EXERCISE INTENSITY & DURATION: SUBSTRATE UTILIZATION (LIPIDS)

Question 1

Outline the dynamics of carbohydrate and fat metabolism during physical activity of various intensities and durations

Answer 1

Low-intensity: Mostly uses fat for energy

Moderate-intensity: Uses a mix of carbs and fat
High-intensity: Primarily uses carbs (from muscle glycogen)

Long-duration: Uses fat as main energy once glycogen depletes

Question 2

Outline the 3 major energy producing systems in cells

Answer 2

ATP-PCr: Immediate energy for short, intense activity (e.g., sprints)

Glycolysis: Breaks down glucose for energy, supports moderate-high intensity

Oxidative (Aerobic): Uses oxygen for long-term, lower-intensity energy (fat, carbs)

Question 3

Understand how fatty acids enter the mitochondria

Answer 3

Fatty acids enter cells, bind to CoA, and use the carnitine shuttle to enter mitochondria for energy production

Question 4

Know the primary lipolytic regulators and how they
change with exercise

Answer 4

Hormone-sensitive lipase (HSL): Breaks down fat (activated by epinephrine, cortisol)

Insulin: Lowers fat breakdown (suppresses lipolysis after meals)

Exercise increases HSL, burning more fat

Question 5

Understand the role of AMPK on regulating substrate use

Answer 5

AMPK activates when energy is low. It helps burn fat, take in glucose, and slow down energy-wasting processes

Question 6

Contrast protein with the other macronutrients regarding fuel use and storage

Answer 6

Carbs: Stored as glycogen, fast energy

Fat: Stored as triglycerides, slow, long-term energy

Protein: Used for muscle repair, not a main energy source unless needed

Question 7

Contrast the speed of energy transfer from carbohydrate

Answer 7

Carbs: Quick energy for high-intensity efforts

Fat: Slower energy, used for long, low-to-moderate intensity activities

Question 8

Summarize the alanine-glucose cycle

Answer 8

Muscles release alanine (from protein breakdown) during exercise

The liver turns it into glucose to keep blood sugar steady

Question 9

Why is the Ratio of ATP to ADP/AMP critical to the energy status of a cell?

Answer 9

Ratio is 500 ATP:1 ADP/AMP

So this means a small change in ATP is a large change in ADP/AMP

This allows cells to detect changes in energy levels quickly. If ATP breaks down, ADP/AMP levels rise, signaling the cell to make more energy.

The cell adjusts which energy sources it uses depending on how fast it needs ATP: if energy is needed slowly, the cell uses fat (lipids), but if it needs quick energy, it uses glycogen.

Question 10

What is the lipolytic rate and how is it impacted by exercise?

Answer 10

At rest, lipids are used

Lipolysis: contributes to ATP during exercise
- indicated by the rate of glycerol appearance in the bloodstream

As duration of exercise increases, the contribution of lipids to generating ATP increases

Positive correlation

Question 11

What happens to substrates with increasing exercise duration? there is a gradual switch in fuel from carbohydrate to lipids

Answer 11

there is a gradual switch in fuel from carbohydrate to lipids

Question 12

How do fatty acids stored in triglycerides (adipose) get to the muscle mitochondria?

Answer 12

1. Lipolysis: breaking fatty acids down in adipocytes through a lipase enzyme
2. Release fatty acids to bloodstream
3. Fatty acids to skeletal muscle
4. Go thru membrane (sarcolemma) into cytoplasm aided by fatty acid transporters (CD36)
5. CPT-1 enzyme gets fatty acids from cytoplasm to mitochondria

Question 13

What does insulin do for carbohydrates and lipids?

Answer 13

Inhibits lipolysis

Promotes carbohydrate oxidation

Reciprocal regulation!!

Question 14

What are the regulators in lipolysis?

Answer 14

Catecholamines stimulate and inhibit lipolysis

Insulin inhibits lipolysis

Question 15

What does glycerol appearance in the bloodstream indicate?

Answer 15

Lipolysis!! / the breakdown of lipids

It increases with exercise, fasting, stress, and low-carb diets

The liver can use glycerol to make glucose (gluconeogenesis) or rebuild triglycerides

Question 16

Why does insulin decrease during exercise? What happens when plasma insulin increases?

Answer 16

insulin stores glucose, so when exercising, the body secretes insulin to use the glucose for energy

As plasma insulin levels increase, the lipolytic rate decreases because it promotes the use of carbohydrates (since they are a faster source of energy production)

Question 17

Know the proportions of substrates at each exercise intensity (i.e. 50% carbs and 50% lipids at 65%)

Know the absolute values of the calories that are burned per minute at each exercise intensity

And also relative values?

Answer 17

relative:
low - 90% Fats, 10% CHOs
moderate - 50/50
high - 20% Fats, 80% carbs

??

Question 18

Steps of lipolysis (how do we communicate to our fat storing cells that we need more fatty acids and glycerol to make more ATP)?

Answer 18

1. Catecholamine (EPINEPHRINE!!) binds Beta-androgenic receptors on the cell membranes of adipocytes
2. Cyclic AMP + protein kinase activate hormone sensitive lipase
3. Lipase results in cleavage of triglycerides; release fatty acid 3 times also releases glycerol backbone

Visceral fat > Subcutaneous fat (contributes more greatly to this process) MORE exercise responsive (during Beta binding!)

NOTE Catecholamine binding to ALPHA-androgenic receptors INHIBITS lipolysis and SubQ>visceral because LESS exercise responsive

Question 19

FAT/CD36 Translocation

Answer 19

1. Muscle contractions activate AMPK
2. AMPK signals FAT/CD36 to move toward the membrane
3. FABP binds to FAT on the membrane (allows fatty acids to be taken up by receptors

Note the fatty acids are now in the muscle, BUT they get OXIDIZED in the mitochondria!!

Enhanced by exercise

*KNOW that AMPK helps moderate this process

*Insulin slows down FAT because it promotes carbohydrate oxidation

Question 20

How does fatty acid transport get into the mitochondria?

Answer 20

1. In the cytosol, the fatty acids is converted into Acetyl-CoA
2. Transported in by CPT-1
3. Converted to fatty acyl-carnitine
4. Then translocated into mitochondria
5. fatty acyl-carnitine converted back to acetyl-CoA THROUGH CPT-II to be broken down
6. Beta oxidation

*Regulated by ATP demand via exercise intensity!! Maximized at 65%

Question 21

How many carbons in fatty acids? Acetyl-CoA?

Answer 21

16-20 ; 6

Question 22

Beta oxidation

Answer 22

The process of breaking down fatty acids into 2-carbon length segments

Occurs in the MITOCHONDRIA!

Question 23

Where do most processes occur?

Answer 23

Mitochondria

Only Glycolysis and ATP-Pcr are in the cytosol (double check)

Question 24

Relative contribution of plasma fatty acids and IMTG at varying exercise intensities

Answer 24

25% - mostly plasma fatty acids (lipids)

65% - half IMTGs half fatty acids (greatest amount of fatty acid oxidation)

85% - levels decrease bc mostly used by CHOs

Question 25

Reciprocal regulation of substrates: high glycolytic flux (fed state)

Answer 25

High CHO demand! to make ATP quickly; producing lactate, pyruvate, and Krebs cycle intermediates (Malonlyl-CoA!!) Malonlyl CoA inhibits CPT-1 which in turn inhibits fatty acids from getting into the mitochondria

Question 26

Reciprocal regulation of substrates: fasted state

Answer 26

Breakdown of fatty acids can keep up with ATP being demanded by contracting muscles (not limiting) high levels of *CITRATE blocks glucose metabolism Glucose and glycogen are spared

Question 27

Why is the storage of triglycerides in muscle beneficial to exercise performance? When would you expect these intramuscular triglycerides to be heavily utilized?

Answer 27

IMTGs! Storing lipid in muscle is beneficial for endurance since having fat next to the site of oxidation (the mitochondria) during moderate intensity exercise helps Utilization is highest at ~65-75% VO₂max, where fat oxidation is maximized IMTGs help spare glycogen, prolonging exercise performance

Question 28

What are ketones? Can we use them for ATP production? Where do they come from?

Answer 28

* Recovery & adaptation Carbon containing molecules that can be transformed into energy (ATP) producing substrates Increase production during prolonged exercise and exercise in the fasted condition Ketones can also fuel the CNS when blood glucose is low

Question 29

Is protein a substrate for ATP production during endurance exercise? If so under what circumstances or context?

Answer 29

No unless glycogen is depleted such as during prolonged exercise or low insulin levels

Question 30

What are the mechanisms that take us from 25% intensity (mainly lipid) to 65% (50/50)?

Answer 30

Accumulation of AMP and activation of AMPK, which drives increased GLUT4 translocation to the membrane and stops glycogen synthesis and promote glycolysis (AMPK) Also helps activate lipid metabolism

Question 31

What amino acid is important during exercise

Answer 31

Alanine (CH3 side chain) because it is used when making glucose It is a byproduct used to make glucose Alanine glucose cycle!!

Question 32

Alanine glucose cycle

Answer 32

Protein breakdown to produce more glucose to fuel muscle contraction Adding an amine to pyruvate creates alanine 1. During prolonged exercise or fasting, muscle protein undergoes catabolism (breakdown) to release amino acids 2. **Pyruvate can undergo a transamination reaction where it is converted into alanine; This reaction involves the transfer of an amino group from glutamate (another amino acid) to pyruvate. 3. Alanine is released from he muscles into the bloodstream and travels to the liver 4. Alanine undergoes deamination, meaning it loses its amino group resulting in the formation of pyruvate, which can enter the **gluconeogenesis pathway 5. Pyruvate is converted into glucose

Question 33

What happens to protein synthesis and degradation during exercise?

Answer 33

Generally protein synthesis is suppressed during exercise, while protein degradation increases.

Question 34

What happens to protein synthesis and degradation during recovery from exercise?

Answer 34

During recovery following exercise, muscle protein synthesis increases and degradation decreases

Question 35

What is important to consider about protein metabolism?

Answer 35

Nitrogen balance is important to consider, athletes may have to adjust protein intake based on training regimen

Question 36

What factors modify protein metabolism?

Answer 36

diet, hormones, age, etc.

Question 37

Why does the contribution of carbohydrate increase with increasing exercise intensity? (List as many reasons/mechanisms as you can that might contribute to these phenomena)

Answer 37

- *rate of CHO breakdown is 2x faster than lipids - AMPK activates pathways that promote CHO breakdown - reciprocal regulation (byproducts such as Malonlyl CoA that increase while breaking down CHOs and limit fatty acid oxidation)

Question 38

What are some of the KEY factors responsible for regulating fat oxidation during exercise? (and how do they work to control fat oxidation?)

Answer 38

intensity & duration ! also epinephrine, AMPK, reciprocal regulation (citrate)