Digestion and absorption of food

When I prayed for success, I forgot to ask for sound sleep and good digestion. -Mason Cooley

Happiness: a good bankaccount, a good cook, and agood digestion. #Jean-Jacques Rousseau

Digestion and Absorption of Food

-Dr. Ganesh

Digestion and Absorption of Food

Contents:

Biological Importance

Medical Importance

Digestion and Absorption – General Aspects

Digestion and Absorption of Carbohydrates

Digestion and Absorption of Proteins

Digestion and Absorption of Lipids


Digestion a chemical process –large molecules in the food arebroken down tosmaller molecules so that they can be absorbed


large molecules small molecules

small molecules

Food

Digestion

Absorption

vitamins, minerals, monosaccharides and free amino acids

BLOOD


• Absorption is transport of molecules, either digested products or other small molecules which do not require

digestion, from the intestinal lumen into blood across the intestinal mucosal cells.

Medical Importance

• Lactose intolerancedeficiency of lactaseintake of milk causes diarrhea

• Hartnup’s disease genetic defect in the absorption of neutral amino acids, especially tryptophan.

Medical Importance

• Steatorrhea excess fat is excreted in feces and is seen in diseases of pancreas, biliary obstruction, etc.

• Chronic diarrhea can cause malabsorption as seen in celiac disease, Sprue, Crohn’s disease, etc.

Digestion – General Aspects

major foodstuffs that require digestion

carbohydrates,

proteins

fats and oils (triacylglycerol)

Amino acids

macromolecules

monosaccharides

glycerol and fatty acids

smaller molecules digestion

Digestion takes place in the aqueous medium of

Site of Digestion Digestive Juices

Mouth

Stomach

Small intestinal lumen

Saliva

Gastric juice

Intestinal juice

various digestive juices –


Pancreatic juice,

Bile


• Digestion involves action of enzymes that are present in different digestive juices. • All digestive enzymes are hydrolases that hydrolyze the anhydride linkages –


carbohydrates,

proteins

fats and oils (triacylglycerol)

Glycosidic linkage

Peptide linkage

Ester linkage

anhydride linkages


• Bile, synthesized in liver and entering into duodenum,• helps in fat digestion and • neutralizes

acidic stomach contents when it enters the duodenum.

Digestion – General Aspects • Cooking hydration of polysaccharides and denaturation of proteins helps digestion of these molecules. • Mastication helps in breaking down of food particles increases solubility and surface area for enzyme action.• Peristalsis also important in breaking down of food particles and mixing them with enzymes.

Absorption– General Aspects

• small intestine main absorptive organ. About 90% of the ingested foodstuffs absorbed through the small intestine• Considerably more water is absorbed in the large intestine, so that the contents, gradually become more solid in the colon.

Absorption – General Aspects Absorption of substances into mucosal cells involves

the passage across the plasma membrane

simple diffusion carrier-mediated transports

passive

(no energy expenditure)

No carrier protein

facilitated transport

(passive)

(requiring energy expenditure)

active transport

Absorption – General Aspects two pathways for the transport of nutrients

hepatic portal blood lymphatic vessels

water-soluble nutrients lipid-soluble nutrients

blood

thoracic ductliver

absorbed by the intestine

blood

Digestion and Absorption of

Carbohydrates

Contents:

• Introduction • Digestion of Starch• Digestion of Disaccharides • Absorption of Carbohydrates• Clinical Significance

Digestion and Absorption of Carbohydrates

Introduction

major carbohydrates in the diet

Monosaccharides

Disaccharides

Polysaccharides

fructose (present in fruits)

starch

Starch - more than 50% of carbohydrates

sucrose lactose maltose

(present in malt, beer)

Small amounts

dietary fibers

Digestion and Absorption of Carbohydrates - Introduction

• Dietary fibers – cellulose, hemicellulose, pectin, lignins, etc. are indigestible. For ‘Dietary fibers’, see Chapters – ‘Chemistry of Carbohydrates’ and ‘Nutrition’)


Human food also contains small amounts of • pentoses, • glucose, • trehalose (disaccharide present in mushroom), and• glycogen (present in liver in animal foods).


• All enzymes of carbohydrate digestion cleave glycosidic bonds by hydrolysis.

• Disaccharides and polysaccharides are digested to their respective constituent monosaccharides units and absorbed in the small intestines along with free monosaccharides present in the food


• The absorbed monosaccharides

hepatic portal circulation

liver cells

systemic circulation.

Fructose

Sucrose

Starch

Lactose

FOOD

DIGESTION

ABSORPTION (Small Intestine)

Glucose

Galactose

Fructose

Digestion and Absorption of Carbohydrates -Introduction

Digestion of Starch1, 2

• Starch on complete digestion

yields

glucose• Digestion of starch

takes place in • mouth,• small intestinal lumen and • small intestinal brush border

(luminal surface of intestinal mucosal cells).


• The enzymes and the steps of digestion of glycogen are same as those of amylopectin component of starch as both have similar structure.

• Cooking hydrates the starch granules making it more susceptible to digestion.


Enzymes required for complete digestion of starch – • amylase (salivary and pancreatic amylase), • maltase and • isomaltase.• Amylase and maltase cleave

-1,4 glycosidic linkages and isomaltase cleaves-1,6 glycosidic linkages of starch. (Amylase hydrolyzes internal -1,4 glycosidic linkages.)

Digestion of Starch• Starch is mainly digested by

pancreatic amylase.

Contribution by salivary amylase is very little since the food remains in the mouth for a very short period of time and the enzyme is inactivated by gastric HCl as it enters stomach.

• Maltase and isomaltase are present on the luminal surface of small intestinal epithelial cells (brush border cells).

Starch (amylose and amylopectin)

Salivary amylase (in mouth)3

Or Pancreatic amylase (in small intestinal lumen

Limit dextrins4 Small unbranched oligosaccharides (e.g., Maltose, Maltotriose, etc)

Isomaltase (brush border cells)Glucose Isomaltose

Maltase Maltase (brush border cells)

Digestion of Starch

Isomaltase

(brush border cells)

• Cl- is an activator of Salivary amylase(ptyalin)

• Limit dextrins derived from amylopectin component by the action of amylase and contain eight glucosyl units with one or two branches with α-1,6-glycosidic bonds.

Digestion of Starch

Digestion of DisaccharidesMajor disaccharides present in the human diet are – • sucrose and • lactoseSmall amounts of • free maltose (present in malt, beer) and • trehalose (disaccharide present in mushroom)

Digestion of Disaccharides

• However, quantitatively the major disaccharide digested in the gut ismaltose

• Most of the maltose in the gut is derived from digestion of starch.

• Isomaltose is another disaccharide derived from starch and is digested by isomaltase. (See ‘Digestion of Starch’).


• Disaccharidases are attached to the surface of the small intestinal brush border cells.

• (For ‘digestion of maltose’, see ‘Digestion of Starch’.)

Digestion of DisaccharidesSite of digestion of disaccharides – small intestinal brush border

maltose

sucrose

lactose

trehalose

disaccharidases

maltase

sucrase (also called invertase)

lactase

trehalase

disaccharides

attached to the surface of the small intestinal brush border cells

2 glucose


maltose

sucrose

lactose

trehalose

maltase

sucrase

lactase

trehalase

glucose + galactose

2 glucose

glucose + fructose

Isomaltose isomaltase

2 glucose

Absorption of Monosaccharides

The major monosaccharides resulting from carbohydrate digestion are –

• D-glucose, • D-galactose and • D-fructose. Absorption is carrier mediated.• Pentoses are absorbed by

simple diffusion.• Monosaccharides are first transported

from the lumen to the small intestinal epithelial cells and then into capillaries of portal venous system.

Absorption of Glucose

from the small intestinal lumen into the intestinal epithelial cells by carrier mediated mechanism involving transporter proteins situated on the luminal surface of intestinal epithelial cells.

• Glucose is absorbed mainly by • 1) Na+-dependent transporter

by secondary active transport and to a less extent by

• 2) Na+-independent transporter by passive transport.

Absorption of Glucosefrom the small intestinal lumen

by carrier mediated mechanism involving transporter proteins 1) Na+-dependent transporter

by secondary active transport and to a less extent by

2) Na+-independent transporter by passive transport

into the intestinal epithelial cells

1. Na+-dependent transporter (SGLT)• This carrier protein carries

glucose or galactose along with sodium ion from the lumen. The driving force for the Na+-dependent transport is derived from the maintenance of low intracellular levels of Na+ by the action of the Na+-K+ATPase (secondary active transport).

2) Na+-independent transporter

Small amounts of glucose, transported by facilitated transportutilizing glucose transporter-5 (GLUT-5).

Transport of glucose from cells to portal venous capillaries:

Glucose is transported from the intestinal epithelial cells into portal venous capillaries by glucose transporter-2 (GLUT-2).

• It is a uniport facilitated transport system, which is sodium independent.

Intestinal Epithelial Cell


Glucose

Glucose

Glucose

GLUT-5

Intestinal Lumen Na+

Na+

Na+ K+

K+

Na+-dependent transporter (SGLT)

GLUT-2

Portal Capillary Blood

ATP

Na+–K+ ATPase

ADP + Pi

secondary active transportfacilitated transport


Galactose

GLUT-5

Na+

Na+

Na+ K+

K+

Na+-dependent transporter (SGLT)

GLUT-2

ATP

Na+–K+ ATPase

ADP + Pi

Galactose

Galactose

Fructose

Fructose

Fructose

Absorption of Other Monosaccharides

• Fructose facilitated trasporter GLUT-5, sharing with glucose.• Galactose

Na+-dependant trasporter (SGLT)secodary active transport sharing with glucose

• Any pentose present in food is absorbed by simple diffusion.

Absorption of Other Monosaccharides

• Both fructose and galactose transported from the intestinal epithelial cells into portal venous capillaries by glucose transporter-2 (GLUT-2), sharing with glucose.

Clinical SignificanceLactose Intolerance • This is a common condition

gastrointestinal symptoms like diarrhea, abdominal cramps and flatulenceafter ingestion of milk or milk-based foods

Clinical SignificanceLactose Intolerance • Cause

deficiency of lactase1.Deficiency may be due to

• genetic (primary/inherited) or

• acquired (secondaryto other causes) The reason for acquired lactose intolerance may be damage to intestinal epithelial cells due to colitis, gastroenteritis, alcohol consumption or sudden change into a milk-based diet.

Clinical SignificanceLactose Intolerance • Cause

lactase enzyme

defective at

birth

early onset lactose intolerance

(inherited lactase deficiency).

A significant number of adults

exhibit

late onset lactase deficiency

(primary low lactase activity)

especially

Asian-, Native- and African-Americans.

Osmotic movement of water from the intestines to the lumen

Biochemical basis of Clinical Manifestation

Lactase deficiency

Accumulation of lactose,

(Produced by action of bacteria on lactose in the gut)

organic acids and gases (CO2

and H2)

diarrheaAbdominal cramps

flatulence

Lactose Intolerance

Treatment • avoiding milk• Curd

is an effective treatment, because Lactobacilli in curd contains the enzyme Lactase

Digestion and Absorption of Proteins

Contents:

• Digestion of protein – general aspects• Reactions of protein digestion• Absorption of amino acids• Clinical significance

Digestion of Proteins – General Aspects

Contents:• Introduction• Sites of protein digestion, GIT juices and proteases• Endopetidases and exopetidases• Specificity of proteases• Proteases - zymogen form and activation• Role of HCl in protein digestion


Introduction

• Digestion of dietary proteins –hydrolysis of peptide bonds catalyzed by a group of enzymes called proteases or peptidasesin the gastrointestinal tract

• • Complete digestion of proteins yields

amino acids.


Introduction

• Dietary proteins denatured on cooking and therefore, cooked proteins more easily digested1. During the process of denaturation unfolding of protein molecule takes place and thus peptide bonds become more accessible for enzyme action.


Sites of Protein Digestion, GIT Juices and Proteases

Protein digestion takes place in • stomach and • intestinal lumen.Enzymes of protein digestion are secreted in• gastric juice, • pancreatic juice and • intestinal juice.


GIT Juices and Proteases

GIT Juices

Gastric juice

Pancreatic Juice

Intestinal Juice

Proteases Present

Pepsin (chief cells of stomach), Rennin2

Trypsin Chymotrypsin Elastase

Carboxypeptidases

Aminopeptidases Dipeptidases

Tripeptidases

Rennin

• Rennin, a protease, active in infants and involved in curdling of milk. Rennin denatures casein of milk to paracasein irreversibly, which then is acted upon by pepsin.

Endopeptidases and Exopeptidases

hydrolyze terminal bond releasing one amino acid at a time.

carboxy terminal

amino terminal

Pepsin

Trypsin

Chymotrypsin

Elastase

Carboxypeptidases

Aminopeptidases

hydrolyze peptide bonds in the interior of the protein chain to

cleave the protein molecule into more than one smaller polypeptides and peptides.


Specificity of Proteases• Endopeptidases hydrolyze specific peptide bonds in protein molecules. Specificity differs from one protease to another3.

Enzyme Hydrolysis of petide bonds formed by carboxyl groups of

Pepsin Phe, Tyr, Trp, Met

Trypsin Arg, Lys (basic amino acids)

Chymotrypsin Phe, Tyr, Trp, Val, Leu (Aromatic, uncharged amino acids)

Elastase Ala, Gly, Ser (small amino acid residues)

Specificity of Proteases


Proteases – Zymogen Form and Its Activation

Proteolytic enzymes are secreted as inactive zymogens/proenzymes, which are converted to their active form in the intestinal lumen4.



Activation of enzymesinvolves cleavage of small peptides so that active sites are exposed. prevents auto-digestion of the secretory acini.



Active Enzyme Inactive Zymogen

Pepsin

Trypsinogen

Pepsinogen

Trypsin

Chymotrypsin Chymotrypsinogen

Elastase Proelastase

Carboxypeptidase Procarboxypeptidase



Pepsinogen Pepsin

(Parietal cells of stomach) HCl

Autoactivation

TrypsinTrypsinogen Enterokinase

Autoactivation

(a protease present on the intestinal mucosal membranes)



Chymotrypsinogen Chymotrypsin

Proelastase Elastase

Procarboxypeptidase Carboxypeptidase

Trypsin

1) Activation of pepsinogen 2) Denaturation of dietary proteins and 3) Providing optimum pH (pH 2 to 3) for the

action of pepsin. • (HCl also kills microorganisms present in food).


Role of HCl in Protein Digestion

Reactions of Protein Digestion(In Stomach and Small Intestinal Lumen)

Dietary Proteins

In Stomach PepsinHCl

Gastric Juice

Proteoses + Peptones (polypeptides and smaller polypeptides)

In Small Intestinal Lumen

Trypsin

Chymotrypsin

Elastase

Pancreatic Juice

Small Polypeptides + Peptides

Dietary Proteins

In Stomach PepsinHCl

Gastric Juice



Trypsin

Chymotrypsin

Elastase

Pancreatic Juice


HCl



Trypsin

Chymotrypsin

Elastase

Pancreatic Juice


Carboxypeptidases Pancreatic Juice

Aminopeptidases

Dipeptidases Intestinal Juice

Amino acids


Absorption of Amino Acidsabsorbed from the intestine into portal blood.• transported by a number of carriers many by secondary active transport –Na+-dependent carrierssimilar to glucose transporter system.

Absorption of Amino AcidsDifferent Na+-dependent carriers are:• Neutral amino acid carrier• Phenylalanine and methionine carrier• Carrier specific for imino acids (proline and

hydroxy proline)There are also Na+-independent carriers specializing in the transport of• Neutral and lipophilic amino acids (e.g. Phe,

Leu)• Cationic amino acids (e.g. Lys)

Clinical Significance

• Allergy to certain food proteins(milk, fish) believed to result from absorption of partially digested proteins.


• Defect in non-tropical sprue is located within the mucosal cells of the intestine and permits the polypeptides (resulting from the peptic and tryptic digestion of gluten, the principal protein of wheat) to be absorbed into the circulation and thus elicit the production of antibodies.


• Defect in the intestinal amino acid transport systems seen in -- Hartnup’s disease, [defect in intestinal neutral amino acid (Trp) carrier], I-- iminoglycinuria, cystinuria etc.


• Acute pancreatitis (acute inflammation of pancreas) caused by autodigestion of pancreas by its proteolytic enzymesa life threatening disorder. Autodigestion is due to unusual conversion of proenzymes into active enzymes by trypsin.

Digestion of Fat (Triacylglycerols)

Contents• Digestion of fat• Digestion of other lipids• Absorption of lipids• Clinical significance

Digestion of Fat (Triacylglycerols)main site Small intestinal lumen

Hydrolysis of ester bonds Fatty acids

Glycerol

Monoacyl glycerols (MAGs)

Lipases

- Pancreatic lipasemain enzyme for digestion of most of the fat

Colipase

a protein secreted by pancreas

cofactor

other lipases

lingual lipase, gastric lipase and intestinal lipase

their contribution is negligible

Bile salts help fat digestion by emulsifying fat

Fat

Role of Bile Salts in Fat Digestion

Bile salts

lower the surface tension

– emulsify fat in the intestine.

– Intestinal peristalsis also helps in this.

present in the bile

Emulsification increases the surface area of

fat droplets

enabling more enzyme (lipase) molecules to

act

and thus

speeding up digestion

Reactions of Digestion of Fat (Triacylglycerol)1

Colipase Pancreatic Juice

Fatty Acid

Triacylglycerol (Fat)

Diacylglycerol (DAG)

Lipase

Colipase

Lipase

Fatty Acid

Monoacylglycerol (MAG)

Glycerol

Colipase

Lipase

Fatty Acid


Digestion of fat

requires another enzyme also

an isomerase

which isomerzes 2-MAG into 1-MAG,

as lipase cannot hydrolyze 2-MAG.

The major end products of digestion of fat are –

• monoacylglycerols (MAGs), • glycerol and • fatty acids


Digestion of Other Lipids

Pancreatic secretion also contains

cholesterol esterase

phospholipase A2

cholesterol ester cholesterol

Fatty acid

lysophospholipid phospholipid

Fatty acid

Water-insolubleDigested Products of Lipids

Water-soluble

Absorption of Digested Products of Lipids

MonoacylglycerolsLong-chain fatty acids Cholesterol

MicellesFat-soluble vitamins

Mixed micelles

MonoacylglycerolsLong-chain fatty acids

CholesterolFat-soluble

vitaminsPhospholipids

Apolipoproteins

Triacylglycerol

Chylomicron

ChylomicronLYMPHATIC VESSELS Glycerol(SMCFA)

Short- and Medium-chain fatty acids(SMCFA)

SMALL INTESTINAL LUMEN

PORTAL BLOOD

Glycerol

Intestinal Epithelial Cell

Absorption of Digested Products of Lipids

• Normally over 98% of the dietary lipid is absorbed.

Clinical Significance• Steatorrhea• Chyluria and Chylothorax

When daily excretion of fat in feces more than 6g per day

• may be due to defective digestion or defective absorption of fat.

• - Defective digestion may be due to absence of or deficiency of pancreatic lipase as in chronic diseases of pancreas or surgical removal of pancreas. bile salt

•

Clinical Significance• Steatorrhea• Defective absorption of fat occurs

when bile salts do not enter the intestine as in biliary obstruction (e.g. due to biliary stone)

• - Defective absorption may also be due to malabsorptive diseases e.g. celiac disease, Sprue, Crohn’s disease, etc or surgical removal of large lengths of the intestine.

MCQS on Digestion and Absorption

1. All of the below are true about digestion, EXCEPT,

A. All digestive enzymes are hydrolases. B. Digestion ensures the absorption of

nutrients. C. Sites of digestion in the GIT are mouth,

stomach, and the lumen of small and the large intestines.

D. Vitamins, minerals, monosaccharides and free amino acids do not need digestion.

2. The anhydride linkages that are broken during digestion are:

A.glycosidic, peptide and ester linkages of carbohydrates, proteins and fats, respectively.

B.ester, peptide and glycosidic linkages of carbohydrates, proteins and fats, respectively.

C.glycosidic, peptide and ester linkages of carbohydrates, fats and proteins, respectively.

D.peptide, glycosidic and ester linkages of carbohydrates, proteins and fats, respectively.

3. Factors, other than enzymes that help in digestion are the following, EXCEPT,

A.Cooking B.Exercise C.Mastication D.Peristalsis

4. All the following are true about absorption, EXCEPT,

A.Considerably more water is absorbed in the large intestine than in the small intestines.

B.Absorption of all substances require carrier-mediated transports.

C.Water-soluble nutrients are absorbed through hepatic portal system.

D.Lipid-soluble nutrients are absorbed through the lymphatic vessels.

5. The full set of enzymes required for complete digestion of starch are –

A.amylase, maltase and isomaltase. B.amylase, maltase and sucrase. C.amylase, maltase, isomaltase and sucrase. D.amylase, sucrase and isomaltase.•

6. Contribution by salivary amylase in digestion of starch is very little because:

A.activity of the enzyme is very low.B.the food remains in the mouth for a very short

period of time. C.the food remains in the mouth for a very short

period of time and the enzyme is inactivated by gastric HCl.

D.the enzyme is inactivated by gastric HCl.

7. Which of the following is FALSE about digestion of starch?

A.The enzymes and the steps of digestion of glycogen are same as those of amylopectin component of starch.

B.Cooking hydrates the starch granules making it more susceptible to digestion.

C.Cl- is an activator of salivary amylase. D.Limit dextrins are derived from amylose

component of starch by the action of amylase.

8. Quantitatively the major disaccharide digested in the gut is

A.sucrose B.maltose C.lactose D.isomaltose

9. The major monosaccharides resulting from carbohydrate digestion are

A.glucose, galactose and ribose. B.glucose, mannose and fructose. C.glucose, ribose and fructose. D.glucose, galactose and fructose.

10. Glucose is absorbed mainly by

A.simple diffusion B.Na+-dependent transporter C.Na+-independent transporter D.passive transport•

11. Enzymes of protein digestion are secreted in

A.gastric juice, pancreatic juice and intestinal juice.

B.saliva, gastric juice and pancreatic juice. C.saliva, gastric juice and intestinal juice. D.saliva, gastric juice and intestinal juice.

12. Proteases secreted in the pancreatic juice are

A.pepsin, trypsin, chymotrypsin and carboxypeptidases.

B.trypsin, chymotrypsin, elastase and carboxypeptidases.

C.trypsin, chymotrypsin, elastase and aminopeptidases.

D.trypsin, chymotrypsin, aminopeptidases and carboxypeptidases.

13. HCl has following roles in protein digestion, EXCEPT.

A.kills microorganisms present in food B.activation of pepsinogen C.denaturation of food proteins D.providing optimum pH for the action of

pepsin

14. Intestinal absorption of amino acids involves all of the following, EXCEPT,

A.simple diffusion B.Na+-dependent transporter C.Na+-independent transporter D.passive transport

15. Which of the following is false about action of bile salts?

A.They help both in digestion of fat and absorption of digested products of lipids.

B.They lower the surface tension and emulsify fat in the intestine.

C.They stimulate pancreatic secretion.D.Emulsification increases the surface area

of the fat droplets.

16. Digestion of fat produces:

A.fatty acids, glycerol and diacyl glycerols.

B.fatty acids and glycerol.C.glycerol and monoacyl glycerols.D.fatty acids, glycerol and monoacyl

glycerols.

17. Lipases other than pancreatic lipase present in the human GIT are:

A.salivary lipase, gastric lipase and intestinal lipase

B.lingual lipase, gastric lipase and intestinal

lipase C.lingual lipase, gastric lipase and hepatic lipase D.gastric lipase, biliary lipase and intestinal

lipase

18. Steatorrhea is a condition when daily excretion of fat in feces is more than:

A.6g B.8g C.10g D.12g

Digestion and absorption of food

Health & Medicine

Transcript of Digestion and absorption of food