Aerobic Metabolism: An Overview

Aerobic Metabolism

• Chapter 3 focuses on aerobic metabolism.
• This unit aims to describe aerobic metabolism, explain oxygen and mitochondria's roles, factors influencing substrate use, oxygen consumption changes, metabolic recovery after exercise, and adaptations enabling greater ATP production.

Initial Energy Systems

• The first 30 seconds of exercise rely on the ATP-PC system, which depletes quickly.
• Anaerobic glycolysis provides energy for the next 2-3 minutes post-ATP-PC usage.
• If exercise extends beyond the ATP-PC system, other systems activate.
• When other systems are involved there is an Increased contribution from glycolysis.

Glycolysis Explained

• Glycolysis involves breaking down glucose or glycogen into pyruvate which is a 3 carbon molecule.
• Anaerobic glycolysis is fast and does not use oxygen whereas aerobic glycolysis slower and uses oxygen.

Key Substrates in Aerobic Metabolism

• Substrates include carbs (glucose/glycogen), fats (fatty acids/triglycerides), proteins (amino acids), and lactate.

Aerobic Metabolism Byproducts

• Carbon dioxide (CO2), which is expired by the lungs, and water (H2O) are byproducts of aerobic metabolism.

Initial Steps of Carbohydrate Metabolism

• Aerobic metabolism of carbohydrates starts with glycolysis.
• Glycolysis occurs in the sarcoplasm.
• Glucose from the blood and glycogen from muscle are used

Blood Glucose Regulation

• Liver glycogenolysis is the breakdown of glycogen to glucose and involves glucagon, epinephrine, and norepinephrine.
• Insulin facilitates glucose uptake by muscle, but its role changes during exercise.
• Muscle contraction during exercise helps glucose enter muscle cells even when insulin is suppressed.

Glycogen Breakdown

• Glycogen phosphorylase cleaves off glucose from glycogen during glycolysis.

Location of Key Processes

• Glycolysis occurs in the sarcoplasm.
• Acetyl CoA production, the Krebs cycle, and the electron transport chain occur in the mitochondria.

Fat Metabolism

• Fat metabolism starts with beta oxidation.
• Lipase enzymes break down stored triglycerides to glycerol, and 3 fatty acids, which are facilitated into the muscle.
• Long-chain fatty acids break down into two-carbon segments to form acetyl CoA.

Acetyl CoA Synthesis

• Fatty acids and glycerol can be synthesized from glucose and acetyl CoA, with implications for excess protein or carbohydrate consumption.

Protein's Role

• Amino acids also contribute as a protein substrate to Aerobic Metabolism
• Amino acids undergo deamination and transamination.

Common Metabolic Pathway

• Acetyl CoA is a common pathway that can use Glucose, Glycogen, Fatty acids, and Amino acids
• Some ATP is produced at the Acetyl CoA phase, a bit more during Kreb's Cycle and lots during the Electron Transport Chain

Krebs Cycle

• The Krebs cycle's function is to remove hydrogen ions (H+), transported to the electron transport chain (ETC) by carrier molecules NAD and FAD.
• It occurs in the mitochondrial matrix.
• The cycle starts and ends with oxaloacetate, producing a small amount of ATP (1 ATP per acetyl CoA).
• For every acetyl CoA molecule, these are the key end-products: 2 CO2 is expired, 3 NADH goes to the electron transport Chain, 1 FADH2: goes to the ETC, and 1 GTP/ATP that is used by the cell.
• Per glucose molecule these products are doubled, and there are more per fatty acid multiplied by the # Acetyl CoA

Electron Transport Chain Function

• The electron transport chain produces the majority of ATP during aerobic metabolism.
• This process is also known as oxidative phosphorylation.
• NAD+ regenerated for glycolysis and the Krebs cycle.
• O2 from the air acts as the final H+ and electron acceptor at the end of electron transport chain
• It takes place in the inner mitochondrial membrane.
• This produces energy: e- and H+ move during this process
• Oxygen is the final electron acceptor, with electrons needing an empty complex ahead to facilitate movement along the chain. A lack of oxygen means ATP can't be produced
• Energy from electron transport pumps H+ from the inner to outer mitochondrial compartment, creating a gradient.

ATP Production

• The total ATP production varies based on the substrate and pathway.

Lactate's Role

• Lactate is produced during high-intensity exercise, and the Cori cycle converts it back into glucose.
• Lactate can be used by other tissues to synthesize glycogen or convert to pyruvate.

Substrate Interactions

• Substrate use during exercise depends on availability, amount of energy produced, and the intensity and duration of the activity.
• The percentage of energy from carbohydrates increases with exercise intensity in contrast with fats
• Anaerobic and ATP-PC systems primarily use glucose.
• Aerobic metabolism primarily uses a mixture of carbohydrates and fats, with its preference influenced by intensity and duration.

Increased Carbohydrate Metabolism

• Carbohydrate metabolism is more efficient.
• Fast-twitch muscle fibers are recruited, having a high presence of gkycolytic enzymes and a low aerobic enzyme count
• Glycolic enzymes are increased via epinephrine.
• Lactate inhibits fat metabolism by reducing triglyceride use.availability.

Substrate Utilization Factors

• Utilization depends on both continuous dynamics.
• Fat is primarily used for Low Intensity, Long Duration exercises
• Carbohydrates are primarily used for High Intensity, Short Duration exercises
• Protein use is low in intensity and Negligible in High Intensity exercise.

Duration's Influence

• Fuel storage influences exercise capacity, shifting utilization when duration exceeds capacity.
• Muscle glycogen depletion depends on the individual and exercise type, often between 60 minutes and 2 hours.
• For Low intensity, long duration activities there is a gradual shift form carb to fat metabolism because the body uses carbs first before switching to fats when carb stores run low
• During this shift, levels of epinephrine, norepinephrine, glucagon increase during exercise.
• Insulin inhibits hormone sensitive lipase, resulting in lowered available fatty acids. with less insulin leading to less inhibition of lipase, and more available fatty acids.
• The decrease in carbohydrate stores leads to increased fat use but the rate of energy is slowed down - known as "hitting the wall" and carb stores being fully depleted, resulting in CNS fatigue and increased RPE.

Carbohydrate Availability and Performance

• Lower carbohydrate stores lead to increased use of fats but at a lower rate of energy supply.
• Carbohydrate loading can help with events lasting over 90 minutes and calls for a consumption of 10-12g/kg of carbohydrates per 24 h for 36-48 h prior
• Muscle glycogen will return with a high carb diet vs a low carb one

Lactate Threshold

• Lactate threshold is another predictor of performance and the exercise intensity at which blood lactate exceeds resting concentration.
• The lactate threshold is higher in trained individuals as opposed to untrained
• It Indicates the point at which lactate production exceeds clearance as higher Lactate Threshold suggests more capacity.
• The untrained levels being at 50-60% VO2Max vs trained levels at 65-80%VO2Max
• The increased accumulation of H+ impacts performance with increased acidity impairing muscle contraction and also effecting enzyme function

Lactate Threshold vs OBLA

• Lactate threshold is the exercise intensity at which blood lactate exceeds resting concentration.
• OBLA is the intensity at which blood lactate exceeds 4 mmol/L

Metabolic Recovery

• Post-exercise, intramuscular PC stores need to be resynthesized, and intramuscular and blood acidity needs to be reduced.
• This increases elevated breathing rate and HR
• O2 is needed for above as aerobic metabolism helps recover from exercise

O2 Deficit and EPOC

• O2 deficit is the difference between O2 consumed and the amount needed if aerobic metabolism could fully meet energy demands.
• Steady-state O2 consumption occurs when all energy needs are met aerobically.
• Excess post-exercise oxygen consumption (EPOC) is when oxygen uptake remains above resting levels after exercise.
• A higher exercise intensity translates into a higher EPOC.
• Active recovery peak lactate will be lower and sooner vs passive recovery and needs to be exercised below lactate threshold

Active Recovery

• Active recovery should be undertaken below lactate threshold.

• As the Lactate is being used for ATP production

• Cycling = ~30-45% VO2Max

• Running = ~55-60%VO2Max

• An active recovery might not be optimal if the person is too tired, or injured

Metabolic Adaptations

• Greater Mitochondrial Density and Mitochondrial Enzyme Activity, that both increase ATP
• The need for greater capacity, substrate and ATP synthesis can also improve performance
• Muscle glycogen stores also assist with this
• Intramuscular trylycerides also increase allowing for more aerobic triglyceride metabolism

Aerobic Adaptations

• The increase in relative intensity and decrease in the reliance of lactate promotes better performance and allows one to uphold the above for a longer time period

Aerobic Exercise Adaptation

• Ability to metabolize lipids.
• Increased enzymatic activity in the Krebs cycle and ETC.
• Increase in capillary density

Energy Systems in Sports

• All energy sources are used at some point and depend on the activity
• ~ 8-10 Second activity will require high Intensity, ATP-PC (100m Sprints)
• 10-30 Second activity will require high Intensity, ATP-PC and anaerobic glycolysis (200m Sprints)
• ~ 30 Second activity to 2/3 min will require High and anaerobic glycolysis (Hockey shift)
• 3+ min = Lower to moderate and aerobic metabolism. (Running, cycling)

Measuring Aerobic Metabolism

• Can be done though Calorimetry or Spirometry
• The O2 Consumption is measured and is directly proportional to aerobic ATP.
• O2 can be seen in both L/min or ml/kg/min with "L/min: Not Accounting for a person's body weight"
• ml/kg/min measures accounting for persons body weight
• Respiratory Exchange Ratio = VCO2/VO2 and is used of oxygen to measure carbon dioxide in the body and the relationship between them.

RER Insights

• There will likely be a mixture of all forms of CHO and TG forms,
• A lower RER will translate to a larger amount the body gets out of fat

Can the RER Exceed 1

• The measure is assumed to Max out at 1.0.
• When the intensities are higher, and require more then aerobic metabolism.
• It can lead to high accumulation levels with the max being at ~1.0
• The use of High intensity exercise will translate to bicarbonate buffering system

200 and 400m Sprints

• Will need rely on anaerobic sources to start
• The transfer from anaerobic to aerobic metabolism is ~ 15 to 30sec

800m

• Will need Percentage of the energy produced because as time goes on
• Will require Aerobic activity

1500

• Endurance events can sometimes increase during periods of anaerobic state (Such as uphill) or during short periods