Carbohydrates Classification, Digestion and Absorption Flashcards
Hydrated Carbon Compounds
- H to O ratio of 2 to 1 (like water)
- the reaction for hydrating carbons is photosynthesis
Glucose is also known as (5)
- hexose
- monosaccharide
- simple sugar
- available carbohydrate
- nutritive sweetener
RDA/AI of carbohydrates for majority of population (g/day)
130 g/day
RDA/AI denotes the essential minimal amount required for brain function
RDA/AI of fibre for majority of population (g/day)
19-38 g/day
- 38g for males (14-50y)
- 26g for females (9-50y)
Carbohydrate AMDRs for children (1-3), children (4-18) and adults
45-65% of energy intake
- based on total energy requirement
Aldose & Ketose
Aldose
- (-ose), contains 1 aldehyde on carbon 1
Ketose
- (-ulose), contains 1 ketone on C 2
examples of mono, di, oligo and polysaccahrides
- mono: glucose, fructose, galactose
- di: lactose, sucrose, maltose
- oligo: raffinose, stachyose
- poly: starch, glycogen, dietary fiber
structure of disaccharides (sucrose, maltose, lactose)
- sucrose: glucose + fructose
- maltose: glucose + glucose
- lactose: galactose + glucose
sucrose is aka table sugar
Who discovered the reactivity of sugars?
Louis Camille Malliard
What causes the build up of advanced glycation end products (AGEs) in collagen-rich tissues, joints, blood vessel walls and lens?
caused by reactions between sugars and proteins
- aldoses and ketoses can react with primary and secondary amines to form N-glycoside
- dehydration (malliard), then a rearrangement occurs at C1 & 2 to form an aminoketose (highly reactive)
- when this compound reacts with amine groups on proteins it can result in cross-linked proteins
the reactivity of glucose with proteins serves as a marker for how glucose has affected tissues over time - 2-3 fold increase in glycoproptein in patients with diabetes mellitus
reducing vs non reducing sugars
reducing: have an anomeric carbon available (lactose, maltose)
non reducing: both anomeric carbons are involved in the glycosidic bond (sucrose, trehalose)
why does dietary fiber remian in the human gastrointestinal tract?
humans lack the digestive enzymes that can hydrolyze β-1,4 glycosidic bonds
- some enzymes from colonic bacteria may degrade fiber
Polysaccharides
polysaccharides can be linear or branched
Digestible Polysaccharides:
1) Amylose:
- alpha 1-4 linkages
- straight chain polymer of D glucose units
- makes up 20% of starch
- less soluble in water (forms a helix)
- can be hydrolyzed (β/⍺ amylases)
- no gel formation
2) Amylopectin:
- alpha 1-4 & alpha 1-6 GL
- branched chain polymer of D-glucose units
- makes up 80% of starches
- more soluble in water
- cannot be hydrolyzed by enzymes completely
- forms a gel when hot water is added
Non-digestible polysaccharides
- aka dietary fiber
- eg. cellulose: has β 1-4 and hydrogen bonds
Two major steps of carbohydrate digestion
1) intraluminal digestion (amylases)
- initial: salivary ⍺ amylases then pancreatic amylases
- salivary amylases deactivated in stomach by acids
- pancreatic juices neutralize acid and hydrolysis continues (inter luminal hydrolysis)
2) membrane digestion (brush border glycohydrolases
- maltose, maltotriose, trisaccharides, oligos and ⍺-limit dextrins require further breakdown
- how quickly you can break carbs down into their single units determines how quickly you get a glucose response
alpha limit dextrins: branched oligosaccharides that result from the incomplete hydrolysis of amylopectin
Digestion of complex polysaccharides
mouth: amylose and amylopectin get broken down into dextrins by salivary ⍺ amylases (amylopectin still contains branches)
alpha 1,4 bonds only
stomach: no further digestion of dextrins occurs
small intestine: dextrins get broken down into maltose (glux2), and limit dextrins (amylopectin) by pancreatic ⍺ amylases.
alpha1,4 bonds only
brush border: maltose is hydrolyzded to free glucose by maltase. limit dextrins are hydrolysed to free glucose by ⍺-dextrinase
alpha 1,6 bonds
fructose: sources, glycosidic bond, membrane enzymes and products
- fruits & honey
- none
- none
- fructose
glucose: sources, glycosidic bond, membrane enzymes and products
- fruit, honey, grapes
- none
- none
- glucose
amylopectin: sources, glycosidic bond, membrane enzymes and products
- potatoes, rice, corn, bread
- ⍺-1,4, ⍺-1,6
- maltase, glucoamylase, isomaltase
- glucose
amylose: sources, glycosidic bond, membrane enzymes and products
- potatoes, rice, corn, bread
- ⍺-1,4
- maltase, glucoamylase
- glucose
sucrose: sources, glycosidic bond, membrane enzymes and products
- table sugar & desserts
- ⍺-1,2
- sucrase
- glucose & fructose
trehalose: sources, glycosidic bond, membrane enzymes and products
- young mushrooms
- ⍺-1,1
- trehalase
- glucose
lactose: sources, glycosidic bond, membrane enzymes and products
- milk & milk products
- β-1,4
- lactase
- glucose & galactose
Digestion: absorption of monosaccharides
- transported processes occur on the apical side of the small intestine epithelial cells
- hepatic portal vein transports the absorbed nutrients to the liver
Glucose/Galactose:
- carrier-dependant, energy-requiring active transport
- sodium dependant
Fructose:
- facilitated diffusion - doesn’t illicit a GI
Rate of CHO assimilation:
- enzymatic hydrolysis at brush border
- transport of sugar
expression of GLUT2 controls the last phase of GI (entrance of glucose into bloodstream)
transport accross the epithelium is the rate-limiting step for carbohydrate digestion
Malabsorption
- lactose from milk requires lactase
- undigested lactose gets fermented
- hydrogen breath test/stool acidity test
- ## hydrolysis defect not absorption defect
why do we need more enzyme expression for oligo-/polysaccharides
CHO digestive enzymes and transporters
SGLT1:
- located on the apical membrane
- requires energy (Na+ gradient dependant) to shuttle glucose into the cell
- expressed in low and high concentrations of glucose
GLUT5:
- located on the apical membrane
- passively transports fructose into the cell
- expressed in low and high fructose concentration
GLUT2:
- located on basal membrane and on apical (when hexose concentration is high) - GLUT2 expression is maximal within 10m after consuming a sugar-rich meal
- passively transports both fructose and glucose into the cell, as well as out of the cell into the bloodstream
