Case 1: GI Tract
vrijdag 20 september 2024 10:59
1. GI Anatomy, histology and physiology
Mouth - chewing to enhance surface area
What is digested?
- carbohydrates
How is this digested?
We have salivary glands
- Parotid glands
- Submandibular glands
- Sublingual glands
And these produce alpha-amylase and anti-microbial proteins
Alpha-amylase is an endopeptidase that breaks down alpha-1,4 and alpha-1,6 linkages in
polysaccharide chains into
- Maltrotriose
- Dextrins
- Maltose
Cells in a salivary glands
- Acinar cells: Ach binds to the M3-receptor/ NE binds to the beta-adenergic receptor
which will lead to an calcium influx and activate NKCC1, NHE1 and AE2 transporters
starting the production of saliva
- Ductal cells: the cells will eventually become hypotonic and the saliva solution is
secreted by the glands
Stomach - mixing of food with enzymes
What is digested?
- Carbohydrates
- Proteins
- Fats
How is this digested?
In the wall of the stomach different cells are present:
- Epithelial cells: produce bicarbonate
- Neck cells: produce mucus
○ H+ in the stomach interact with bicarbonate in the mucus layer to form H2O and
CO2
- G cells: produce gastrin which stimulates the parietal cells to secrete HCl and the chief
cells to secrete pepsinogen
- Parietal cells: release hydrochloric acid (H+ Cl-) which activates intrinsic factor (IF)
(important for Vitamin B12 absorption) HCl makes the stomach acidic, this is needed to:
○ Kill bacteria
○ Denature proteins
○ Stimulates pepsinogen to pepsin
- Chief cells: pepsinogen is activated into pepsin in the presence of HCl (pH 1.8-3.5).
Pepsin is responsible for the breakdown of proteins.
- Enteroendocrine cells: release regulatory hormones, serotonin and histamine that in the
presence of HCl and allow muscle contraction
Small intestine - Digestion and Absorption
What is digested?
- Carbohydrates -> brush border enzymes present here
- Proteins
- Fats
How is this digested?
The small intestine has a large surface with villi and the villi have microvilli (glucocalyx).
Different cells are present:
In the villus:
- Enterocyte
- Tuft cells: inflammatory cells
- Enteroendocrine cells (EEC): secrete peptide hormones
- Goblet cells: mucus producing cells
3013 Diet health and nutrition Pagina 1
, - Enterocyte
- Tuft cells: inflammatory cells
- Enteroendocrine cells (EEC): secrete peptide hormones
- Goblet cells: mucus producing cells
In the crypt:
- Crypt based columnar cells (CBC): stem cells
- Paneth cells: involved in the release of WNT and NOTCH signalling
Colon - water absorption, vitamin synthesis
The colon consists of different parts:
- Cecum
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
NO villi present
Two important mechanisms in the colon
- Peristalsis
- Segmentation
2. Digestion and absorption of macronutrients
Digestion of carbohydrates
Carbohydrates are consumed as starch or disaccharides and starts at the oral cavity with
salivary amylase. Then it travels down the oesophagus to the stomach. In the stomach salivary
amylase loses its function due to low ph. Then in the small intestine pancreatic juice contains
pancreatic amylase to further breakdown the carbohydrates to oligosaccharides and
disaccharides, like:
- Maltose: 2x glucose (enzyme: maltase)
- Sucrose: 1x glucose and 1x fructose (enzyme: sucrase)
- Lactose: 1x glucose and 1x galactose (enzyme: lactase)
But only monosaccharides can be absorbed so the disaccharides are broken down into
monosaccharides: glucose, fructose and galactose.
- Glucose and galactose: absorbed via cotransport with sodium ions (Na+) in the SGLT
symporter into the lumen. Then they enter the blood via GLUT2 transporter. (both
active)
- Fructose: is sodium independent and enters the cell via the GLUT5 transporter by
diffusion. Then it enters the bloodstream also via the GLUT2 transporter.
Digestion of proteins
The digestion starts in the stomach. The protein is folded and this stimulates G cells to secrete
gastrin. Gastrin will stimulate the parietal cells to secrete HCl And the chief cells to secrete
pepsinogen. These are endopeptidases (protease) meaning they attack the peptide bonds to
break down larger polypeptide chains into smaller ones, but still quite long. HCl creates an
acidic environment pH 1-2 that:
1. Kills bacteria
2. Denatures proteins (unfolds)
3. Stimulates the transition of pepsinogen to pepsin
Now the smaller chains enter the small intestine. Here the release of secretin and
cholecystokinin CCK is stimulated and HCO3- (bicarbonate) is secreted to counter the effects
of HCl. The pH is now 5-6. CCK allows the release of zymogens (enzymes) that are
exopeptidases that release single amino acids from the peptide chain (most important are
carboxypeptidases (remove COO-) and aminopeptidases (remove NH2)). Elastase is
responsible for the chemical breakdown if elastin and trypsin and chymotrypsin further break
down the polypeptide.
This all causes the breakdown to amino acids and some di- or tripeptides. Which can now be
absorbed.
3013 Diet health and nutrition Pagina 2
,This all causes the breakdown to amino acids and some di- or tripeptides. Which can now be
absorbed.
There are 3 ways proteins can enter which are mainly secondary active meaning they are not
directly dependent on metabolic activity, but their co-transport molecules are.
1. Co-transport in PEPT1 with H+ for di- and tri-peptide → apical transport
2. Co-transport with Na+ for single amino acids → apical and basolateral transport
3. Endocytosis for longer peptides, they are transported intact in a vesicle
- In enterocyte - 10% remains intact
- In M-cell - 50% remains intact
Digestion of lipids
In the oral cavity, lingual lipase is added which starts the emulsification process
of the fats with water. Then it moves down to the stomach where gastric lipase
is added, which targets the breakdown of triglycerides. Then in the small
intestine bile is secreted (+lecithin) and the main digestion can occur. Fats are
hydrophobic and lipophilic. When bile salts are added, which are hydrophobic
on one side and hydrophilic on the other it starts to form a coat around it. Fat +
bile salts = micelle. The pancreatic lipase starts to break down the triglycerides
into monoglycerides and fatty acids, without sticking to other fats due to the coat
surrounding it. The micelle now contains fatty acids and monoglycerides and
can be absorbed.
In the micelles are: bile salts, monoglycerides (MAG), fatty acids, phospholipids
and cholesterol (due to low pH).
Absorption:
1. Non-ionic diffusion to form chylomicrons
2. Incorporated into the plasma membrane (collision)
3. Transport through carrier-mediated transport, for this the uptake FAT/CD36
etc. are the main transporters
Where do they go?
3. Digestion and absorption of micronutrients
Vitamins
- Water-soluble vitamins -> Vitamin B and Vitamin C
- Absorbed with water, excreted via urine
1) Vitamin B12 (antioxidants)
2) Vitamin C (antioxidants)
- Fat-soluble vitamins -> Vitamin A, Vitamin D, Vitamin E and Vitamin K
3013 Diet health and nutrition Pagina 3
, - Fat-soluble vitamins -> Vitamin A, Vitamin D, Vitamin E and Vitamin K
- Incorporate into micelles and enter the liver via endocytosis
1) Vitamin D (calcium and phosphor maintainance in blood and bone)
Minerals
Macrominerals
- Calcium
- Phosphor
- Potassium
- Sulphur
- Sodium (natrium)
- Chloride
- Magnesium
Trace minerals (examples)
- Iron
- Copper
- Zinc
4. Calcium and Vitamin D
Calcium is for bone strength, and muscle function and is needed for message transduction (E.g.
the production of saliva) and to give strength to structures
3013 Diet health and nutrition Pagina 4
vrijdag 20 september 2024 10:59
1. GI Anatomy, histology and physiology
Mouth - chewing to enhance surface area
What is digested?
- carbohydrates
How is this digested?
We have salivary glands
- Parotid glands
- Submandibular glands
- Sublingual glands
And these produce alpha-amylase and anti-microbial proteins
Alpha-amylase is an endopeptidase that breaks down alpha-1,4 and alpha-1,6 linkages in
polysaccharide chains into
- Maltrotriose
- Dextrins
- Maltose
Cells in a salivary glands
- Acinar cells: Ach binds to the M3-receptor/ NE binds to the beta-adenergic receptor
which will lead to an calcium influx and activate NKCC1, NHE1 and AE2 transporters
starting the production of saliva
- Ductal cells: the cells will eventually become hypotonic and the saliva solution is
secreted by the glands
Stomach - mixing of food with enzymes
What is digested?
- Carbohydrates
- Proteins
- Fats
How is this digested?
In the wall of the stomach different cells are present:
- Epithelial cells: produce bicarbonate
- Neck cells: produce mucus
○ H+ in the stomach interact with bicarbonate in the mucus layer to form H2O and
CO2
- G cells: produce gastrin which stimulates the parietal cells to secrete HCl and the chief
cells to secrete pepsinogen
- Parietal cells: release hydrochloric acid (H+ Cl-) which activates intrinsic factor (IF)
(important for Vitamin B12 absorption) HCl makes the stomach acidic, this is needed to:
○ Kill bacteria
○ Denature proteins
○ Stimulates pepsinogen to pepsin
- Chief cells: pepsinogen is activated into pepsin in the presence of HCl (pH 1.8-3.5).
Pepsin is responsible for the breakdown of proteins.
- Enteroendocrine cells: release regulatory hormones, serotonin and histamine that in the
presence of HCl and allow muscle contraction
Small intestine - Digestion and Absorption
What is digested?
- Carbohydrates -> brush border enzymes present here
- Proteins
- Fats
How is this digested?
The small intestine has a large surface with villi and the villi have microvilli (glucocalyx).
Different cells are present:
In the villus:
- Enterocyte
- Tuft cells: inflammatory cells
- Enteroendocrine cells (EEC): secrete peptide hormones
- Goblet cells: mucus producing cells
3013 Diet health and nutrition Pagina 1
, - Enterocyte
- Tuft cells: inflammatory cells
- Enteroendocrine cells (EEC): secrete peptide hormones
- Goblet cells: mucus producing cells
In the crypt:
- Crypt based columnar cells (CBC): stem cells
- Paneth cells: involved in the release of WNT and NOTCH signalling
Colon - water absorption, vitamin synthesis
The colon consists of different parts:
- Cecum
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
NO villi present
Two important mechanisms in the colon
- Peristalsis
- Segmentation
2. Digestion and absorption of macronutrients
Digestion of carbohydrates
Carbohydrates are consumed as starch or disaccharides and starts at the oral cavity with
salivary amylase. Then it travels down the oesophagus to the stomach. In the stomach salivary
amylase loses its function due to low ph. Then in the small intestine pancreatic juice contains
pancreatic amylase to further breakdown the carbohydrates to oligosaccharides and
disaccharides, like:
- Maltose: 2x glucose (enzyme: maltase)
- Sucrose: 1x glucose and 1x fructose (enzyme: sucrase)
- Lactose: 1x glucose and 1x galactose (enzyme: lactase)
But only monosaccharides can be absorbed so the disaccharides are broken down into
monosaccharides: glucose, fructose and galactose.
- Glucose and galactose: absorbed via cotransport with sodium ions (Na+) in the SGLT
symporter into the lumen. Then they enter the blood via GLUT2 transporter. (both
active)
- Fructose: is sodium independent and enters the cell via the GLUT5 transporter by
diffusion. Then it enters the bloodstream also via the GLUT2 transporter.
Digestion of proteins
The digestion starts in the stomach. The protein is folded and this stimulates G cells to secrete
gastrin. Gastrin will stimulate the parietal cells to secrete HCl And the chief cells to secrete
pepsinogen. These are endopeptidases (protease) meaning they attack the peptide bonds to
break down larger polypeptide chains into smaller ones, but still quite long. HCl creates an
acidic environment pH 1-2 that:
1. Kills bacteria
2. Denatures proteins (unfolds)
3. Stimulates the transition of pepsinogen to pepsin
Now the smaller chains enter the small intestine. Here the release of secretin and
cholecystokinin CCK is stimulated and HCO3- (bicarbonate) is secreted to counter the effects
of HCl. The pH is now 5-6. CCK allows the release of zymogens (enzymes) that are
exopeptidases that release single amino acids from the peptide chain (most important are
carboxypeptidases (remove COO-) and aminopeptidases (remove NH2)). Elastase is
responsible for the chemical breakdown if elastin and trypsin and chymotrypsin further break
down the polypeptide.
This all causes the breakdown to amino acids and some di- or tripeptides. Which can now be
absorbed.
3013 Diet health and nutrition Pagina 2
,This all causes the breakdown to amino acids and some di- or tripeptides. Which can now be
absorbed.
There are 3 ways proteins can enter which are mainly secondary active meaning they are not
directly dependent on metabolic activity, but their co-transport molecules are.
1. Co-transport in PEPT1 with H+ for di- and tri-peptide → apical transport
2. Co-transport with Na+ for single amino acids → apical and basolateral transport
3. Endocytosis for longer peptides, they are transported intact in a vesicle
- In enterocyte - 10% remains intact
- In M-cell - 50% remains intact
Digestion of lipids
In the oral cavity, lingual lipase is added which starts the emulsification process
of the fats with water. Then it moves down to the stomach where gastric lipase
is added, which targets the breakdown of triglycerides. Then in the small
intestine bile is secreted (+lecithin) and the main digestion can occur. Fats are
hydrophobic and lipophilic. When bile salts are added, which are hydrophobic
on one side and hydrophilic on the other it starts to form a coat around it. Fat +
bile salts = micelle. The pancreatic lipase starts to break down the triglycerides
into monoglycerides and fatty acids, without sticking to other fats due to the coat
surrounding it. The micelle now contains fatty acids and monoglycerides and
can be absorbed.
In the micelles are: bile salts, monoglycerides (MAG), fatty acids, phospholipids
and cholesterol (due to low pH).
Absorption:
1. Non-ionic diffusion to form chylomicrons
2. Incorporated into the plasma membrane (collision)
3. Transport through carrier-mediated transport, for this the uptake FAT/CD36
etc. are the main transporters
Where do they go?
3. Digestion and absorption of micronutrients
Vitamins
- Water-soluble vitamins -> Vitamin B and Vitamin C
- Absorbed with water, excreted via urine
1) Vitamin B12 (antioxidants)
2) Vitamin C (antioxidants)
- Fat-soluble vitamins -> Vitamin A, Vitamin D, Vitamin E and Vitamin K
3013 Diet health and nutrition Pagina 3
, - Fat-soluble vitamins -> Vitamin A, Vitamin D, Vitamin E and Vitamin K
- Incorporate into micelles and enter the liver via endocytosis
1) Vitamin D (calcium and phosphor maintainance in blood and bone)
Minerals
Macrominerals
- Calcium
- Phosphor
- Potassium
- Sulphur
- Sodium (natrium)
- Chloride
- Magnesium
Trace minerals (examples)
- Iron
- Copper
- Zinc
4. Calcium and Vitamin D
Calcium is for bone strength, and muscle function and is needed for message transduction (E.g.
the production of saliva) and to give strength to structures
3013 Diet health and nutrition Pagina 4
