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Should you measure Lactate Threshold for Performance Gains?

What fuel source do you use at various exercise intensities?  How do we create energy?

uel sources and exercise physiology methods for creating ATP based on different exercise intensity zones:

What is ZONE two that we keep hearing about?  How do you know what YOUR current zone two is for you right now??

Zone two training, also known as aerobic base training or low-intensity steady-state (LISS) training, involves exercising at a moderate intensity where the body primarily relies on aerobic metabolism for energy production. This intensity typically corresponds to around 60-70% of maximum heart rate or a perceived exertion level where conversation is possible (i.e., you can speak comfortably while exercising).

Benefits of Zone Two Training:

  1. Improves Aerobic Capacity: Zone two training enhances the body’s ability to utilize oxygen efficiently, leading to improvements in aerobic capacity and endurance.
  2. Enhances Fat Metabolism: Training in zone two promotes the utilization of fats as a primary fuel source, which is beneficial for endurance athletes as it conserves glycogen stores and delays the onset of fatigue during prolonged exercise.
  3. Mitochondrial Adaptations: Zone two training stimulates mitochondrial biogenesis, increasing the number and density of mitochondria within muscle cells. This improves the capacity for oxidative phosphorylation, leading to greater ATP production and endurance performance.
  4. Increases VO2 Max: Training in zone two helps to improve VO2 max, which is the maximum amount of oxygen that an individual can utilize during intense exercise. By enhancing the efficiency of oxygen delivery and utilization, VO2 max improvements contribute to better endurance and performance.
  5. Supports Endurance Training: Zone two training serves as the foundation for endurance training programs, laying the groundwork for more intense and specific workouts. It builds aerobic endurance and resilience, allowing athletes to sustain higher intensities for longer durations.
  6. Promotes Longevity: Regular aerobic exercise, such as zone two training, has been associated with numerous health benefits, including reduced risk of cardiovascular disease, improved metabolic health, and increased longevity. It supports overall well-being and longevity by enhancing cardiovascular function and metabolic efficiency.

How Zone Two Training Helps Mitochondria and Improves Longevity:

  • Mitochondrial Biogenesis: Zone two training stimulates the growth and proliferation of mitochondria within muscle cells, enhancing the capacity for oxidative metabolism. This adaptation increases the efficiency of energy production and utilization, supporting endurance performance.
  • Improved Cellular Function: Increased mitochondrial density and function not only enhance athletic performance but also contribute to overall cellular health. Mitochondria play a crucial role in cellular energy production, metabolism, and regulation of cellular processes.
  • Longevity Benefits: Regular aerobic exercise, including zone two training, has been linked to improved mitochondrial function, reduced oxidative stress, and enhanced cellular repair mechanisms. These factors contribute to overall health and longevity by supporting optimal cellular function and tissue maintenance.

Zone two training is a foundational component of endurance training programs, promoting aerobic adaptation, mitochondrial biogenesis, and overall health and longevity. By exercising at a moderate intensity over sustained periods, individuals can improve their aerobic capacity, endurance, and cellular function, leading to better performance and well-being.

What are the fuel sources used in the five heart rate zones?

Zone 1:

  • Intensity: Very low intensity, often associated with warm-up or cool-down periods.
  • Fuel Sources:
    • Primarily aerobic metabolism.
    • Fats are the predominant fuel source.
  • Exercise Physiology:
    • Fat oxidation is the main pathway for ATP production.
    • Oxygen consumption is relatively low.

Zone 2:

  • Intensity: Low to moderate intensity, sustainable for longer durations (e.g., endurance training).
  • Fuel Sources:
    • Aerobic metabolism.
    • Balanced utilization of fats and carbohydrates.
  • Exercise Physiology:
    • Increased reliance on fat oxidation for ATP production.
    • Carbohydrate (glycogen) stores are also utilized but to a lesser extent than fats.
    • Oxygen consumption increases compared to zone 1.

Zone 3:

  • Intensity: Moderate to high intensity, approaching lactate threshold (the point at which lactate accumulation exceeds clearance).
  • Fuel Sources:
    • Increasing reliance on anaerobic metabolism.
    • Carbohydrates become the primary fuel source.
  • Exercise Physiology:
    • Increased reliance on glycogen stores for ATP production.
    • Anaerobic glycolysis becomes more prominent, leading to lactate accumulation.
    • Oxygen consumption continues to rise but may plateau.

Zone 4:

  • Intensity: High intensity, near or above lactate threshold.
  • Fuel Sources:
    • Predominantly anaerobic metabolism.
    • Carbohydrates are the main fuel source.
  • Exercise Physiology:
    • Maximal utilization of glycogen stores for ATP production.
    • Anaerobic glycolysis is the primary pathway, leading to significant lactate accumulation.
    • Oxygen consumption may reach its maximal capacity (VO2 max).

Zone 5:

  • Intensity: Maximum intensity, typically unsustainable for more than a few minutes.
  • Fuel Sources:
    • Primarily anaerobic metabolism.
    • Rapid utilization of available ATP and creatine phosphate stores.
  • Exercise Physiology:
    • Near maximal to maximal effort.
    • Reliance on anaerobic glycolysis and ATP-PC (phosphocreatine) system for ATP production.
    • Oxygen consumption may be insufficient to meet energy demands.

In summary, exercise intensity influences the balance between aerobic and anaerobic metabolism, with lower intensities relying more on aerobic pathways and higher intensities shifting towards anaerobic pathways. The utilization of different fuel sources and metabolic pathways varies across these exercise intensity zones.

What is Aerobic Glycolysis?

  • Definition: Aerobic glycolysis is the process of breaking down glucose to produce ATP in the presence of oxygen.
  • Mechanism:
    1. Glycolysis: Glucose is converted into pyruvate through a series of enzymatic reactions in the cytoplasm.
    2. Pyruvate Oxidation: Pyruvate enters the mitochondria and undergoes oxidation to form acetyl-CoA, which enters the Krebs cycle.
    3. Krebs Cycle: Acetyl-CoA is further oxidized in the Krebs cycle, generating NADH and FADH2, which carry electrons to the electron transport chain (ETC).
    4. Electron Transport Chain: NADH and FADH2 donate electrons to the ETC, leading to the production of ATP through oxidative phosphorylation.
  • Role:
    • Aerobic glycolysis is efficient and sustainable for providing ATP during low to moderate-intensity exercise when oxygen is readily available.
    • It is the primary pathway for ATP production during endurance activities and helps maintain energy levels over extended periods.

What is Anaerobic Glycolysis?

  • Definition: Anaerobic glycolysis is the process of breaking down glucose to produce ATP in the absence of oxygen.
  • Mechanism:
    1. Glycolysis: Glucose is converted into pyruvate in the cytoplasm, similar to aerobic glycolysis.
    2. Lactate Formation: In the absence of oxygen, pyruvate is converted into lactate by the enzyme lactate dehydrogenase, regenerating NAD+ to sustain glycolysis.
  • Role:
    • Anaerobic glycolysis is the primary pathway for ATP production during high-intensity exercise when oxygen demand exceeds supply.
    • It provides a rapid but limited source of ATP, contributing to short bursts of energy required for activities like sprinting or weightlifting.
    • Lactate accumulation during anaerobic glycolysis contributes to muscle fatigue and can lead to the sensation of “burning” in muscles.

What is the Role of Lactate for Fuel?

  • Lactate Production:
    • Lactate is produced as a byproduct of anaerobic glycolysis when pyruvate is converted into lactate in the absence of oxygen.
  • Lactate Utilization:
    • Contrary to the common misconception, lactate is not just a waste product but can serve as a fuel source.
    • Lactate produced during exercise can be taken up by various tissues, including skeletal muscles, the heart, and the liver.
    • In skeletal muscles and the heart, lactate can be oxidized back to pyruvate and used as a substrate for ATP production through aerobic metabolism.
    • In the liver, lactate can be converted back into glucose through gluconeogenesis, contributing to the maintenance of blood glucose levels during prolonged exercise.
  • Role in Energy Metabolism:
    • Lactate serves as an important metabolic intermediate, facilitating the redistribution of energy substrates and helping to sustain ATP production during intense exercise.
    • Its conversion to pyruvate and subsequent utilization in aerobic metabolism represents an efficient mechanism for energy recycling and metabolic flexibility.

Low Intensity (Zone 1 and Zone 2):

Fuel Sources:

  • Fats (Free Fatty Acids):
    • Mechanism: Free fatty acids are released from adipose tissue and transported to muscle cells.
    • Action: Fatty acids undergo beta-oxidation in mitochondria, producing acetyl-CoA, which enters the Krebs cycle to generate ATP through oxidative phosphorylation.
  • Carbohydrates (Glycogen):
    • Mechanism: Glycogen stored in muscles and liver is broken down into glucose.
    • Action: Glucose is converted into pyruvate through glycolysis. Pyruvate enters the mitochondria for further oxidation in the Krebs cycle.

Exercise Physiology:

  • Fat Oxidation:
    • Mechanism: Aerobic metabolism of fatty acids.
    • Action: Provides sustained energy production, especially during prolonged, low-intensity exercise.
  • Carbohydrate Utilization:
    • Mechanism: Aerobic glycolysis.
    • Action: Supplies additional ATP during moderate-intensity exercise, supporting fat oxidation.

Moderate Intensity (Zone 3):

Fuel Sources:

  • Carbohydrates (Glycogen):
    • Mechanism: Increased reliance on glycogen stores.
    • Action: Glycogen is rapidly broken down into glucose, which undergoes glycolysis to produce ATP, with some lactate produced due to anaerobic glycolysis.

Exercise Physiology:

  • Anaerobic Glycolysis:
    • Mechanism: Increased glycolytic activity.
    • Action: Provides ATP at a faster rate than aerobic metabolism but also generates lactate, contributing to fatigue.

High Intensity (Zone 4 and Zone 5):

Fuel Sources:

  • Carbohydrates (Glycogen):
    • Mechanism: Primary reliance on glycogen.
    • Action: Glycogen is rapidly converted into glucose for ATP production through anaerobic glycolysis.

Exercise Physiology:

  • Anaerobic Glycolysis:
    • Mechanism: Predominant pathway for ATP production.
    • Action: Generates ATP quickly but produces lactate as a byproduct, contributing to fatigue.
  • ATP-PC System (Phosphocreatine):
    • Mechanism: Creatine phosphate donates a phosphate group to ADP to rapidly regenerate ATP.
    • Action: Provides immediate energy for short bursts of high-intensity activity, such as sprinting or weightlifting.


  • Low to Moderate Intensity:
    • Reliance on aerobic metabolism.
    • Utilization of fats and carbohydrates for ATP production.
  • Moderate to High Intensity:
    • Increasing reliance on anaerobic glycolysis.
    • Shift towards carbohydrates as the primary fuel source.
  • High Intensity (Near Maximal to Maximal Effort):
    • Predominant use of anaerobic pathways.
    • Rapid depletion of glycogen stores and production of lactate.
    • Contribution from the ATP-PC system for immediate energy needs.

Lactate can be used as a fuel during exercise primarily in situations where the demand for energy exceeds the body’s ability to supply oxygen to muscles, leading to anaerobic glycolysis and lactate production. This typically occurs during high-intensity exercise, such as sprinting or intense weightlifting, where oxygen availability is limited and the muscles rely heavily on anaerobic metabolism for ATP production.

The lactate threshold is the exercise intensity at which lactate production exceeds its removal, leading to a rapid increase in blood lactate concentration. It is often associated with a transition from predominantly aerobic to increasingly anaerobic energy metabolism. The lactate threshold is an important physiological marker as it reflects the point at which the body’s ability to clear lactate from the bloodstream is overwhelmed by its production, resulting in a rise in lactate levels.

How to Measure the Lactate Threshold:

  1. Blood Lactate Testing: This is the most direct method and involves taking blood samples at various exercise intensities to measure lactate concentrations. Typically, blood lactate levels are measured using a portable lactate analyzer or taken to a laboratory for analysis. The lactate threshold is often identified as the point at which blood lactate concentration begins to increase nonlinearly or exceeds a predetermined threshold (e.g., 2-4 mmol/L above resting levels).
  2. Incremental Exercise Testing: This involves progressively increasing exercise intensity (e.g., speed or workload) while monitoring physiological responses such as heart rate, oxygen consumption, and lactate levels. The lactate threshold is often determined based on changes in lactate concentration relative to changes in workload or exercise intensity.

Personalizing a Training Program with Lactate Threshold Data:

Once the lactate threshold is determined, it can be used to personalize training programs in several ways:

  1. Training Intensity Zones: Training zones can be defined based on the lactate threshold to optimize specific physiological adaptations. For example, training at or slightly above the lactate threshold can improve lactate clearance and tolerance, while training below the lactate threshold can focus on aerobic endurance.
  2. Interval Training: Interval training can be structured using lactate threshold data to target specific energy systems and improve performance. For instance, intervals near or above the lactate threshold can enhance anaerobic capacity and lactate buffering, while recovery intervals below the lactate threshold promote aerobic adaptations and recovery.
  3. Progression Monitoring: Regular lactate threshold testing allows for tracking changes in fitness over time and adjusting training programs accordingly. Increases in the lactate threshold indicate improved aerobic capacity and endurance, while deviations may signal overtraining or inadequate recovery.

By incorporating lactate threshold data into training program design and monitoring, athletes and coaches can optimize training adaptations, enhance performance, and reduce the risk of overtraining or injury.

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