10. Carb Digestion

Question 1

Carbs

Answer 1

-main source of carbohydrate in world population: Half of the world’s population live on a staple diet of rice
Carbs provide ~50% of calories in most diets
Glucose major carbohydrate
Main source of Glucose in world population is starch: Rice, Pasta etc.

Table sugar=fructose/levulose- sweetness value 173
Vs sucrose- sweetness value 100
So companies use fructose to sweeten food bc of higher sweetness value

Question 2

Mouth

Answer 2

saliva-salivary amylase:
Starch and glycogen; disaccharides not digested

Mucus: lubricates
Bicarbonate buffers to keep alkaline (pH)
Optimum for salivary alpha-amylase (glycogen, starch have alpha 1,4 linkages)

Question 3

Stomach: Peristalsis

Answer 3

No carbohydrate digestion
Drop in pH due to stomach HCl
-stops salivary amylase activity
Resulting acidic mixture: chyme

disaccharides not digested

Question 4

Intestinal Lumen

Answer 4

(acidic) Chyme from stomach is neutralized by bicarbonate secreted by pancreas
Pancreatic amylase: starch
From pancreatic juice
Disaccharidases (eg lactose, sucrose):
Membrane bound
Brush border of mucosal cells
Monosaccharides are released to portal system->liver

Question 5

Disaccharide Digestion:

Answer 5

Apical surface (faces lumen of the enterocytes) have enzymes that hydrolyze disaccharides 
cell membrane-bound: Transmembrane proteins of brush border on luminal surface of mucosal lining of upper jejunum monosaccharides

Disaccharidases:
Maltase: bd 14 glucose: maltose
Lactase (b-galactosidase)
b14 in lactose glucose and galactose (only beta linkage broken eg don’t break beta linkages btwn glc-glc in cellulose (fibers))
Sucrase:
12glucose and fructose (sucrose)
Isomaltase: 16 in isomaltose (Single protein w 2 diff activities)

Question 6

Amylases: Glycogen and Starch

Answer 6

Endoglucosidase-
Hydrolyze internal a 14 linkages at random intervals (break in middle, not end)
mixture of short, branched a(16) and unbranched oligosaccharides (mixed limited-dextrins)
Do not hydrolyze disaccharides
Do not hydrolyze a 16 (only isomaltase breaks 1,6)
Do not hydrolyze cellulose b 14

Question 7

Oral and Pancreatic Amylases

Answer 7

2 Types of Amylases

Salivary
Salivary glands secrete ~1L/day into mouth
Mixes with food during mastication

Pancreatic
Pancreas: Pancreatic juice (~1.5L/day)
Secreted into the duodenum via the pancreatic duct after chyme (from stomach) has been neutralized by bicarbonate
Alpha 14 linkages
Alpha-limited dextrins (have branches)

Why are 2 amylases necessary in the body to degrade starches and glycogen?

Salivary amylase begins digestion in the mouth but is inactivated when it reaches the stomach by HCl
Pancreatic amylase is secreted into intestines with neutralizing bicarbonate so it can further degrade starches and glycogen

Question 8

Absorption and Transport of Glucose

Answer 8

Absorption Intestines and Kidney: Na+ symport
Sodium Glucose Transporters: SGLT-1 and SGLT-2
Transport in/out of tissues to blood: e.g. Liver
GLUT Transporters

Absorption of Monosaccharides from Intestines

All digestible carbohydrates are absorbed before material enters large intestines

(SI) Duodenum and upper jejunum absorb most of the monosaccharides transported to the portal system (blood)liver

Different sugars have different mechanisms for absorption

Question 9

Intestinal Glucose Absorption

Answer 9

Disaccharidases on lumen

Sodium-dependent glucose/galactose cotransporter (SGLT1) on apical surface of membrane-

Flows with [Na+] and against [Glucose] (glc higher inside cell than outside while Na higher outside than inside)

Na/K ATPase: maintains Na+ gradient on basolateral side of membrane: 1 (primary) Active transport (use ATP for pumping) (ATP req 3 Na out, 2 K in) (reason why Na higher outside than inside bc pumping Na against conc gradient)

secondary active transport (based on conc gradient): SGLT-1 (doesn’t directly use ATP; primary active transport allows for secondary AT to happen

Question 10

Renal Glucose Reabsorption:SGLT-2, SGLT-1

Answer 10

Kidney: SGLT-2, SGLT-1: Glucose reabsorption

SGLT-2:
In Proximal Convoluted Tubule (PCT)
Reabsorbs ~90% of filtered glucose

SGLT-1:
In Proximal Straight Tubule (PST)
Reabsorbs ~10% of filtered glucose

2ndary active transport: Na/K ATPase
Healthy pt- kidney reabsorbs all of ~180g/d glc

Question 11

SGLT-2 Inhibitors:treatment of hyperglycemia

Answer 11

Reduce renal tubular glucose reabsorption reduction of blood glucose without stimulating insulin release

SGLT-2 inhibitors:
effective treatment of type 2 diabetes (most common form)
weight loss
reduction of blood pressure
increase in high-density lipoproteins(HDL)
Diabetes, Obesity and Metabolism 16: 457–466, 2014.

A reasonable choice for second-line or third-line diabetes therapy
Canagliflozin, Dapagliflozin
Diabetes CareMarch 2015vol. 38no. 3352-354

Question 12

GLUT Transporters

Answer 12

Proteins that transport glucose and other sugars
Glu cannot cross the cell plasma membrane without GLU transporters
Membrane bound
Facilitated transport with the concentration gradient (nonactive)
Change conformation upon substrate binding

Question 13

Monosaccharide Intestinal Absorption/Transport

Answer 13

Facilitative fructose transporter (GLUT5) in the brush border membrane lining the lumen (intestinal lumen -> epithelial cell).
Energy and substrate-independent

GLUT2 in the basolateral membrane ensures sugar exit into the blood stream when blood glucose levels are lower.
Flows with [Glucose] W CONC GRADIENT
Glu, Gal and Fructose are transported into portal circulation by GLUT2  Liver.

Question 14

rev

Answer 14

Why is intestinal transport and renal reabsorption of Glucose a secondary ACTIVE transport?

SGLT-1 and SGLT-2 in intestines and kidney (on apical surface membrane) use a Na+ to transport Glucose AGAINST its concentration gradient and with the Na+ gradient.
This is an example of 20 active transport
The Na+ gradient is maintained by a Na/K ATPase which used energy (ATP) located on the basolateral surface of the epithelial membrane, a 10 active transporter.

SGLT-1: Na+ symport of glucose into intestinal cells (kidney): ATP dependent

SGLT-2 : Na+ symport of glucose for renal reabsoprtion: ATP dependent
SGLT-2 inhibitors: reduce glucose absorption in kidney reduce blood glucose

GLUT-1: RBC and Brain; Basal uptake of glucose in tissues
GLUT-2: (energy-independent transporter) Transports glucose into and out of liver and kidney cells, into b-pancreatic cells
GLUT-3: Transports into neurons
GLUT-4: Most abundant in adipose and muscle glucose uptake: insulin sensitive (a uniporter transports glucose with its concentration gradient.)
GLUT-5: Fructose absorption into intestinal cells

Question 15

Normal Carbohydrate Digestion

Answer 15

All digestible carbohydrates is absorbed by the time it reaches lower jejunum before material enters large intestines

Question 16

Abnormal Degradation of Disaccharides

Answer 16

Incomplete digestion of carbohydrates–>
osmotically active carbon compounds enter large intenstine
Water is drawn from the mucosa into large intestine osmotic diarrhea

Bacterial fermentationosmotically active compounds, CO2, H2(can be measured), lactic acid (acidity) diarrhea, cramps and flatulence

Can measure acidity of stools in infants

Question 17

Digestive Enzyme Deficiencies

Answer 17

Hereditary
Lactase deficiency (b-galactosidase): most common
digestion of lactose (dairy products)

Intestinal Diseases
Colitis: inflammation of the colon
Gastroenteritis: inflammation of the gastrointestinal tract (stomach and duodenum)
Cystic Fibrosis (more of problem w pancreatic ducts): lactase activity can be lower in CF mucosa

Malnutrition
Kwashiorkor (protein malnutrition) (common in third world countries- rice=main staple, too little protein)
Cannot produce enough digestive enzymes

Drugs
Augmentin, Clindamycin:destroy intestinal flora which help digest fibers and undigested Carbohydrates that the body failed to digest and absorb.

Acquired
severe diarrhea  loss of enzymes

Question 18

Lactose Intolerance

Answer 18

Lactase deficiency: Primary
Presents with diarrhea, bloating, cramps
H2O comes in, bacteria digest
Can increase with age
More than three quarters of the world’s adults are lactose intolerant
Can vary within different ethnic populations
90% of adults of Asian or African descent
Up to 75% of the world population has some lactose intolerance
25% of Northern European descent (e.g. Dutch, Scandinavian)

Lactase deficiency: Secondary
Due to intestinal injury
First activity lost and last to recover

Why does Lactose intolerance present with diarrhea?

Normally all carbohydrates should be digested by the time they reach the large intestines
In lactose intoleranceincomplete digestion of lactoseosmotically active carbon compounds
Water is drawn from the mucosa into large intestine osmotic diarrhea