Section 2: Carbohydrates - FNH 350

What is the role of carbohydrates in human nutrition?

• main source of energy in the average human diet

• supply half or more of the total caloric intake

Describe the major classes of carbohydrates

• simple CHO

  • mono-

  • di-

• complex CHO

  • oligo-

  • poly-

Describe monosaccharides

• simplest sugar

• one sugar unit

• most important in human nutrition

Why are carbs optically active?

they have 1 or more chiral carbon atoms in the molecule

Describe pentose

• little source of dietary energy

• transformed into metabolically important compounds

Describe disaccharides

• two sugar units

Which type of glucose can’t be digested? L or D?

L-glucose is not phosphorylated

Describe oligosaccharides

• made of 3-9 sugar units

• they must be digested to be used

• no human enzyme available to absorb

  • need intestinal bacteria to digest them

Describe dextrins

• can be oligo- or polysaccharides

• made of GLU

• from hydrolysis of starch or glycogen

• used as thickeners

What is starch?

• polysaccharide

• exists in the form of amylose + amylopectin

What bonds are in amylose?

• held by α-1,4 glycosidic bonds

• linear

What bonds are in amylopectin?

• held by α-1,4 and α-1,6 bonds

• α-1,6 bonds makes branches

What is glycogen?

storage form of GLU (liver, skeletal muscle)

What is cellulose?

• fibrous

• not digestible

• dietary fibre

• not energy source

• has a role in the speed of digestion

What are food sources of simple carbs?

Mono-

GLU → honey

FRUC → fruits

GALAC → doesn’t occur in isolated form in our diet

Di-

LAC → dairy

SUC → table sugar

MAL → grains

What are food sources of complex carbs?

Oligo-

Raffinose, stachyose, verbacose → grains, legumes, potatoes

Poly-

Starch, glycogen → plants, animals

Cellulose → vegetables

What is the carb intake in Canada?

• 50% of the diet

• ↑ age, ↓ CHO intake

What is the sugar intake in Canada?

↓ added sugar intake throughout the years

What are free sugars?

mono- and di- added to foods by the manufacturer, cook, or consumer + sugars naturally present in honey, syrups, fruit juice concentrates, and fruit juices

What are added sugars?

Sugars added to food and drinks during food processing

What’s the difference between unrefined and refined cards?

unrefined:

• whole grains, beans, fruit, vegetables

• original, nutrient dense

refined:

• simple sugars, processed grains

• limited natural fibre and nutrients

Which organs are involved in carb digestion?

• oral cavity / salivary glands

• pancreas

• small intestine

What enzymes does saliva have?

• α-1,4 amylase aka glycosidase hydrolyzes the α-1,4 glycosidic bonds

• optimum pH 6.9

• break randomly into smaller chains

What enzyme does the pancreas make? Where does it get released?

• pancreatic α-1,4 amylase hydrolyzes the α-1,4 glycosidic bonds

• released in small intestine

What else does the pancreas release? Why?

• pancreatic bicarbonate

• to increase pH so amylase is in optimum pH

Describe pancreatic α-1,4 amylase

• fast and efficient enzyme

• increased amount released after eating a high-carb diet

• secretion is regulated by cholecystokinin

What happens in the intraluminal phase?

• most complex carbs are broken into simple sugars

• digestion of starch and dextrins in duodenum occur in the interior of the lumen

What does the brush border contain?

oligo-saccharidases = glycoprotein enzymes

What are the enzymes that are in the brush border to break disaccharides?

• Maltose enzyme - maltase → GLU + GLU

• Isomaltose enzyme - isomaltase → GLU + GLU

  • Isomaltase breaks the alpha-1,6 bonds

• Lactose enzyme - lactase → GLU + GALAC

• Sucrose enzyme - sucrase → GLU + FRUC

Using active transport, how does GLU/GALAC go from the lumen, through the mucosal cell, and into the blood stream?

• Sodium-glucose transporter 1 (SGLT1) binds to Na+ which changes the conformation. Glucose or galactose binds to SGLT1 and enters the mucosal cell

  • ATPase takes out Na+ to regulate Na+ levels and for SGLT1 to keep working

• 15% of GLU/GALAC will leak back into the lumen

  • Can be absorbed again at another mucosal cell

• 25% will enter blood stream by simple diffusion

• 65% will enter blood stream by facilitated diffusion with GLUT2

Using no energy, how does GLU/GALAC go from the lumen, through the mucosal cell, and into the blood stream? Why doesn’t this need energy?

• GLUT2 will transport GLU/GALAC via facilitated transportation

• The protein binds to GLU/GALAC, changes conformation, pulls it to the other side

How does FRUC go from the lumen, through the mucosal cell, and into the blood stream?

• GLUT5 will transport FRUC via facilitated transportation into the mucosal cell

• GLUT2 will transport FRUC via facilitated transportation into the blood steam

How do glucose, galactose, and fructose move?

down the concentration gradient

After leaving the mucosal cell, where does GLU/GALAC and FRUC go? What happens then?

• goes to the liver via portal vein

• taken up by hepatocytes to be phosphorylated

What is the PH issue with increased FRUC intake? How can you help this?

• not everyone can absorb FRUC well

• ↓FRUC ↑GLU to reduce symptoms

Explain the GLUT1 deficiency syndrome

• rare genetic disorder

• less GLUT1

• Transports GLU through blood brain barrier so brain doesn’t get enough glucose

• cognition, behaviour, movement symptoms

What issues happen when there is a problem with GLUT4?

metabolic conditions

What is the healthy range for blood [GLU] before and 2 hours after a meal?

4 - 5.4 mmol/L

up to 7.8 mmol/L 2 hours after a meal

What do you not want to see blood [GLU]?

prolonged shifts in elevated GLU levels

How do you measure blood GLU?

acute parameter:

• fast before measuring

• check plasma glucose concentration with glucometer

long-term parameter:

• check if Hb have been glycosylated

• don’t want high levels of Hb that have been glycosylated

• HbA1c

What is hyperglycemia? What are the acute symptoms and chronic symptoms?

• blood glucose levels above normal range

• dehydration

• glycosylation of proteins, organ damage

What is hypoglycemia? What are the acute symptoms and chronic symptoms?

• blood glucose levels below normal range

• fatigue, dizzy, unconscious, seizures

• brain damage

What is the pathway when blood glucose is elevated?

• pancreas release insulin

• glucose is transported to cells + conversion of glucose to glycogen

What is the pathway when blood glucose is low?

• pancreas release glucagon

• breakdown glycogen to glucose + increased synthesis of glucose

What is glycogenolysis?

glycogen breakdown

What is gluconeogenesis?

glucose synthesis

What is glycogenesis?

glycogen synthesis

Explain glycemic index and how it is calculated

Shows how quickly and how much blood glucose levels increase

• Check increase in a 2-hour period after eating 50g of CHO compared to sugar or white bread

• GLU = area under the curve (AUC) for 50g

• GI = (AUC apple / AUC glucose) x 100

What are the numbers/categorization of the glycemic index?

• Low: ≤ 55

  • Oatmeal, sweet potato, fruits

• Moderate: 56 - 69

  • Whole wheat, brown rice

• High: ≥ 70

  • White bread, cereal, white rice

Explain glycemic load and how it is calculated

Shows how quickly and how much blood glucose levels increase considering the standard serving size

• Considers quantity and quality of CHO in a meal

• GL = GI x (CHO g / serving g)

What are the numbers/categorization of the glycemic load?

• Low: < 10

  • Best food choice 

• Moderate: 10 - 20

  • Moderate effect on blood GLU

• High: > 20

  • May lead to spikes in blood GLU and insulin