CHO: digestion and absorption

how much of our typical diet comes from CHO in %

45-80% of energy

1,000kcals or 250g

what are the 2 main categories of CHO?

simple and complex

what are simple carbs

monosaccharides and disaccharides

what are complex carbs

polysaccharides including digestible, partially, and non-digestible

according to dietary guidelines, how much added sugar should be in your diet

<10% (200kcals, 12tsp for

OVERVIEW OF CHO DIGESTION

in what forms does our body intake CHO (types of carbs)

Starch, lactose, sucrose, nonstarch polysaccharides, and oligosaccharides

where does digestion begin

in the mouth

in our mouth, we begin to break down what

starch

starch is broken down into what?

starch dextrins and amylose

what enzyme in the mouth breaks down starch?

salivary a-amylase

in the stomach, what takes place with CHO

no digestion

why is there no digestion of CHO in stomach?

Low pH of stomach will limit the action of salivary alpha-amylase

where does the most digestion and absorption of CHO occur

small intestines

in the SI, starch is broken down into what?

 starch dextrins, maltotriose, and maltose

what enzyme breaks down starch into  starch dextrins, maltotriose, and maltose in the SI

pancreatic a-amylase

Starch dextrins, maltotriose, and maltose get broken down into what

glucose

how do Starch dextrins, maltotriose, and maltose get broken down into glucose (by what)

 sucrase-isomaltase; maltase-glucoamylase

complexes

sucrase-isomaltase and maltase-glucoamylase complexes include what which is of most importance

isomaltase and maltase

sucrose in broken down into what? by what enzyme?

fructose and glucose

sucrase

lactose in broken down into what? by what enzyme?

• glucose and fructose

• Enzyme: sucrase

trehalose in broken down into what? by what enzyme?

•  glucose and glucose

• Enzyme: trehalase

where are the enzymes that break down carbs in the SI located?

in the villi (apical side/ brush border)

absorption of CHO occurs via

transporters

carbs are absorbed as what structure?

monosaccharides!!

glucose is absorbed into enterocyte via? into blood?

• gets absorbed into enterocytes via SGLT1

• Enters the blood via GLUT2

galactose is absorbed into enterocyte via? into blood?

• gets absorbed into enterocytes via SGLT1

• Enters the blood via GLUT2

fructose is absorbed into enterocyte via? into blood?

• gets absorbed into enterocytes via GLUT5

• Enters the blood via GLUT2

in the colon, what enters there?

Undigested/unavailable carbs (fiber)

what will occur to the Undigested/unavailable carbs (fiber) that enters the colon

will be fermented by bacterial enzymes

what are the types of Undigested/unavailable carbs that reach the colon?

• resistant starch

• nonstarch polysaccharides

• indigestible polysaccharides

• malbasorbed carbs

bacterial enzymes will ferment the CHO in colon to produce what?

SCFA

what are the 3 main SFA’s made via fermentation in the colon

butyrate, propionate, acetate

butyrate importance

MAJOR energy source for colonocytes

what happens to the SCFA’s

enter portal blood to have different actions on peripheral tissues

what will happen to Undigested/unfermented carbs?

excreted via feces

CHO DIGESTION AND ABSORPTION IN DEPTH

What are available CHO

those that are glycemic →Those that are digested by endogenous enzymes and utilized fully in the SI

available CHO include

• Monosaccharides

• Disaccharides

• Some oligosaccharides

• Polysaccharides (mostly starch)

where does our CHO come from in our diet?

• About half are found in grains and some vegetables

• Mono and disaccharides from fruits and vegetables

• About 50g from added sugars

why is available = glycemic

they contribute to our blood glucose concentrations

what is unavailable CHO

not glycemic —> those that are not digested in the SI by endogenous enzymes, do not contribute to BG, usually called fiber

CHO digestion begins with what enzyme?

amylase

what does amylase do? (bond)

Cleaves alpha 1,4 linear bonds

what are the 2 types of amylase?

salivary and pancreatic amylase

salivary amylase production is based on what?

Amylase production is based on saliva flow rate

salivary amylase is produced where?

parotid gland

salivary amylase production is related to what (what triggers it)

 smell, sight, taste of food

salivary amylase is active at what pH

neutral

pancreatic amylase is released from what cells?

pancreatic acinar cells (exocrine cells)

when is pancreatic amylase released? (what triggers)

neural or hormonal signals

pancreatic amylase is released in what form?

zymogen

when is pancreatic amylase activated?

higher (more neutral) pH of duodenum = activates enzyme

what helps increase the pH of the SI to activate pancreatic amylase?

Bicarbonate from pancreatic juice and bicarbonate from Brunner cells

starch is made up of what 2 components?

amylose and amylopectin

how does amylase act upon amylose

 will break the alpha-1,4 bonds of amylose

amylase acting on amylose will result in what products

maltotriose and maltose molecules

how does amylose act on amylopectin

break alpha-1,4 bonds from amylopectin

amylase acting on amylopectin results in what products

a-limit dextrins, maltotriose, and maltose 

what enzyme will act upon amylopectin due to branches

isomaltase

isomaltase acts upon what bonds?

alpha 1,6 bonds

what does isomaltase do to amylopectin?

when a branch point is reached, will break the alpha 1,6 bonds of the a-limit dextrins

once we have the maltose, maltotriose, and alpha-dextrins what will happen to them

undergo more processing to eventually break it down to glucose (or monosaccharides)

for a alpha-dextrin, what enzyme will act on it when brach has 2+ monomers

maltase

what will maltase do?

cleave a glucose by breaking a-1,4 bond (repeat until reach the branch point)

once our alpha-dextrin is broken down to only the glucose monomer making the branch, what enzyme acts on it?

isomaltase

what does isomaltase do?

 cleave off the branched glucose attached via a-1,6 bond

the resulting maltotriose will be acted upon by

maltase

what will maltase do to the maltotriose?

remove a glucose via a-1,4 cleavage resulting in a disaccharide

the resulting disaccharide will be acted upon by what enzyme?

maltase (if maltose)

sucrase (if sucrose)

lactase (if lactose)

for starch breakdown, what will be the resulting dissacharide

maltose (glu+glu)

what is the sucrase-isomaltase complex?

 digestive enzyme composed of 2 subunits derived from a precursor protein

sucrase-isomaltase complex is usually composed of what 2 subunits?

sucrase and isomaltase

when is the sucrase-isomaltase complex activated?

it will be cleaved between the subunits via pancreatic protease

the sucrase-isomaltase complex is part of what part of the cell?

cell membrane of the enterocyte in the villi

lactase is present in what form

as lactase phlorizin hydrolase complex

what does lactase do?

breaks the b-1,4 bond between glucose and galactose

lactase prevalence

• northern Europe

• Asia and Africa

• full term infants

• pre-mes

• High in Northen European populations

• Low in Asian and African populations

• High in full-term infants

• lower in pre-me infants

lactase varies through

different population groups

what occurs to lactase in our bodies by age 5

decreases by 90%

what is the maltase-glucoamylase complex

our maltase complex composed of 2 subunits

what does maltase do?

breaks alpha-1,4 bonds

where is most of starch digested in the body (specific body part)

lumen of duodenum

where does complete hydrolysis or oligosaccharides and disaccharides into monosaccharides occur in the body?

jejunum

how are our digestive enzymes produced in the SI made and activated?

made within the cell and then shuttled to apical side of cell where they will move their way up the villi and become activated

enzymes are produced where (cellular)→ go where→ and finally reach what part of the cell?

 produced at the ER--> further processed in Golgi--> taken to apical side of cell

how are digestive enzymes activated (referring to our complexes that digest CHO)

• At the crypts, the cells are inactive and not functional (inactive enzymes)

• As the cells mature and reach the top of villi, they will gain more function

• the cells at the very top of the villi have the most absorptive and enzymatic activity (enzymes will be activated via cleavage)

how is SI (sucrase-isomaltase complex) activated?

 will be cleaved to be activated once it reaches the top of the villi

CHO absorption occurs primarily in the

SI

what structure of the SI allows for for catalytic ability?

villi

how are the villi related to catalytic activity?

Cells will mature as they move towards the top of the villi (gain catalytic activity)

the villi contain what that allows for absorption?

 capillaries --> monosaccharides will enter capillaries to then enter portal vein when absorbed

what line the villi?

enterocytes

enterocytes will use what for energy

glucose and also butyrate

glucose and galactose enter the enterocyte via

SGLT1

SGLT1 does what?

takes 2Na and 1 glucose/galactose into the cell

taking in 2 different molecules into the cell makes it a

cotransporter

SGLT 1 is what type of protein

integral protein

SGLT1 is what type of transport

facilitative active transport

SGLT1 is what type of active transport

secondary active transport