Carbohydrates

FNN200

Carb classifications

• simple sugars

  • monosaccharides (Glucose, fructose, galactose, allulose, ribose)

  • Disaccharides (sucrose, lactose, maltose)

• Oligosaccharides (3-10 monosaccharide units)

  • starchyose raffiones

• Polysaccharides (10+ monosaccharide units)

  • starch and glycogen (amylose, amylopectin)

  • Fiber and resistant starch (cellulose, pectin, hemicellulose)

  • Glycosaminoglycans (chondroiti sulphate, hylauronic acid

Carbohydrates

• hydrates of carbon

• MONOSACCHARIDES ALL C6H1206

Isomers

• identical molecular formulas but with different structures

Epimers 

• different configuration around only one C 

  • D-glucose and D-galactose

Enantiomers

• mirror images of eachother 

• Lglucose and D-glucose 

Glycosidic bonds

• can be alpha or beta depending on bond orientation

• the body is unable to break down b-glycosidic bonds 

• EG. Maltose is 2 glucose units connected by a a1-4 bond

Most prevelant disaccharides

• sucrose 

• maltose 

• lactose

Total sugars

• natural sugars - naturally occuring in fruits

  • the other things in the food controls how the body digests them 

• Free sugars 

  • added suggars are added to food in processing and preparation 

  • sugars in honey, syrups, juices and juice concentrate also are free sugars 

CFG and sugar

• does nto have a specific number 

• just says that most sugars should come from naturally occuring sources 

• say to limit free sugars to less than 10% of daily calories 

Front of package nutrition labels

• containing amount of total sugars meeting or ex ceeding 15% of the daily value/serving 

Complex carbohydrates (oligosaccharides)

• Common in dried beans, peas, lentils, whole grains

• cannot be digested 

  • they are foods for gut microbiota 

• human digestive enzymes cant digest their glycosidic bonds - become prebiotics

Glucose polysaccharides

• linear a1-4 = amylose which is a linear chain 

• branched a1-4 + a1-6 = amylopectin 

  • several different connnections makign it more complex and therefore easier to break down 

• Glycogen is also branched a1-4 and a1-6 bonds 

  • easily hydrolyzed for a quick release of glucose 

  • allows it to be broken from many points 

Branching of polysaccharides -

• increases the surface area allowign a larger number of glucose molecules to be cleaved 

Glycosidases

• specific for a-glycosidic bonds

• B1-4 bonds cannot be hydrolyzed by mammalian digestive enzymes 

• cellulose - not an energy source becomes it contains the beta bonds 

Dietary fiber

• things like cellulose which cannot be broken by our digestive enzymes because they contain B-bonds 

• eg. cellulose

Digestion and absorption of carbs

• Mouth

  • starches → breakdown by salivary alpha amylase → makes alpha dextrins

• Stomach 

  • does not do digestion 

• Small intestine 

  • Dextrins → pancreatic a-amylase breaks down → Maltose, dextrins and maltotriose

• BBM Microvilli 

  • maltose → broken down by a-glicosidase (maltase) → glucose 

  • Limit dextrios → a-limitdextrikinase → glucose

Allergy vs intollerance

• allergy = immune response to proteins in milk 

  • attack proteins 

• Intolerance 

  • digestive system cannot break down lactase sugar

Lactose intolerance

• lactase contains a B1-4 linkage 

  • this is the only one that humans are usually able to break down 

Families of glucose transporters

• Sodium glucose co-transporters (SGLT)

  • energy dependent - active transport 

  • function coupled with sodium co-transport and ATP hydrolysis 

  • SGLT1 → intestine

  • SGLT2 → Kidney 

• Glucose transporters (GLUT)

  • Facilitated diffusion 

  • transport can be bidirectional → GLUT2

  • 14 isoforms distributed througout the body 

Monosaccharide absorption impacted by 

• concentration 

• affinidty and capacity of transporter 

• tissue localization 

• mode of transport 

Small intestine absorption of glucose and galactose 

• SGLT1

  • uses sodium cotransport to bring from intestine to enerocyte 

• GLUT2 

  • facillitated diffusion to the hepatic portal vein 

sucrose transport

• sucrose broken down into 1 fructose molecule and one glucose by sucrase 

• GLUT5 transports into enterocyte 

• GLUT2 transports into hepatic vein 

GLUT2 at brush boarder membrane

• when intestinal lumen glucose is really high 

• the GLUT2 will transport over to the brush boarder membrane 

  • transient expression 

• the rate of transport is dependent on blood glucose contentration because it is facilitated diffusion 

  • insulin causes GLUT2 to leave membrane and return to the intracellular storage vesicle 

Renal reabsorption of glucose

• kidney 

  • SGLT2 enables body to retain glucose and prevents its loss through urine 

  • has GLUT2 on the other side to brign into the blood

Enzyme kinetics

• Km value (michaelis constant)

• Km = the concentration of substrate when the reactin is at ½ it max velocity 

Km

• concentration of substrate when the reaction is at ½ its max velocity 

• Low Km = high affinity for substrate 

• High Km = low affinity for substrat3e 

  • takes longer to reach the fullness 

Affinity of GLUTS

• GLUT1 = low Km cause we want it to be fast in

  • blood brain barrier, fetal placenta 

• GLUT2 = high kM low affinity 

  • regulation of glucose concentrations 

SGLT1

• glucose and galactose 

• small intestine

• uses active transport

SGLT2

• glucose 

• kidney 

• active transporter 

GLUT1

• glucose 

• BBB, placenta, fetal tissue

• low Km

GLUT2

• glucose 

• liver, small intestine, basolateral side 

  • can be translocated 

• Highest Km so that it can regulate blood glucose better 

  • want to have more substrate to keep glucose in line

GLUT3

• glucose

• Brain, placenta, testes 

• low KM 

  • brain needs to be cative 

GLUT4

• glucose 

• muscle and adipose tisue 

• moderate Km 

Glut 5

• Fructose transporter

• small intestine 

• Low Km

GLUT4 Translocation

• insulin sensitive transporter

• Process - on muscle and adipose

  • insulin receptor binds insulin 

  • activates a cascate which will translocate GLUT4 to teh outside fo the cell to bring in glucose from outside the cell 

• Muscle contraction also stimulates the translocation of GLUT4 independent fo insulin through a different signalling pathway 

Monosaccharides post digestion 

• Glucose → oxidized for energy or put into storage as glycogen

  • 30-40% will go to the liver and the remaiing will go to blood 

• Galactose → glycogen storage 

• Fructose → oxidized for energy and excess may be converted to triglycerides 

Glycemic response 

• there is a spike after consuming glucose in plasma glucose 

• Glycemic index is the area under the curge in test food/reference food x 100

• glycemic load = GI x g of digestible carb in a serving /100

Regular blood glucose levels

• Fasted = 4.5-5.5 mmol/L 

• Fed = 7.8 mmol/L

Low GI and GL

• GI = <55

• GL = <10

Medium GI nd GL

• GI = 56-69

• GL = 11-19

High GI and GL

• GI = >70

• GL = >20

Low GI foods

• Bran ceral 

• apple juice 

• apples 

• banannas 

Low GL foods

• all bran ceral 

• apples 

• beets 

• peanuts 

Foods that have high GI but low GL 

• watermelon 

  • while it has a high GI the actual amount in a serving is low and therefore it is not actually goign to have such a high spike

Food with low GI but a high GL

• Macaroni 

• there is a GI of 47 which is considered low, but the GL is 23 which is high

Criticisms of GI and GL for food labeling

• the reality depends on what else you eat

• what activities you d

• how much of the foods you eat etc 

• High gi would be a quick energt boost 

Carbs and hormonal regulation

• anabolic → insulin 

  • triggers: blood glucose, hormones, incretins

• Catabolic → glucagon 

  • epinephrine/norepinephrine 

  • triggers: hypoglycaemia 

Monosaccharide darivitives

• amino sugars, acetyl amino sugars, glyconic acids 

• almost all carb in cells is a derivitive 

D and L monosaccharides

• the position of the hydroxyl group on the asymmetric carbon is used to designate D andL isomers 

• OH on right is D on the Left is L 

Allulose

• found in some fruits but is rare

• less sweet than sucrose but is still able ot be processed less making it less sweet

• derivative of D fructose and is not efficiently metabolize for energy makign it low calorie

Sucrose

• fructose and glucose 

• cane sugar, beet sugar, table sugar 

Lactose

• glucose and galactose

• milk, aids in calcium absorption and supports beneficial bacteria growth 

Maltose

• two glucose units 

• found only briefly in a plant usually in the seets 

• an intermediate product of the digestion of complex starches in the guy and part of partial hydrolysis of starch in some foods like beers 

Sweetness relative to sucrose

• Fructose corn syrip is higher

  • also frucsose higher

• All other sugars are less sweet 

• Sugar alcohols are all much sweeter than sugar 

  • used in sugarless gums, dietetic candies, sugarless candies etc 

• High intensity sweetners 

  • hundreds of times stronger than sucrose

Carb consumption 

• often a critical issue for those trying to loose weight or control blood sugar levels 

• most carb comes from cerial grains and refined sugar because of the increased production of sucrose 

Cereal grains

• large source of starch because of the endosperm

• most of hte starch is attributed to amylose and amylopectin 

AMDR for Carbs

• 45-65% of total energy consumption which allows people to plan their dietary carb levels on their lifestyle 

• encouraged to consume more whole grain and whole food products 

• DRI  for carb is 130g/day for all individuals to prevent ketosis 

Ketosis

• state which ketone body production exceeds metabolism resulting in the accumulation of ketones in the blood and tissues 

Dietart fiber AI

• 38g for men and 25 for women 

• goes down with age 

Why does no digestioin occur in the stomach

• because tha amylase can operate at 6.6-6.8pH so the low pH in the stomach causes the amylase to not function properly 

starch breakdown 

• occurs in the brush boarder 

• uses an enzyme a-dextrinase or isomaltase

• starch as a branched mixture of dextrines contianing a1-6 linkages

  • breaks down into a-dextrins, maltose and maltrioses

• the remainder occurs in the intestinal surface

• Enterocytes on the small intestine contain

  • maltase, lactase and sucrase to break down sucrose, maltose and a-dextrins

Fructose absorption 

• facilitative diffusion through GLUT5 transporter 

• fructose must be bound to a membrane protein carrier as well as move down the concentration gradient 

euglycemia 

• the term relating to the maintenance of optimal fasting blood glucose levels despite changing nutrition and metabolic states

Glycemic Response

• degree and duration to which blood glucose level is elevated after consuming a portion of food that would provide 50g of digestibel carb an dmeasured for the next 2 hours under the curve 

Glycemic index

• comparison of a foods glyfemic response to a food standard based on studies 

• glucose and white bread are the standards 

  • blood glucose of 50% that of glucose would be a GI of 50 

Glycemic load

• GI normalized to a serving standards 

• multiply the GI by the amount of digestible carb in a serving then divide by 100 

Impacts of high GI and GL on blood sugar

• high GI food can worsen hyperglycemic state and glucose managemnet is essential to reduce health risks 

Glucose transport into cells

• uses GLUT 

• GLUT1 is the most widely expressed especiually in epithelial tissues and the BBB 

• GLUT 2 is in hepatocytes, pancreatic B-cells and basolateral membranes of intestinal and renal epithelial cels

• GLUT3 goes grlucose in neurons and has a low Km to ensure that there is always a glucose supply

• GLUT4 is in muscles and is regulated by insulin

• GLUT5 is the small intestine and is the fructose transporter

• GLUT7 acts to transport free glucose to cytosol after the action of glucose-6-phosphate important for glucose liberation and release into circulation

Monosaccharide activation

• must be phosphorylated in order to be metabolized within a cell 

• this locks the monosaccharides within certain cells like in hepatoxytes 

Hormones in carb metabolism

• insulin 

• glucagon 

• epinephrine 

• cortisol 

Insulin role in carb

• increase uptake fo glucose in skeletal muscle and adipocites 

• increase the synthesis of glycogen in skeletal muscle and liver hepatocytes 

• increases fatty acid synthesis from excessive carbs 

Glucagon

• increases glycogen breakdown in liver 

• increases liver glycogen derived glucose release into blood 

• increases glucose manufacturing in liver 

Epinephrine

• increases glycogen breakdown in liver hepatocytes and skeletal muscle

• increases liver glycogen-derived glucose release into blood 

• increases fat breakdown and mobilization from adipocytes 

Cortisol

• increased muscle protein breakdown into aminoa cds 

• increase gluconeogenesis and liver glucose release into blood 

• increase fat breakdown and mobilization from fat tissues 

• increase liver glycogen 

Insulin

• B-cells from pancrease 

  • 2 chains disulphide bonded 

• Produced in the pancreas 

• Golgi has proinsulin then it is converted to mature insulin through the stretching of amino acids 

INsulin secretion

• insulin release is energy dependend and stimulated by plasma glucose concentration

• release is promoted by certain amino acids and epptides 

Insulin mediated glucose uptake

• insulin promotes the lowering of blood glucose through 

  • adipose tissue and skeletal muscle taking in glucose through GLUT4 which is upregualted by the insulin 

Metabolic roles of insulin

• increases glycolysis and the pentose phosphate pathway

• glycogen formation and fatty acid synthesis 

• Inhibits glucose formation via glycogen breakdown and a conversion from non carb molecules 

• also inhibits fat breakdown and fatty acid oxidation in adipocytes 

Glucagon cells

• from a-cells of pancreatic islets 

• circulates in the plasma 

• inactivated in the liver 

• Gluconeogenesis occurs because of amino acids whihc stimulate glucagon 

  • protein containing meals are likely to stimulate glucagon release 

• mostly acts on the liver and adipose tissues 

• binding of glucagon to its receptors results in increase intracellular cAMP

• promotes the conversion of glycogen to glucose and gluconeogenesis 

• breaking down of fats also occurs in adipose tissues

  • activation of adentl cyclase 

• skeletal muscle does not have glucagon receptors so does not influence glycogen breakdown 

Non-nutritive sweetener guidelines

• non sugar sweeteners should not be used as means of achieving weight control or reducing risk of diseases 

• people should consider reducing free sugars through consumption of naturally occuring sugars 

Personalized microbiome effects of non-nutritive sweeteners study

• Objective

  • determine the impact of NNS on glycemic responses 

• what did they do? 

  • RCT on 120 adults 

  • consumed NNS for 2 weeks (less than acceptable daily intake dose)

  • vehicle was glucose, the control was no subbplement 

• What did they find

  • saccharin and sucralose impared glucose tolerance compared to vehicle and control groups

  • there were distinct changes in microbiome

NNS effects on microbiome

• responders

  • developed impared glucose response with NNS consumptuon (saccharin and sucralose)

• Non-responders

  • no impared glucose response wiht NNS consumption 

• When microbes from responders where transplanted, the microbiome did result in a different impared response in mice 

Key findings from NNS study

• NNS can change gut microbiome and change hormonal regulation in glucose response

  • microbiome changed in responders 

• our microbiome diversity/balance can affect how we respond to NNS 

  • personalized factors changin response 

• May change depending on metabolic conditions of individuals being tested