metabolism ii

Incremental Exercise and Metabolism

🫁 VO2max: Aerobic Power Explained

VO2max is the maximum ability to draw oxygen into the body, deliver it to working muscles, and utilize that oxygen to generate ATP.

Here is an image of a VO2 max graph.

VO2max is difficult to achieve in untrained adults. The reasons people stop exercising are more related to metabolites and muscle fatigue.

While VO2max broadly correlates with athletic performance, it isn't the only factor. Elite athletes can have average VO2max test results on different machines.

To ensure a true VO2max is reached during testing, look for these criteria:

1. Plateau in O2 uptake: Less than 2.1 ml/kg/min increase in VO2 from one stage to the next.

2. R > 1.10: Where R is the respiratory exchange ratio.

3. HR max within 10 bpm: Heart rate max should be within 10 beats per minute of age-predicted HR max (220 - age).

4. RPE 18-20: Rating of perceived exertion on a scale.

5. Post-exercise Blood Lactate > 8 mmol/L

If less than 4 out of the 5 criteria are reached, it is considered VO2 peak. All criteria must be met for a true max test.

젖산 Lactate Threshold: A Better Assessment

Lactate threshold is a more helpful assessment of metabolism and exercise for athletes.

Here is an image of a lactate threshold graph.

• Appears at approximately 50 to 60% VO2max in untrained subjects.

• Occurs at higher work rates (65 to 80% VO2max) in endurance-trained subjects.

The lactate threshold is also referred to as:

• Anaerobic threshold

• Onset of blood lactate accumulation (OBLA)

• Exercise intensity at which blood lactate levels reach 4 mmol/L

An increase in exercise intensity or duration leads to the accumulation of lactate in the blood. Exercise training delays the onset of the lactate threshold.

Here is an image showing the relationship between lactate and metabolism.

Factors affecting the Lactate Threshold:

• Reliance on glycolysis

• Low Tissue Oxygen (Hypoxia)

• Too much NADH/Not enough NAD: Shuttles that move NADH from the sarcoplasm into the mitochondria get overwhelmed, and we need NAD

• Contribution of fast-twitch muscle fibers: Unable to deliver sufficient oxygen to working muscle leads to Anaerobic glycolysis.

• Reduced lactate clearance from the bloodstream: Tissues involved in clearance (Liver) get busier.

The conversion of Pyruvate to Lactate via Lactate Dehydrogenase (LDH) is REVERSIBLE.

• Slow-twitch (Type I) muscle, cardiac muscle, and lactate-removing tissues (Liver) have a type of LDH that favors the conversion of Lactate to Pyruvate.

• Fast-twitch (Type IIa, Type IIx) fibers have a type of LDH that favors the conversion of Pyruvate to Lactate.

Here is an image that shows the factors that influence lactate threshold.

🏃‍♀ Lactate: The Misunderstood Energy Source

Lactate is not a bad guy, it is an energy source! While Pyruvate is STUCK inside the muscle, Lactate can get out and move around via the Lactate Shuttle.

The Lactate Shuttle moves lactate from Type IIx fibers (fast twitch) to slow-twitch fibers, where it is converted to pyruvate for ATP production.

Here is a diagram of the lactate shuttle.

Here is an image of a muscle cell.

There is no such thing as lactate receptors. Lactate is cleared from the blood within 60 minutes of exercise.

It was once assumed that lactate causes soreness, but that is incorrect.

Training at or slightly above the Lactate Threshold has huge benefits, acting as the point at which there is enough overload of the aerobic system to induce positive adaptations.

🧪 Measuring Fat and CHO Use

The Respiratory Exchange Ratio (RER, or R) is used to calculate the ratio of carbon dioxide expired to oxygen inspired:

R=VCO2VO2R=VO2​VCO2​​

The RER utilizes bioenergetics of fat and carbohydrates (CHO) and measurements of carbon dioxide expired and oxygen inspired.

The RER of Glucose:

R=VCO2VO2=6CO26O2=1.00R=VO2​VCO2​​=6O2​6CO2​​=1.00

The RER of Fat:

R=VCO2VO2=16CO223O2=0.70R=VO2​VCO2​​=23O2​16CO2​​=0.70

Here's a table to show the relationship between R value, % of Fat, and % Carbohydrate:

Values range from 0.70 (low intensity, 100% Fat oxidation) to 1.00 (high intensity, 100% CHO oxidation). An R value of 0.85 indicates a 50:50 split between fat and CHO being used as fuel.

The RER is an indicator of what substrate (CHO/Fat) is being used at various exercise intensities. In a steady state, it is reflective of O2 and CO2 exchange in the tissues.

Protein is ignored as it only plays a small role.

⛽ Fuel Selection During Exercise

Low-intensity exercise (<30% of maximum aerobic capacity) results in a reliance on fat as the primary fuel source. High-intensity exercise (>70% of maximum aerobic capacity) leads to a reliance on carbohydrate as the primary fuel source. As exercise intensity increases, there is a point where you 'Cross-over' and begin to use more carbohydrate than fat.

Three factors contribute to how fuel is regulated and selected:

• Exercise Intensity

• Exercise Duration

• Fuel Availability

The vast majority of bioenergetic fuel regulation/selection comes down to the interplay between Carbohydrate and Fat. Protein is the credit card of bioenergetics, used sparingly because you have to pay it back.

The Crossover Point is not fixed and is highly responsive to training, making it one of the most important exercise training adaptations.

The longer the duration of exercise, the more exaggerated the intensity response becomes, shifting the % bioenergetic use towards fat.

While Fat supplies are basically unlimited, Carbohydrate supplies are DEFINITELY NOT.

Why does this crossover happen? It's because carbohydrates provide access to ATP faster via muscle glycogen, and more fast-twitch muscle fibers are recruited.

Carbohydrate supplementation introduces CHO into the bloodstream (glucose), which is then delivered to and taken up by working muscle.

It is absolutely possible to run out of carbohydrate, which is a significant contributor to fatigue and poor performance. Consume 60 grams/hour if performing:

• Continuous, submaximal intensity (60-70% max) activity for longer durations (>90 minutes).

• Continuous, high intensity (>75% max) activity for longer than 45 minutes.

Carbohydrate Supplementation leads to:

• Reduced rate of glycolysis and production of pyruvate.

• Reduced citric acid (Krebs) cycle intermediates.

• Reduced fat oxidation.

• Fats are metabolized by the Krebs cycle.

• "Fats burn in the flame of carbohydrates."

• Glycogen is depleted during prolonged high-intensity exercise.

Here is a graph of the relationship between energy expenditure and exercise.

🏋 Specific Sources of Steady-State CHO and FAT

There are four sources of CHO and FAT in the body that can be used for exercise:

• Muscle Glycogen (CHO)

• Intramuscular Triglycerides (FAT)

• Circulating Free Fatty Acids from Adipose tissue (FAT)

• Circulating glucose from Liver glycogen and diet/supplementation (CHO)

Longer Duration shifts the contribution away from the muscle and towards the other areas.

Here is a graph showing the relationship between fuel source and exercise.

Intensity and Duration of Exercise impact fuel selection within CHO and FAT sources in a fairly predictable way.

💪 Exercise Training and the Crossover Concept

Exercise training shifts the crossover point to the right. It allows you to utilize fat more efficiently at higher intensities of exercise. It also shifts in fuel utilization from CHO to fat, sparing Muscle Glycogen for when you really need it.