Transcript Digestion and Nutrient Absorption

Digestion and Nutrient
Absorption
Presented by:
Professor Steven Dion, Teresa Ward & Kelly Baker
Salem State College - Sport, Fitness and Leisure Studies Dept.
The Odyssey
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The gastrointestinal tract:
 Includes the esophagus, gall bladder, liver,
stomach, pancreas, small intestine, large
intestine, rectum, and anus (which are both
part of the large intestine).
 Its entire length from the mouth to the anus
is 6-8 meters long.
 It is the tube that supplies nutrients and
water to the body.
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The GI Odyssey
The nutrients are delivered to the liver by
way of the hepatic-portal vein and then they
are distributed throughout the body by the
circulatory system.
 It takes the GI tract 1-3 days to eliminate the
ingested food.

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The Small Intestine
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Approximately 90% of digestion (and essentially all
lipid digestion) takes place in the first two sections of
the 3 meter long small intestine.
It is a coiled structure with three sections called the
duodenum, jejunum and the ileum.
There are tiny fingerlike protrusions along the walls
of the small intestine called villi.
These structures absorb the carbohydrates, proteins,
lipids, water, vitamins and minerals.
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The Small Intestine
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The intestinal walls are made up of smooth muscle
that contract and relax moving the food or chyme
forward then slightly backward.
 This gives the intestine additional time for
absorption.
 Here the pancreas secretes 1.2-1.6 liters of alkali
containing juice to help buffer the hydrochloric
acid that mixes with the chyme and enters into
the small intestine from the stomach.
Neutralizing this acid is crucial otherwise it causes
ulcerations or ulcers.
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The Large Intestine
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This is the final digestive structure.
It consists of the ascending and descending colon,
transverse colon, sigmoid colon, rectum and anal
canal. It is 1.2 meters long.
By the time the digested food or chyme reaches
the large intestine, most of the nutrients have been
absorbed.
The primary role of the large intestine is to
convert chyme into feces for excretion.
Here the colon absorbs water from the chyme,
changing it from liquid to solid. The large
intestine does not contain villi.
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The Digestive Process
Carbohydrate digestion and absorption
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Carbohydrates start breaking down in the mouth.
The enzyme salivary amylase starts breaking up the
starches reducing it into smaller glucose molecules
where in the stomach it continues to be broken
down further.
Upon entering the small intestine the pancreas
releases the enzyme pancreatic amylase to help
complete the hydrolysis of starch into smaller
chains of glucose molecules – monosaccharides,
which is 1 molecule of sugar.
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The Digestive Process
(Carbohydrate digestion continued)
 The monosaccharides are absorbed into the
small intestine and delivered to the liver by
way of the hepatic portal vein.
 After the liver processes the nutrients, the
nutrients enter into the blood stream
circulating throughout the body.
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The Digestive Process
Lipid or fat digestion and absorption
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In the stomach fats are broken down into glycerol
and fatty acids.
The digestion of fats starts in the stomach when
mixed with the enzyme lipase.
The major part of the breakdown takes place in
the small intestine.
In the duodenum the enzyme pancreatic lipase
furthers the process by breaking the fats down
from triglycerides to monoglycerides (which is 1
fatty acid instead of 3 fatty acids connected to a
glycerol molecule).
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The Digestive Process
(Lipid/fat digestion and absorption continued)
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Bile is produce in the liver and secreted by the gall
bladder which increases the lipids solubility,
breaking it down into droplets, making it easier
for the small intestine to absorb.
When foods with high lipid content enter the
stomach, the hormone – gastric inhibitory peptide
is released, slowing down movement flow out of
the stomach.
This is why we feel full after eating high fat foods.
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The Digestive Process
Protein Digestion and Absorption
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Proteins are split into linked amino acids called
peptides and then into individual amino acids.
In the stomach the enzyme pepsin starts the
breakdown of proteins into smaller units called
polypeptides and peptides.
In the duodenum of the small intestine the
pancreatic enzymes trypsin and chymotyrpsin also
split proteins into polypeptides and peptides.
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The Digestive Process
(Protein Digestion and Absorption continued)
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In the jejunum of the small intestine an enzyme created
by the small intestine called peptidase splits the large
peptides into smaller peptides and than into amino acids.
All of these smaller protein fragments go directly to the
liver by the hepatic portal vein.
Once in the liver one of three things happens to the
proteins:
 1. It converts to glucose,
 2. It converts to fat or
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3. It is directly released into the blood as amino acids.
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The Digestive Process
Vitamin Absorption
 Vitamins are absorbed mainly in the
jejunum and ileum sections of the small
intestine. A, D, E, and K are the fat soluble
vitamins and they must be absorbed in
combination with fat.
 Fat is the transport for which the vitamins
are delivered to the liver.
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The Digestive Process
(Vitamin Absorption Continued)
 The B’s and C vitamins are water soluble
vitamins.
 They start breaking down in the stomach
and then throughout the small intestine.
 They do not remain in the body’s tissues
very long and the excess is passes on into
the urine.
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The Digestive Process
Mineral Absorption
 Intestinal absorption of minerals increases
when the body is lacking the particular
nutrient.
 Intestinal absorption of minerals decreases
when the body is not lacking the particular
nutrient.
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The Digestive Process
Water Absorption
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Saliva, gastric secretions, bile, pancreatic acids
and intestinal secretions take up approximately 7
liters of water, when combined with an intake of
an average of 2 liters of water ingested, the body
is processing approximately 9 liters of water daily.
72% is absorbed in the first half of the small
intestine, 20% is absorbed in the lower half of the
small intestine, and 6% is absorbed in the large
intestine.
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Digestion & Absorption
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Water and ions
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Carbohydrate (CHO)
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Protein
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Fat
Vocabulary: trypsin/ogen, enterokinase, proteolytic, chylomicron,
chyme, micelle
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Digestion & Absorption
GENERAL PRINCIPLES
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Breakdown of complex molecules
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Enzymes (pH)
Absorption into gut cells
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Mechanism of absorption
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Active [energy]
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Passive [no direct energy ]
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carriers
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Bulk flow of liquid in gut
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Input
 Ingestion ~ 2 litres per day
 Secretion (gut) ~ 7 litres/day
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Output
 Faeces ~100 ml/day
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Conclude ~ 9 litres/day absorbed
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Absorption of Water
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Passive
Osmotic gradient
+
 Due to Na [active]
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‘Leaky’ tight junctions (paracellular)
 Regulated
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~95% absorption in small intestine
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Carbohydrate (CHO) Digestion
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Carbohydrate (CHO) Digestion
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CHO Digestion — Summary
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Polysaccharides to disaccharides (gut amylases)
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Disaccharides to monosaccharides (brush border)
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Glucose, galactose enter cells by
energy-dependent secondary active transport
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i.e. ‘piggy-back on Na+ gradient (active)
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Carbohydrate Absorption
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Glucose, galactose enter capillaries down
concentration gradient
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Fructose enters capillaries via passive
carrier mediated transport
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Protein Digestion
Essentially similar to CHO
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Proteins to peptides
 Gastric pepsinogen
 Activated by HCl AND pepsin
 Pancreatic proteases (trypsin)
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Protein Digestion contd
Essentially similar to CHO
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Peptides to amino acids (brush border)
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Absorbed by secondary active transport
 Depends on Na+ transport
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Pancreas
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Endocrine
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Insulin,glucag
on
Exocrine
 Enzymes
(acini)
 Bicarbonate
(ducts)
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Digestion and Absorption of Fat
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Emulsification by
bile salts
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Pancreatic lipase
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Micelles are watersoluble
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Digestion and Absorption of Fat
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Monoglycerides and FFA enter
cells by diffusion
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Triglyceride synthesis
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Add protein
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Chylomicrons
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To lacteal (lymph)
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Fat Digestion & Absorption
Fat to triglycerides (pancreatic lipase)
 Bile salts emulsify (surface area)
 Bile salts — micelles containing
monoglycerides and free fatty acids (FFA)
 Enter passively
 Triglyceride synthesis — chylomicrons
 Exocytosis and thence to lacteals
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Lipid Digestion
Bile salts emulsify lipids by:
• Chain length
• Saturation
• Placement
Lingual Lipase:
Mouth:
• Important largely for neonates
• Acts on milk fats - preemulsified fats
O
O
–O–C–R1
O
R2 –C–O–
O
H2O
–O–C–R3
O
–O–C–R1
R2 –C–O–
O
+ R3–C–OH
–OH
Stomach:
• Little to none
• Churning creates a course lipid emulsion
• Fat slows the release of food from the stomach
Small intestine
• Performs 90% of all lipid digestion
• Primarily performs hydrolysis and de-esterification
• Mix bile salts to convert coarse emulsion into micelles
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Lipid Digestion
Pancreatic Lipase
• long > short, unsaturated > saturated
• inhibited by bile salts
• enhanced by Ca++ – Colipase
R1
H2O
R2
R1
H2 O
R2
R3
OH
R2
OH
FFA
OH
FFA
TAG
DAG
2-MAG
Phospholipase A2 = Lecithinase
• acts at C1, 2 position of PL
R1
H2O
R2
Cholesterol Esterase
R1
H2 O
HO
P–R3
• removes FA at low pH - lumen
• adds FA at higher pH - mucosa
FFA
OH
HO
P–R3
FFA
P–R3
H+
O
OH–
Cholesterol + FFA
O
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Lipid Absorption
•
•
•
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Lipid Absorption
Liver
Emulsion
500 nM
Gall
Bladder
PL
4-14
CE
micelle 5 nM
+ CCK
duodenum
Fatty acids:
DAGs
MAGs
Cholesterol
50 nM
bile salts
ATP
Digestion & Absorption
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Bile Acid Synthesis
OH
COOH
COOH
OH
HO
HO
Cholic acid
OH
Chenodeoxycholicacid
OH
COOH
COOH
HO
Lithocholicacid
HO
Deoxycholic acid
COOH
O
–O–SO
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Absorption
Lipid
+
Bile salts
+
Enzymes
Bile salts
FAs
DAGs
MAGs
cholesterol
Directly
absorbed
into
mucosal cell
Re-esterify
to
TAG
chol–FA
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Circulate
to lymph
as
chylomicron
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Lipid Absorption
glucose
-glycerol–P
glucose
cholesterol
cholesterol
fatty acids
MAG
cholesterol ester
FA-CoA
TAG
Chylomicrons
lysolecithin
Lymphatics
lecithin
apoproteins
short chain
FAs </= C10
FA
+
albumen
Blood
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