῳ1 ῳ2 ῳ3 ῳ4 ῳ5

9. Classification of fatty acids
1. Depending on Total No. of carbon
atoms:
a. Even chain, having carbon atoms 2,4,6 and similar
series. Most of the naturally occurring lipids
contain even chain fatty acids.
b. Odd chain, having carbon atoms 3,5,7, etc. Odd
numbered fatty acids are seen in microbial cell
walls. They are also present in milk.

10. 2. Depending on Length of
hydrocarbon chain:
a. Short chain with 2 to 6 carbon atoms
e.g. Acetic acid (2 C) CH3-COOH
Butyric acid (4 C) CH3-CH2-CH2-COOH
b. Medium chain with 8 to 14 carbon atoms.
c. Long chain with 16 and above, usually up to 24
carbon atoms.
e.g. Palmitic acid (16 C) CH3-(CH2)14- COOH
Stearic acid (18 C) CH3-(CH2)16- COOH

11. Depending on Nature of hydrocarbon chain:
1. Saturated fatty acids
• They have the general structural formula
CH3-(CH2)n-COOH. contain single bond.
• They are named by adding the suffix ‘anoic’
after the hydrocarbon.
• The two carbon acetic and 4 carbon butyric
acid are important metabolic intermediates.
• C16 palmitic and C18 stearic acids are most
abundant in body fat.

12. 2. Unsaturated fatty acids
• They are named by adding the suffix ‘enoic’ after
the systematic name.
• which may be sub- classified into Mono-
unsaturated (mono-enoic) having single
double bond or polyunsaturated (poly-enoic)
with 2 or more double bonds.

13. Clinical significance of PUFA
• Linoleic and linolenic acids are polyunsaturated
fatty acids. Linoleic acid has 2 double bonds;
Linolenic acid has 3 double bonds and Arachidonic
acid has 4 double bonds.
• Unsaturated fatty acids are also designated ῳ3
(omega 3) family (Linolenic acid); ῳ6 family
(Linoleic and Arachidonic acids) and ῳ9 family
(Oleic acid).
• Arachidonic acid is the precursor of
prostaglandins.
• The pentaenoic acid present in fish oil is of great
nutritional importance (ῳ3 unsaturated fatty
acid).

14. BILOGICAL CLASSIFICATION
1- Essential fatty acids
• Poly unsaturated fatty acids with more than double
bond and not synthesize by mammals.
• Linoleic (ὠ6, 18C, two double bonds) and Linolenic
acid (ὠ3, 18C, three double bonds) are the only
fatty acids which cannot be synthesized in the body.
• They have to be provided in the food; hence they
are essential fatty acids.
• Arachidonic acid can be formed, if the dietary
supply of linoleic acid is sufficient.

15. N.B. Eicosanoids are 20C compounds derived from
arachidonic acid.
• Normal dietary allowance of PUFA is 2-3% of total
calories.
2- Non-essential fatty acids
Saturated fatty acids or fatty acids contain one double
bond and synthesize by mammals.
e.g. Stearic and Oleic acid.

16. Neutral fats or Triacylglycerols (TAG)
or Triglycerides
• These are esters glycerol with fatty acids.
1. Nomenclature of Carbon Atoms:
• The carbon atoms of glycerol are designated as α, β
and ἀ or as 1, 2, 3
α (1) CH2-O-CO-R
I
Β (2) R-CO-O-C-H
I
ἀ (3) CH2-O-CO-R

17. 2. Storage of energy as fat:
• The triglycerides are the storage form
of lipids in the adipose tissue.
• When stored as TAG, water molecules
are repelled and space requirement is
minimal.
• Excess fat in the body leads to obesity.

18. 6. Rancidity of fat:
• Fats and oils have a tendency to become
rancid.
• The term rancidity refers to the appearance of
an unpleasant smell and taste for fats and oils.
• PUFA are more easily oxidized; so vegetable
oils with a high content of PUFA are usually
preserved with addition of antioxidants.

19. • Fats and oils are preferred cooking media.
However, overheating and repeated
heating would lead to the formation and
polymerization of cyclic hydrocarbons.
• These will impart an unpleasant taste and
color to the oil.
• Coconut oil having medium chain saturated
fatty acids will withstand such
polymerization.

20. Prostaglandins (PGs)
• PGs are present in almost all tissues.
• They are the most potent biologically active
substances; as low as one nanogram/ml of
PG will cause smooth muscle contraction.
• The diverse physiological roles of
prostaglandins confer on them the status of
local hormones.
• depending on number of double bonds on the
side chains they are denoted by a subscript after
the capital letter, e.g. PGE1, PGE2, PGE3, etc
• In most tissues, PGE increases cAMP (cyclic AMP)
level.

21. Effects of PGs on CVS: (cardiovascular system)
• Prostacyclin or PG12 is synthesized by the vascular
endothelium. Major effect is vasodilatation.
• It also inhibits platelet aggregation and has a protective
effect on vessel wall against deposition of platelets.
• Thromboxane (TXA2) is the main PG produced by
platelets.
• The major effects are vasoconstriction and platelet
aggregation.
• Prostacyclin and thromboxane are opposing in activity.
• Prostaglandins increase the contractility and lowers the
blood pressure.
• Hence, it may be used in the treatment of hypertension.

22. Effects on ovary and uterus:
• PGF2 stimulates the uterine muscles.
• Hence, PGF2 may be used for medical
termination of pregnancy.
• Yet another use is in inducing labor and
arresting postpartum hemorrhage.
• Effects on respiratory tract
• PGE is a potent bronchodilator.
• PGE is used in aerosols for treating
bronchospasm.

23. Phospholipids
• They contain glycerol, fatty acids and a
nitrogenous base.
• They have both hydrophobic and hydrophilic
portion in their molecule
Biomembranes:
• The molecules align themselves to form
monolayers with the polar heads pointing in
one direction and the nonpolar tails in the
opposite direction.
• In vivo, they act as pulmonary surfactants.

24. Liposomes:
• Liposomes are microscopic spherical vesicles.
• When mixed in water under special conditions, the
phospholipids arrange themselves to form a bilayer
membrane which encloses some of the water in a
phospholipid sphere.
• Drugs, proteins, enzymes, etc. may be encapsulated
by the liposomes which act as carriers for these
substances to target organs.
• Liposomes have important applications in cancer
chemotherapy, antimicrobial therapy, gene therapy,
vaccines and diagnostic imaging.

25. • DIGESTION OF LIPIDS
• The major dietary lipids are triacylglycerol,
cholesterol and phospholipids.
• Digestion in Stomach
• i. The lingual lipase from the mouth enters stomach
along with the food. It has an optimum pH of 2.5-5.
The enzyme therefore continues to be active in the
stomach. It acts on short chain triglycerides (SCT).
SCTs are present in milk butter, ghee and coconut oil.
The action of lingual lipase is observed to be more
significant in the newborn infants.
• ii. Gastric lipase is acid stable, with an optimum pH
about 5.4. It is secreted by Chief cells, the secretion is
stimulated by Gastrin.

26. • Digestion in Intestines
• Emulsification is a pre-requisite for digestion of lipids.
The lipids are dispersed into smaller droplets; surface
tension is reduced; and surface area of droplets is
increased.
• Bile Salts are Important for Digestion of Lipids
• The bile salts present in the bile (sodium glycocholate
and sodium taurocholate) lower surface tension. They
emulsify the fat droplets in the intestine. The
emulsification increase the surface area of the particles
for enhanced activity of enzymes.
• Lipolytic Enzymes in Intestines
• Pancreatic lipase with Co-lipase will further
hydrolyze the neutral fats. The bile (pH 7.7) entering
the duodenum serves to neutralize the acid come from
the stomach and provides a pH favorable for the action
of pancreatic enzymes.

27. • Digestion of Triglycerides
• i. Pancreatic Lipase can hydrolyze the fatty acids at
the 1st and 3rd carbon atoms of glycerol. The
products are 2-mono acylglycerol (2-MAG) and two
fatty acid molecules.
• ii. Then an isomerase shifts the ester bond from
position 2 to 1. The bond in the 1st position is then
hydrolyzed by the lipase to form free glycerol and
fatty acid.
• iii. The major end products of the digestion of TAG
are 2-MAG, 1-MAG, glycerol and fatty acids.
Thus, digestion of TAG is partial (incomplete).

29. • Absorption of lipids by cells in the small intestine.

30. Chylomicron:
• A small fat globule composed of protein and
lipid.
• The chylomicrons are synthesized in the mucosa
(the lining) of the intestine.
• Chylomicrons are found in the blood and
lymphatic fluid where they serve to transport fat
from its port of entry in the intestine to the liver
and to adipose (fat) tissue.
• After a fatty meal, the blood is so full of
chylomicrons that it looks milky.

31. • Fate of absorbed Fat
• i. The absorbed (exogenous) triglycerides are
transported in blood as chylomicrons. They are
taken up by adipose tissue and liver.
• ii. Liver synthesizes endogenous triglycerides.
These are transported as VLDL (very low density
lipoproteins) and are deposited in adipose tissue.
• iii. Triglycerides In adipose tissue are lysed to
produce free fatty acids. In the blood, they are
transported, complexed with albumin.
• iv. Free fatty acids are taken up by the cells, and
are then oxidized by β-oxidation to get energy.