Carbohydrates Flashcards
What are glycemic carbohydrates?
Glycemic carbohydrates are digested (hydrolyzed by enzymes) to sugars (monosaccharides) in the small bowel and absorbed and metabolized
What are nonglycemic carbohydrates?
Nonglycemic carbohydrates are fermented in varying degrees to short-chain fatty acids (SCFAs), carbon dioxide, hydrogen, and methane in the large bowel. Absorbed SCFAs are metabolized in colonic epithelial, hepatic, and muscle cells.
Name 4 important roles of carbohydrates
● sources of metabolic fuels and energy stores
● structural components of cell walls in plants and of the exoskeleton of arthropods
● parts of RNA and DNA in which ribose and deoxyribose, respectively, are linked by N-glycosidic bonds to purine and pyrimidine bases
● integral features of many proteins and lipids (glycoproteins and glycolipids), especially in cell membranes where they are essential for cell–cell recognition and molecular targeting.
What is DP 3-9
oligosaccharides
What is DP>9?
Polysaccharides
How much dietary fibers are recommended a day?
25-35g
What is starch?
A polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds.
It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight
Difference in satiety between sugar and rye?
Sugar reach the blood, trigger insulin, is produced and push down the sugar fast, and as soon as the glucose level is down you feel hungry again
When eat rye bread, takes a long time, glucose evenly distributed, as long as there is glucose In the blood you feel satiety
Linkage of starch and glycogen vs cellulose?
alpha-1,4 linkage which can be digested
beta-1,4 linkage which we cannot digest
Describe the structure of amylose
1-4 glucose units – spiralshaped
Describe the structure of amylopectin
1-4 glucose and then branches with 1-6
What is resistant starch and give examples
Starch that escapes digestion in the small intestine and enters the colon
- RS1 - within intact cell structures
- RS2 raw starch granules
- RS3 - retrograded amylose
What enzymes are involved in the digestion of carbohydrates to monomers?
α-amylase, Sucrase-isomaltase
(brush border, Amyloglucosidase reducing end
What is the role of β-Amylase?
Is an enzyme that catalyses the hydrolysis of starch into sugars.
Working from the non-reducing end, β-amylase catalyzes the hydrolysis of the second α-1,4 glycosidic bond, cleaving off two glucose units (maltose) at a time
What is the role of α-Amylase?
Is an enzyme that catalyses the hydrolysis of starch into sugars.
By acting at random locations along the starch chain, α-amylase breaks down long-chain saccharides, ultimately yielding either maltotriose and maltose from amylose, or maltose, glucose and “limit dextrin” from amylopectin.
Because it can act anywhere on the substrate, α-amylase tends to be faster-acting than β-amylase.
Where are the hydrolytic enzymes that break down carbohydrates to monomers secreted?
within the mouth, from pancreas and on the apical membrane of enterocytes
Which are the major carbohydrase secreted by the salivary glands and by the acinar cells of the pancreas?
endoglycosidase α-amylase, which hydrolyzes (digests) internal α-1,4-linkages in amylose and amylopectin molecules to yield maltose, maltotriose, and dextrins.
What is sucrase–isomaltase?
Sucrase–isomaltase is a glycoprotein anchored via its amino-terminal domain in the apical membrane (located on the brush border of the small intestine) that hydrolyzes all of the sucrose and most of the maltose and isomaltose. The resulting monomeric sugars are then available for transport into the enterocytes.
(breaks down sucrose to glucose and fructose)
How are glucose and galactose absorbed from lumen?
Glucose and galactose are transported active against concentration gradient into enterocytes from lumen sodium–glucose transport protein-1 (SGLT1), a process that is powered by Na+/ K+-ATPase on the basolateral membrane
How is Na+ involved in the transportation of sugar molecules?
Na+ is bonded to sugar molecule and sugar in transported inside because the concentration of Na+ is lower inside the cell
What is the role of GLUT2?
It is the facilitated transporter on the basolateral membrane which shuttles all three monosaccharides from the enterocyte towards the blood vessels
What is transported by the membrane-spanning GLUT5 protein?
Fructose (run by gradient)
What happens to glucose, galactose and fructose when they have been absorbed in the blood?
All three main sugars absorbed from the gut (glucose, galactose, and fructose) are transported via the portal vein to the liver.
but only glucose appears in significant concentrations in the peripheral circulation. Most of the galactose and fructose is removed during first pass through the liver
Why is some glucose absorbed in the liver ?
To refill glycogen reserve in liver
How do fructose and galactose contribute to energy?
By being converted to intermediates and used in glycolysis and the citric acid or Krebs cycle pathways.
Where is gastric inhibitory peptide (GIP) secreted?
from enteroendocrine cells within the mucosa of the small bowel
What is the role of GIP?
As the glucose concentration in blood rises above 5 mM after a meal, these peptide hormones amplify the response of the β-cells of the endocrine pancreas, resulting in the discharge of the hormone insulin from secretory granules which fuse with the cell membrane.
Hence trigger insluin secretion.
What is the role of GLP-1?
GLP-1 is an incretin; thus, it has the ability to decrease blood sugar levels in a glucose-dependent manner by enhancing the secretion of insulin.
What do GIP and GLP-1 have in common?
trigger insulin secretion from pancreas when glucose level in blood exceed.
What receptor is stimulated by insulin to take up glucose in cells?
GLUT4 - membrane in muscles and fat tissues
What is the glucose requirement of the brain (g/day)
120 g/day
What happens when the body need to form glucose from noncarbohydrate sources?
Gluconeogenesis (90% in liver but also some in kidneys)
Can use substrates as pyruvate, lactate, glycerol and amino acid (by catabolism of bodies protein, all AA except lysine ans leucine)
Also triacylglycerols from adipose tissue are catabolized to release glycerol.
How is gluconeogenic processes triggered?
By fall in blood glucose concentration below about 5 mM and are signaled to tissues by secretion of glucagon and glucocortiod hormones.
What is glycemia?
blood glucose concentration?
What concentration does the blood glucose seldom go under?
5 mM
What is the blood glucose concentration during fasting?
And how much does it increase after a meal?
Fasting: 3.5-5.5mmol/L
Increases: 2.5mmol/L after a meal
What determines the glucose concentration?
The blood glucose concentration is determine by glucose influx from e.g. intestine and glucose absorption to peripheral tissues.
What is GLUT4 and how does it act?
The glucose transporter that respons to insulin.
Insulin sit on the receptor on cell – when signal from the receptor come inside the cell, the transportprotein (which is then in the cell) goes to the membrane and sit there until glucose comes and then the GLUT4 brings the glucose inside the cell
Where can glucose be converted to fat (TAG)?
In liver and adipose tissue
What is produced as a result from higher synthesis of fat in the liver?
Causes increase in VLDL, the way liver export fat
Why may fructose cause higher fat concentration in the blood?
Because fructose is metabolised faster than glucose
Why may higher glucose availability lead to obesity?
Because glucose preferred fuel so higher glucose availability can prevent catabolism of fat
What does insulin do?
- stimulate uptake glucose by muscle and adipose tissue!
- stimulate glycogen synthesis in the liver – hormonal regulation – because energy is needed to fill tank for later
Insulin also send signal to stop gluconeogenesis
How does the pathway for fructose and glucose differ?
fructose bypass phosphofructokinase-1 (PFK-1) which regulates glucose metabolism.
therefore fructose is metabolised very fast
Why is fructose metabolised much faster than glucose?
It may be explained by the fact that, because it is phosphorylated in hepatocytes, it bypasses 6-phosphofructokinase, one of the key regulatory enzymes in glycolysis.
Why is is that the consumption of foods and beverages containing fructose produces smaller postprandial insulin excursions than does consumption of glucose-containing carbohydrate
Because fructose does not stimulate insulin secretion from pancreatic β cells
Why can excessive amount of fructose lead to insulin resistance?
Because the metabolism of excessive amounts of fructose leads to fatty liver, which is a key step in the development of insulin resistance
What affect influx of glucose to blood?
Food (from intestines, which depends on carbohydrate content of food ans factors affecting digestion and absorption), gluconeogenesis and glycogenolysis
What affect the outflux of glucose to blood?
Uptake by different tissues
In more detail, how can the influx vary depending on the food factors and consumer factors?
Food factors:
particle size, macrostructure and microstructure of food (esp whether cell walls are intact), amylose-amylopectin ratio starches, lipid content of food, presence (or otherwise) of enzyme inhibitors
Consumer factors:
- degress of communition in the mouth
rate of gastric emptying, small bowel transit time
How come we digest rye and wheat differently?
In wheat it is separated, easier for the enzymes to work
- Rye bread, it is more compact, harder for enzymes to work
What can methane in breath be a sign of?
Consumtion of dietary fibers
How much energy can be extracted from fermented dietary fibers?
8 kJ per gram
How is a bulk effect generated?
from fibers that have absorbed water
Which SCFs is the main energy source for intestinal cells?
Butyrate
it also has protective effects
What can happen if food is low in fiber content
Other compounds can be used as substrate for flora which can change the flora to more harmful flora
What are the three most common SCFs?
Butyrate, propionate and acetate
What can H2 in breath determine?
the extent of
fermentation by gut flora
What defines prebiotics?
Prebiotics are compounds that stimulate the growth of healthy flora (e.g. inulin)
What defines probiotics?
Probiotics are living microorganism that can generate
health effect in host
What is the effect of soluble fibers?
They can generate gel and therefore prevent
gastric emptying and reduced cholesterol absorption (e.g.
β-glucan) (100 oats or 30-50 oats bran)
What is the largest source of dietary fibers?
Bread
What is the different in dietary fiber intake between a white and whole grain bread?
Dietary fiber content in white bread is 1/4 of whole grain bread
What does the bran contain?
High content of minerals and vitamins
How much carbohydrates are recommended by NNR and WHO?
NNR: 50-60E%
WHO: 55-75E%
What is recommended intake of refined sugar?
10E% to reassure intake of essential nutrients
How does millard reaction occur and why is it important in food products?
Reaction between reducing sugar and amino acids
Create flavour and aroma
(Can also reduce lysine which is an essential amino acid)
What are sweetening agents used for and give examples
To reduce energy intake, in case of diabetes, and to reduce dental problem.
Can include fructose which is sweeter than sucrose or alcohol sugar which are absorbed less in intestine (2-3kcal/g)
also alcohol sugars are not fermented /mouth hygiene e.g. sorbitol and mannitol.
What are synthetic sweetening agents and give examples
have different nature than carbohydrates but interact with taste receptors. Ex. saccharine and cyclamate (E-number)
NNR carbohydrates?
45-60%
