Carbohydrate Metabolism

Question 1

Why are carbohydrates the preferred fuel in high-intensity exercise?

Answer 1

• More efficient than fat (uses less oxygen)
• Main fuel source during high-intensity exercise

Question 2

What limits the use of carbohydrates during exercise?

Answer 2

Limited muscle and liver glycogen stores

Question 3

How do athletes manage carbohydrate availability?

Answer 3

• Maximise glycogen stores by fuelling before and refuelling after exercise
• Use exogenous carbs (like energy bars) during exercise

Question 4

What happens to muscle glycogen at 30% VO₂max?

Answer 4

• Mostly type I muscle fibres used
• Glycogen depletion is slow, stores remain high in Type II fibres

Question 5

How does muscle glycogen change at 60% VO₂max?

Answer 5

• Both type I and type II fibres recruited
• Glycogen use increases and plateaus as exercise continues

Question 6

What occurs to muscle glycogen during 120% VO₂max (maximal effort)?

Answer 6

• Heavy use of both type I and II fibres
• Rapid glycogen depletion causes early exercise termination

Question 7

How does a high-carb diet affect muscle glycogen and exercise duration + what’s this relationship?

Answer 7

Increases muscle glycogen stores + extends exercise performance time

• Linear

Question 8

What is glycogen supercompensation?

Answer 8

After low glycogen levels, a high-carb diet leads to greater glycogen uptake and storage

Question 9

How does exercise intensity affect fuel source preference?

Answer 9

Higher intensity → more reliance on muscle glycogen and blood glucose

• Less fat oxidation due to higher oxygen cost

Question 10

Why is fat less efficient than carbohydrates as a fuel?

Answer 10

Requires more oxygen for oxidation (~5.5% more than carbs)

Question 11

What are the three main monosaccharides?

Answer 11

Glucose, fructose, galactose

Question 12

Which monosaccharide is commonly used in sports nutrition and which uses a different transport system?

Answer 12

Glucose is in most sports products

Fructose uses a different intestinal transport system

Question 13

What are common disaccharides?

Answer 13

Maltose, sucrose, lactose

Question 14

What is maltodextrin and why is it used?

Answer 14

Oligosaccharide (8–12 glucose units) with a high GI

• Less sweet → more tolerable in sports nutrition

Question 15

What are the two types of polysaccharides?

Answer 15

Amylopectin (starch) and amylose

Question 16

Why does amylopectin have a higher glycaemic index than amylose?

Answer 16

Amylopectin has more branches → breaks down faster → higher GI

Question 17

Where does glucose come from in the fasted state?

Answer 17

From liver glycogen breakdown → released into blood

Question 18

What is the primary destination of glucose in the fasted state + where else does it go?

Answer 18

The brain, to support normal function

• adipocytes
• muscles

Question 19

Which glucose transporters are active in the fasted state?

Answer 19

Liver & brain: GLUT-2

Muscle: GLUT-4

Question 20

What happens to plasma glucose concentration in the fed state?

Answer 20

Increases from ~4–5.5 mmol/L to ~6–8 mmol/L

Question 21

Why does plasma glucose conc increase in a fed state?

Answer 21

Now getting an influx of glucose from the small intestine as we have ingested food

Question 22

How does insulin affect glucose uptake in the liver during the fed state?

Answer 22

Increases glucose uptake for storage

• Reduces glycogen breakdown

Question 23

How does insulin affect muscle glucose uptake?

Answer 23

Promotes translocation of GLUT-4 to cell membrane → increases glucose uptake

Question 24

What effect does insulin have on adipocytes?

Answer 24

Increases glucose uptake into fat cells

Question 25

Where is insulin secreted from?

Answer 25

Pancreas

Question 26

Is gastric emptying a limiting factor for carbohydrate use during exercise?

Answer 26

Rehrer et al. (1992) - found no difference in oxidation rates despite different gastric emptying speeds - Suggests gastric emptying is not the limiting factor

Question 27

What are the two key transporters for glucose absorption in the intestine?

Answer 27

SGLT-1: transports glucose with sodium into intestinal cells GLUT-2: transports glucose from cells into bloodstream

Question 28

What is the saturation limit of the SGLT-1 transporter?

Answer 28

Saturated at ~1.0–1.2 g/min - Limits glucose absorption to ~60 g/hour

Question 29

How can athletes increase carbohydrate intake beyond 60 g/hour during exercise - what's the ratio?

Answer 29

Add fructose, which uses a different transporter - Old = 2:1 (glucose:fructose) - New = 1:0.8 (e.g. 60g glucose : 48g fructose)

Question 30

Which transporter is responsible for fructose absorption into the intestine?

Answer 30

GLUT-5 moves fructose into the intestinal wall GLUT-2 transports it into the bloodstream

Question 31

What limits exogenous carbohydrate oxidation during exercise?

Answer 31

Saturation of transport proteins, mainly SGLT-1 - remember its at 1-1.2 g/min

Question 32

Why is using multiple transportable carbohydrates beneficial during exercise?

Answer 32

Allows use of both SGLT-1 (glucose) and GLUT-5 (fructose) - Increases total carbohydrate uptake without causing GI distress

Question 33

How does glucose enter muscle cells from the bloodstream?

Answer 33

Via GLUT4 transporters

Question 34

What happens to glucose after entering the muscle cell?

Answer 34

Rapidly phosphorylated by hexokinase into glucose-6-phosphate (G6P) - G6P then enters glycolysis or undergoes glycogenesis

Question 35

What's the concentration of glucose like in a myocyte (muscle cell)

Answer 35

Very low because its phosphorylated rapidly

Question 36

What triggers GLUT4 translocation to the muscle cell membrane?

Answer 36

Exercise and insulin both trigger GLUT4 movement towards cell wall to increase glucose uptake

Question 37

Briefly outline how insulin initiates GLUT4 translocation

Answer 37

1. Insulin binds to receptor → activates IRS-1 (insulin receptor substrate -1) 2. IRS-1 activates PI3-kinase (by creating a docking protein for it) 3. PI3K activates PDK, which activates Akt 4. Akt causes GLUT4 to translocate to membrane

Question 38

Which increases glucose uptake more—insulin or exercise?

Answer 38

Exercise increases uptake more due to much higher blood flow (but has low A-V difference) Insulin has a greater A–V difference, but lower blood flow - compensatory effect means that blood flow leads to greater increase in glucose uptake

Question 39

Does muscle glucose uptake limit exogenous carbohydrate oxidation?

Answer 39

No — Hawley et al. (1994) - Infusion of glucose to 10 mmol/L increased oxidation rates

Question 40

What is the possible + main limiting factor for exogenous carbohydrate oxidation?

Answer 40

- Gastric emptying DOES NOT limit - Muscle glucose uptake DOES NOT limit - Intestinal CHO absorption DOES limit

Question 41

How can exogenous carbohydrate oxidation be assessed during exercise?

Answer 41

Using stable isotope tracers (e.g. ¹³C-labelled carbs) in a drink - ¹³C mixes with endogenous stores (mostly ¹²C), and CO₂ samples are analysed to determine usage

Question 42

Why must athletes deplete carbohydrate stores before tracer studies?

Answer 42

To minimise endogenous ¹³C interference - Ensures all measured ¹³CO₂ comes from the drink, not stored glycogen or hidden ¹³C (e.g. from corn-based food)

Question 43

What technique analyses expired air in tracer studies?

Answer 43

Gas Chromatography Isotope Ratio Mass Spectrometry (GC-IRMS) - Measures δ¹³C: the ratio of ¹³C to ¹²C, reported in parts per thousand - Reference: Pee Dee Belemnite (PDB) standard

Question 44

What were the three test conditions in the stable isotope study of CHO intake?

Answer 44

1. Glucose at 1.2 g/min (rate-limiting) 2. Glucose at 1.8 g/min (above rate limit) 3. Glucose + fructose mix (1.2 g glucose + 0.6 g fructose = 1.8 g total)

Question 45

What did the tracer study reveal about exogenous CHO oxidation?

Answer 45

Glucose alone: no increase above 1.2 g/min → saturation + rate-limiting step Glucose + fructose: higher exogenous oxidation, as both transport systems (SGLT-1 and GLUT-5) are used

Question 46

What is glycogenesis and where does it occur?

Answer 46

The process of storing glucose as glycogen - Occurs in liver and muscle cells for future energy needs

Question 47

What conditions are required for glycogenesis to occur?

Answer 47

- Glucose must be in the cell - Insulin must be elevated to promote uptake - Key enzyme: Glycogen synthase

Question 48

What happens to liver and muscle glycogen after an overnight fast?

Answer 48

Liver glycogen slightly decreases Muscle glycogen remains relatively unchanged

Question 49

Outline the key steps in glycogen elongation

Answer 49

1. Glucose → G6P (via hexokinase) 2. G6P → G1P (via phosphoglucomutase) 3. G1P + UTP → UDP-glucose (catalysed by UDP-glucose pryophosphorylase) 4. UDP-glucose added to other glucose molecules by glycogen synthase

Question 50

Why is glycogen highly branched?

Answer 50

Can only be stored as branches of glucose polymers - More branches = more end points = faster breakdown - Enhances rapid access to stored glucose during exercise

Question 51

How are branches formed in glycogen?

Answer 51

1. Glycogen synthase only forms straight bonds 2. SO a branching enzyme breaks an α-1,4 bond 3. Reattaches part of the chain to form an α-1,6 glycosidic bond 4. Branching starts after ≥11 glucose residues in a chain

Question 52

Where does fructose metabolism occur and why?

Answer 52

In the liver, not muscle - Muscle lacks fructokinase, needed to convert fructose → fructose-1-phosphate

Question 53

Outline the metabolic pathway of fructose in the liver

Answer 53

1. Fructokinase → fructose-1-phosphate 2. F1P→ dihydroxyacetone phosphate (DAP) + glyceraldehyde by aldolase enzyme 3. Used for gluconeogenesis, glycolysis, or converted to fatty acids

Question 54

Why is fructose metabolism considered unregulated?

Answer 54

Fructose bypasses key regulatory steps (unlike glucose) - Leads to uncontrolled entry into pathways → ↑ FA production + triglyceride storage if consumed excessively - sport aspect = provides different way of getting carbs to muscles

Question 55

What happens when glucose is co-ingested with fructose?

Answer 55

Fructose-derived F1P shifts glucokinase receptor protein's affinity towards F1P and away from G6P - This frees up glucose phosphorylation, enhancing liver glycogen storage

Question 56

What are the approximate metabolic fates of fructose-derived triose phosphates?

Answer 56

50% → glucose (gluconeogenesis) 15–20% → glycogen 25% → pyruvate (oxidised / converted to lactate)

Question 57

How does insulin regulate glycogen synthase?

Answer 57

1. Insulin activates Akt 2. Akt inactivates GSK-3 (glycogen synthase kinase-3), reducing phosphorylation 3. Result: ↑ glycogen synthase activity

Question 58

How does adrenaline affect glycogen synthase and metabolism?

Answer 58

1. Activates PKA 2. PKA ↓ glycogen synthase (inhibits glycogenesis) 3. ↑ lipolysis and ↑ glycogenolysis

Question 59

How quick is muscle glycogen storage post ingestion?

Answer 59

Very quickly - 4 hours post ingestion to maximally store glycogen - due to increase glycogen synthase activity by insulin

Question 60

What is glycogenolysis?

Answer 60

The breakdown of glycogen into glucose - want to convert branched glycogen into a straight one

Question 61

What are the end products of glycogen breakdown?

Answer 61

~90% → glucose-1-phosphate ~10% → free glucose

Question 62

What enzyme initiates glycogen breakdown?

Answer 62

Glycogen phosphorylase cleaves α-1,4 bonds forming G1P

Question 63

How is G1P used after it's formed?

Answer 63

Converted to G6P by phosphoglucomutase - G6P used in glycolysis (muscle) or glucose metabolism (liver)

Question 64

What happens when only 4 glucose units remain at a branch point?

Answer 64

Breakdown stops - Glucan transferase moves 3 to the main chain - Debranching enzyme releases the last unit as free glucose

Question 65

What is the purpose of glycogenolysis during intense exercise?

Answer 65

Provides rapid energy, especially in events like 800m run or 2000m rowing - Highly active in skeletal muscle

Question 66

How does adrenaline stimulate glycogenolysis?

Answer 66

1. Adrenaline attached to receptor on cell membrane 2. Activates G-protein 3. Activates adenylate cyclase which converts ATP → cAMP 4. Activates PKA which activates phosphorylase kinase which activates glycogen phosphorylase 5. This converts glycogen to G1P

Question 67

How fast can glycogenolysis occur + why?

Answer 67

Within 1 second - cAMP cascade is slow, but... - Ca²⁺ acts as allosteric activator of phosphorylase kinase → speeds process