Download Digestion and Absorption of Lipids

Lipids
Lipids are heterogeneous group of water insoluble
organic molecules.
An adult ingests about 60 to 150 g of lipids per day.
Dietary fat Composition
 More than 90% are triglycerides, the other are
 Cholesterol,
 Cholesteryl esters,
 Phospholipids and
 Unesterified fatty acids.
Animal Sources
Dairy products- butter, ghee
Meat, Fish, Pork and eggs
Vegetable Sources
Cooking oils- Sun flower oil, Mustard oil, Ground nut
oil
Fats from other vegetable sources
? Challenge
Lipids are insoluble or sparingly soluble in
aqueous solution.
The digestive enzymes, however, are present in
aqueous medium.
Fortunately, the digestive tract possesses specialized
machinery to
1 . Increase the surface area of lipids for digestion;
2. Solubilize the digested products for absorption.
Digestion in Stomach
LINGUAL LIPASE
 The lingual lipase from the mouth enters stomach along with
the food.
 It has an optimum pH of 2.5 – 5. (Acid stable lipase)
 The enzyme therefore continues to be active in the stomach.
 It acts on short chain triglycerides. (SCT)
 SCTs are present in milk, butter and ghee.
 The action of lingual lipase is observed to be more significant
in the newborn infants.
GASTRIC LIPASE
 Gastric lipase is acid stable, with an optimum pH about 5.4.
 It is secreted by chief cells, the secretion is stimulated by
gastrin.
 Upto 30% digestion of triglycerides occurs in stomach. (SCT
and MCT)
Digestion in Intestine
 Emulsification:
 It is a pre requisite for digestion of lipids. (LCFA)
 The lipids are dispersed into smaller droplets, surface tension is
reduced, and surface area of droplets is increased.
 Emulsification is important since the lipolytic enzymes can act only
on the interfacial area between the aqueous and lipid phase.
 Emulsification is favored by:

Bile salts (detergent action)

Peristalysis (mechanical mixing)

Phospholipids (surfactant action)
Bile salts
 Bile salts interacts with the dietary lipid particles and the aqueous
duodenal contents, thereby stabilizing the particles as they become
smaller from peristalsis and preventing them from coalescing.
 There by increasing the surface area of the particles for enhanced
activity of enzymes.
DIGESTION IN INTESTINE
Co- Lipase
 The co-lipase is secreted by
the pancreas as an inactive
zymogen.
 It is activated by trypsin.
 The binding of co-lipase to the
TAG molecule at the oil water
interface is obligatory for the
action of lipase.
 Co-lipase helps in anchoring
and stabilizing lipase.
Control of Lipid Digestion by intestinal hormones
 Cholecystokinin – small peptide hormone.
 Released from mucosal cells of lower duodenum and jejunum.
 Decreases gastric motility.
 Causes contraction of gall bladder , releasing bile and mixture
of bile salts.
 Secretin – small peptide hormone
 Released from intestinal cells in response to the low pH of the
chyme entering the intestine.
 Increases bicarbonate secretion from pancreas, helps to
neutralise the pH of intestinal contents and provides a pH
favorable for the action of pancreatic enzymes.
PHYSIOLOGICALLY IMPORTANT LIPASES
S.NO
LIPASE
SITE OF
ACTION
PREFERRED
SUBSTRATE
PRODUCTS
1
Lingual lipase
Stomach
SCT
FFA + DAG
2
Gastric Lipase
Stomach
SCT & MCT
FFA+DAG
3
Pancreatic lipase
Small intestine
LCT
FFA + 2DAG
4
Phospholipase A2
Small intestine
Phospholipids
Lysolecithin
5
Lipoprotein lipase
Capillary wall
TAG in
chylomicron
and VLDL
FFA + Glycerol
6
Hormone sensitive
lipase
Adipocyte
TAG stored in
adipose tissue
FFA + DAG
ABSORPTION OF LIPIDS
Mixed micelle formation
 The products of digestion, namely 2MAG, FFA,
Cholesterol, Phospholipids, fat soluble vitamins and
bile salts are incorporated into aggregates to form
mixed micelles.
 The micelles are spherical particles with a hydrophilic
exterior and hydrophobic interior core.
 Due to their amphipathic nature, the bile salts help to
form micellar aggregates.
 Micellar formation is essential for the absorption of lipid
and fat soluble vitamins such as Vitamin A,D,E & K.
 The micelles are aligned at the microvillous surface of
the jejunal mucosa.
 FA, 2MAG and other digested products passively diffuse
into the intestinal mucosal cell.
 The bile salts are left behind which are mostly
reabsorbed from the ileum and returned to the liver to
be re-excreted (enterohepatic circulation).
Re- esterification inside the mucosal cell
Chylomicrons
 The TAG, cholesterol ester and phospholipid molecule along
with apoproteins B48 and apoA are incorporated into
chylomicrons.
 The chyle (milky fluid) from the intestinal mucosal cells loaded
with chylomicrons are transported through lacteals into the
thoracic duct and then emptyed into systemic circulation.
 The serum may appear milky after a high fat meal(post prandial
lipemia) due to the presence of chylomicrons in circulation.
 Normally the lipemia clears with in a few hours by the uptake of
chylomicrons by peripheral tissue and liver.
SCFA absorption is different
 SCFA (milk, butter, ghee) and MCFA (coconut oil and
mothers milk) do not need re-esterification.
 They can directly enter into blood vessels, then to
portal vein and finally to liver where they are
immediately utilized for energy.
 Their absorption is rapid.
 They are better absorbed than LCFA.
Summary
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