1. FHB-(Digestion-Absorption-Metabolism) Sandwich Conference 2019
Drs. Choudhry, Mignery and Simmons

2. Peanut Butter and Jelly Sandwich
= Carbohydrate + Fat + Protein

4. The Fate of 60 g of Carbohydrates
Carbohydrates constitute ~50% of typical American diet.
Carbohydrates ingested are polysaccharides, disaccharides and very little amount of monosaccharides.
Only monosaccharides are absorbed through the intestinal epithelial cells.
Thus all ingested carbohydrates must be digested to monosaccharides to be absorbed through the intestine.

5. The Fate of 60 g of Carbohydrates
Major dietary carbohydrate is starch – Polysaccharide - is a mixture of both straight and branched-chain polymers of glucose.
Amylose – are the straight chain polymers.
Amylopectin - are the branched-chain polymers.
The disaccharides in food are – trehalose, sucrose and lactose.
Trehalose – consists of two molecules of glucose.
Sucrose – consists of glucose and fructose.
Lactose – consists of glucose and galactose.
Monosaccharides are – glucose, fructose and galactose.
Cellulose (b1,4-linkage) no enzymes available to hydrolyze this linkage – cellulose is excreted.

6. The Fate of 60 g of Carbohydrates
Digestion of starch begins with α amylase.
Amylase hydrolyzes simple and complex oligosaccharides (starch) into smaller sugar molecules
Mouth: Saliva contains amylase.
Facial (VII) and Glossopharyngeal (IX) cranial nerves and to a minor extent sympathetic innervation – stimulates salivary secretion.
Salivary amylase starts the process , but plays an insignificant role in a healthy individual, since amylases are inactivated by low gastric pH.

7. The Fate of 60 g of Carbohydrates
Small Intestinal Lumen:
2. Pancreatic amylase (most significant) - digests internal α-1,4-bonds in starch – yielding
α-limit dextrins, maltose and maltotriose.
3. Intestinal brush border enzymes – digest these to monosaccharides
α-dextrinase (isomaltase), maltase and sucrase.
The product of each of these is glucose (monosaccharide) – absorbed by the epithelial cells.

8. The Fate of 60 g of Carbohydrates

9. The Fate of 60 g of Carbohydrates
SGLT1 transports glucose and galactose by coupling their transport to that of Na+ (active process).
Glucose and galactose - extruded across basolateral membrane by facilitated diffusion by GLUT2.
Fructose - transported across apical & basolateral membranes by facilitated diffusion
GLUT5 (apical) - fructose specific
GLUT2 (basolateral).
Absorption

11. The Fate of 23 g of Fat
Major categories of lipid in the diet:
Triglycerides (major pool) – most of these have long-chain fatty acids (> 12 carbons) esterified to glycerol backbone.
Phospholipids
Cholesterol
Vitamins A, D, E and K (not lipids but fat soluble)
These are insoluble in water – requires special processing to be digested and absorbed in the GI tract.

12. The Fate of 23 g of Fat Lipid Digestion
Emulsification and Solubilization of Lipids:
Lipid in a fatty meal floats on the surface of gastric contents.
This limits the area of interface between aqueous and lipid phases – thus access to lipolytic enzymes are restricted (enzymes are soluble in aqueous phase).
Emulsification – the mixing action of the stomach churns the dietary lipids into a suspension of fine droplets – this greatly increases the surface area for digestive enzymes.
In the stomach, lipid droplets are emulsified by dietary proteins.
In the small intestine, the primary emulsifying agents are bile acids.

13. The Fate of 23 g of Fat
1. Lipid digestion starts in the Oral cavity.
Gastric lipase and Lingual lipase – hydrolyzes ~10% of the dietary triglycerides to glycerol and free fatty acids.
2. Most of the lipid digestion occurs in the small intestine.
Pancreatic juice contains three important lipolytic enzymes that can work at neutral pH.
Pancreatic lipase
Phospholipase A2
Cholesterol ester hydrolase

14. The Fate of 23 g of Fat
Phspholipase is released in an inactive form and is activated by trypsin like other protein enzymes

15. The Fate of 23 g of Fat
Lipid Absorption
Apo B, β-Lipoprotein; Chol, cholesterol; Chol E, cholesterol ester; FFA, free fatty acids; LysoPL, lysolecithin; MG, monoglycerides; PL, phospholipids; TG, triglycerid

17. The Fate of 12 g of Protein
1. Protein digestion starts in the stomach with the action of pepsin.
2. It is completed in the small intestine with pancreatic and brush-border proteases.
There are two classes of proteases:
Endopeptidases
Exopeptidases
Endopeptidases – hydrolyze the interior peptide bonds of proteins- Example – pepsin, trypsin, chymotrypsin, elastase.
Exopeptidases – hydrolyze one amino acid at a time of proteins and peptides from either
the C-terminal end (carboxypeptidase) or
the N-terminal end (aminopeptidase)

18. The Fate of 12 g of Protein

19. The Fate of 12 g of Protein

20. The Fate of 12 g of Protein
Pancreatic proteases digest dietary protein into
amino acids
di-, tri-peptides and
larger peptides called oligo-peptides.
Only amino acids, di- and tri-peptides are absorbable.
Oligo-peptides are not absorbable.
Oligo-peptides are further hydrolyzed by brush-border proteases – to smaller absorbable molecules.

21. The Fate of 12 g of Protein
Wide number of amino acid transporters (neutral, acidic, basic and imino amino acids).
Some of those transporters (not all) symport amino acids and Na+.
Peptide transporters (example PepT1) work in a symport mode with H+.
Peptide transporters uptake some drugs (clinically relevant).
Inside the cell, most of the peptides are hydrolyzed to amino acids by cytosolic peptidases.
Amino acids exit the cell by facilitated diffusion.