Carbohydrates Flashcards
What determines the digestibility of carbohydrates?
The types of bonds present – alpha or beta (indigestible)
Discuss the monosaccharides, disaccharides and polysaccharides found in the diet (with examples of food sources), their characteristic structure, absorption and metabolism pathways, including relevant enzymes
Glucose:
Hexose
Consumed as sucrose (table sugar) and starch-containing foods
Absorbed via GLUT1-4
GLUT2 translocated to the brush border when high glucose concentration in intestinal lumen. As BGL rises, insulin is releases, causing GLUT2 to be translocated back into the cell (impaired in insulin resistance)
Contributes to BGL (what is measured)
Insulin-dependent cellular uptake = GLUT4
Fructose:
Hexose. Carbonyl group on C2 (glucose is C1)
Component of sucrose. Found in fruit, honey, vegetables
Absorbed via GLUT2 and GLUT5
Cellular uptake insulin independent
Broken down via fructokinase to fructose-1-phosphate. This is cleaved via aldose B (rate limiting) and its products can form glycerol and pyruvate
Galactose:
Hexose. OH orientation on C4 different to glucose
Component of lactose, found in dairy. Also present in some plant products (honey, beets, cherries etc.)
Broken down in the liver via galactokinase. Fed into glycolysis
Galactosaemia is the inability to break down galactose. Important in pregnancy. Inborn error of metabolism
Sucrose: Glucose + fructose a-1,2 gycosidic bond Found in table sugar Broken down via sucrases
Lactose: Glucose + galactose b-1,4 bond Found in dairy products Broken down via lactases
Maltose:
Glucose + glucose
a-1,4 bond
Broken down via maltases (easiest for body to digest)
Cellulose:
b-1,4 linkages
Unbranched
Found in plant cell walls
Amylose:
a,1-4 linkages
Unbranched
Found in plants (starch)
Salivary a-amylase hydrolyses a-1,4 bonds in mouth. Amylose –> dextrins
Gastric juices destroys amylase activity in stomach
Pancreatic a-amylase hydrolyses a-1,4 bonds in SI. Dextrins –> maltose
Maltase hydrolyses maltose at the brush border. Maltose –> glucose
Amylopectin;
a-1,4 and a-1,6 linkages
Branched every 20 subunits
Found in plants (starch)
Salivary a-amylase hydrolyses a-1,4 bonds in mouth. Amylose –> dextrins
Gastric juices destroys amylase activity in stomach
Pancreatic a-amylase hydrolyses a-1,4 bonds in SI. Dextrins –> maltose
a-dextrinase hydrolyses a-1,6 bonds at the brush border. Dextrins –> glucose
Maltase hydrolyses maltose at the brush border. Maltose –> glucose
Glycogen:
a-1,4 and a-1,6 linkages
Branched every 10 subunits
Found in animals
Glycogen phosphorylase releases glucose-1-phosphate from linear chain. De-branching enzymes hydrolyse the branching points
Liver glycogen used for BGL homeostasis principally
Muscle glycogen is the substrate for energy production in muscle, not the BGL. The muscle tissue is lacking glucose-6-phospatase (G6P) required in the last step to glucose
Glucose-1-phosphate –Glucose-1-phosphatase–> Glucose-6-phosphate –> direct entry to glycolysis
Why would fructose be considered a good sugar for diabetics? (an old nutrition myth)
When looking at the metabolism of fructose, why is fructose associated with fatty liver (non-alcoholic fatty liver disease)?
Excessive consumption is linked with fatty liver disease. Used to be recommended to people with diabetes as it didn’t raise BGL. However its rapid breakdown to pyruvate and subsequently acetyl-CoA increases lipogenesis
What is high fructose corn syrup? How is it different to table sugar? Why is it preferred by the food industry?
HFCS produced by processing corn via the use of enzymes. Preferred by the food industry as it is inexpensive and greatly enhances sweetness. Different from table sugar in that it can contain more or less fructose depending on the type
Explain the concept of “carbohydrate quality” using the glycaemic index and glycaemic load. Know how to calculate the glycaemic loads of foods
GI refers to the BGL response of a particular CHO-containing food in comparison to pure glucose (or white bread) gram-for-gram over 2hrs post-ingestion. Low <=55. High >=70
GL takes GI and accounts for the quantity of food consumed: GL = (GI x g)/100
Low <=10. High >=20
What determines the degree of fermentability of indigestible carbohydrates?
Degree of fermentability of indigestible CHO increased with solubility
Briefly define dietary fibre and functional fibre; provide some examples
Dietary = found in whole foods; oats, lentils Functional = supplemental; capsules, psyllium husk
How are carbohydrate foods involved in blood glucose level (BGL)?
Broken down to glucose = increase in BGL
What are the mechanisms that regulate high and low BGL? Why can muscle glycogen not be used for BGL?
Insulin is released by pancreas in response to rising BGL. Causes translocation of GLUT2 from the brush border into the cell and increases expression of GLUT4 for cellular uptake. Also increased conversion of glucose to glycogen
Glucagon is released by the pancreas in response to lowering BGL. Causes breakdown of glycogen to glucose and increases gluconeogenesis
What is the complete recommendations for carbohydrate and fibre intake in the NRVs?
CHO = 45-65% of EER, predominantly from low GI foods. Limit sugar and increase fibre
No EAR for fibre but an AI. 25g for women. 30g for men
How would you determine the quality of carbohydrate and fibre intake in the NIP?
Can’t determine CHO quality on NIP. Look for high fibre and no added sugar
Discuss the health benefits of meeting the fibre requirements daily, by explain the mechanisms of action for these benefits
- Promotes bowel health
- Increases faecal bulk, preventing diverticulosis/diverticulitis
- Faecal bulk activates stretch receptors, promoting peristalsis
- Reduces obesity and weight gain risk
- Slows gastric emptying, thus increasing satiation, less risk of overeating = appetite control
- BGL control
- Slows gastric emptying = slows CHO digestion and glucose absorption, and rise of BGL
- Makes digestible CHO less easily accessible to enzymes = less free glucose
- Reduces plasma cholesterol
- Fibre binds cholesterol and bile (made from cholesterol), increasing excretion and preventing resorption
- Promoting gut and overall health
- Fibres act as substrates for bacterial fermentation by gut micro-organisms
- Feeds the bacteria in the gut –> produces SCFA (acetate, butyrate, propionate) –> provides a range of benefits (increased mucosal layer, decreased cell proliferation)
Is honey a healthier option to table sugar? Explain your answer
No. Honey is pure fructose. Fructose is rapidly converted to pyruvate –> acetyl CoA –> increased lipogenesis
Discuss lactose intolerance
What is meant by lactase persistence? Explain the mechanism and the outcome
Lactose intolerance is the inability to digest lactose via lactase post-weening. Lactase is an enzyme produced by the enterocytes and located in the cell membrane of the brush border (microvilli)
Primary lactose intolerance is the lack of lactase production in adulthood = the default program in humans. Some populations have developed lactase persistence via a mutation/polymorphism in the promoter region of the lactase gene
Secondary lactose intolerance = in lactase persistence phenotypes, LI is associated with damage of the lactase producing cells (e.g. in IBD)
Common symptoms include nausea, cramping, distension, gas, diarrhoea. Begin approx. 30mins after consumption
What would yoghurt be more tolerated in lactose intolerance?
The fermentation to make yoghurt and some cheeses “digests” some of the lactose. The bacteria used in the fermentation also produce lactase. In other cheese-making processes the lactose is removed
