carbohydrate digestion and absorption

what are the only sugars that can be absorbed by the intestinal mucosa cells?

monosaccharides

do monosaccharides need to be digested?

no

how are disaccharides disgested?

by specific enzymes in the small intestine

where does digestion of polysaccharides begin?

the mouth, utilizes similar enzymes throughout the GI tract

what specific enzymes digest carbohydrates?

glycosidases and carbohydrases

what are the key dietary disaccharides?

• maltose

• sucrose

• lactose

• trehalose

where does digestion of disaccharides take place?

• primarily occurs in the upper small intestine: duodenum and jejunum

• occurs within the brush border of the intestinal mucosa

• does not occur in the mouth or stomach

what enzyme is needed for digestion of lactose?

lactase

what enzyme is needed to digest sucrose?

sucrase

what enzyme is needed to break down maltose?

maltase

what enzyme is needed to break down trehalose?

trehalase

what is lactase?

equal parts glucose and galactose

what is sucrase?

equal parts glucose and fructose

what is maltase?

2 glucose molecules

what is trehalase?

2 glucose molecules

what are the final products of digestion?

monosaccharides, which are absorbed in this form

what is the rate of digestion dependent on?

the concentration of disaccharides and the amount of enzymes in the brush border

what population is lactase enzyme activity high in?

human infants, but decreases with age

what happens if there isn’t enough lactase enzyme to digest lactose?

it cannot be absorbed

lactose intolerance

• undigested lactose moves through the GI tract

• pulls water into the lumen due to osmotic pressure

• arrives intact in the colon, where bacteria digest it and produce gas

what are symptoms of lactose intolerance?

• diarrhea

• gas

• bloating

• abdominal discomfort

who is lactose intolerance the highest in?

• native americans

• asian and african populations

• individuals from middle eastern descent

what is the difference between lactose intolerance and a milk allergy?

those with lactose intolerance can digest some lactose because they have some lactase. those with a milk allergy usually cannot tolerate any milk or milk products

why can some with lactose intolerance tolerate yogurt and hard cheeses?

bacteria in cultures digest the lactose

what is sucrase insufficiency?

a genetic disorder that is much less common than lactose intolerance

what are key features of sucrase insufficiency?

• limits the ability to digest sucrose

• same symptoms as lactose intolerance

• avoid foods containing sugar, high fructose sugars, and fruits containing a lot of sucrose

what causes trehalose to be malabsorbed?

consuming it in large amounts, it overwhelms the enzymes available and results in bacterial overgrowth

how can issues with adequate enzyme activity be detected?

by looking for reducing substances in stool

do polysaccharides contain short long chains of glucose?

long

where do digestion of polysaccharides begin?

begin in the mouth and continue in the small intestine

what polysaccharides begin digestion in the mouth?

amylose and amylopectin

what enzyme breaks down starch into dextrin in the mouth?

salivary amylase

what happens to salivary amylase when it reaches the stomach?

acid destroys its activity

what percentage of carbohydrate digestion takes place in the mouth?

5

what happens to starch when it reaches the small intestine?

it is further digested by pancreatic amylase

what is amylose broken down to?

maltose

what is amylopectin broken down to?

maltose, maltotriose, and a few dextrins

what part of the small intestine does digestion of polysaccharides take place?

lumen

what does maltase break down to?

maltose

what is maltotriose broken down to?

glucose

what does alpha dextrinase break dextrins down to?

glucose

what is the end product of all carbohydrate digestion?

glucose and other monosaccharides

what are the only type of carbohydrate that can be absorbed?

monosaccharides

how are polysaccharides initially digested?

with the enzyme amylase to smaller, more manageable chains

how does polysaccharide digestion finish?

by brush border enzymes produced in the intestine

where do sugars and carbohydrates go that are not fully digested or absorbed?

the colon to feed colonic bacteria and can result in GI symptoms or diarrhea

how are monosaccharides absorbed?

through transport proteins

can monosaccharides diffuse across mucosal membranes by themselves?

no, they must cross the plasma membrane two times

what are the two types of transport systems?

sodium glucose cotransporters (SGLTs) and glucose transports (GLUTs)

do sodium glucose cotransporters requires energy?

yes

what form of absorption do glucose transports do?

facilitated diffusion

what do SGLT1 absorb?

glucose and galactose

what do GLUT2 and GLUT5 absorb?

fructose

where SGLT expressed?

in the brush border and their primary role is to absorb glucose and galactose

where does glucose and galactose absorption take place?

occurs in the villi and microvilli of the brush border

what happens to glucose and galactose after digestion?

they are absorbed using both SGLT (active) and GLUT transporters (facilitated diffusion)

do GLUT transporters increase with lower or higher amounts of carbohydrate consumption?

higher

what does active transport require?

• ATP

• sodium to be present for glucose to attach to the transporter

• 1 glucose and 2 sodium

• sodium concentration is typically low in the cell

what happens when sodium and glucose are attached?

• they are transported into the cell and released

• sodium is pumped back out of the cell requiring energy for the sodium potassium pump

• when glucose concentrations get high enough in the cell, glucose binds with GLUT2 and is transported via facilitated diffusion into the blood

what is the relationship between sodium and glucose in the intestine?

they enhance the absorption of each other

what happens during facilitated diffusion?

• when glucose levels increase in the lumen, some glucose and galactose are transported into the cell by GLUT2

• when glucose levels are high within the cell GLUT2 transports glucose into the blood

• feedback loop from blood levels. when bloo sugar rises, insulin is released

• insulins stimulates GLUT2 to move away from the brush border and glucose absorption decreases

how is fructose absorbed?

through facilitated diffusion in either GLUT2 or GLUT5

does absorption of fructose require energy?

no

is the rate or fructose absorption slower or faster than glucose and galactose?

slower

can all of fructose be absorbed?

yes, through normal intakes

how many grams is the normal intake of fructose?

20 - 50 grams

individuals with milk allergies can tolerate small amounts of dairy products

false

oligosaccharides found in beans and peas cannot be digested by human enzymes but are broken down by bacterial enzymes in the intestines

true

alpha amylase digested both amylose and amylopectin

true

fructose can only be absorbed using the active transport SGLT1 system

false

digestion of disaccharides begins in the mouth

false