Debbie Potts Coaching

Zone Two Training or HIIT Training?

Mitochondria play a crucial role in fueling the body during exercise, particularly at different intensities.

Here’s how their function varies across different exercise intensities and how markers like mitochondria density and capacity are involved:

  1. Mitochondrial Function at Different Intensities:
    • Low Intensity (Zone 2): At lower exercise intensities, such as those experienced during Zone 2 training (around 60-70% of maximum heart rate), mitochondria primarily utilize fatty acids as a fuel source. Oxygen availability is sufficient for aerobic metabolism, and mitochondria efficiently oxidize fatty acids to produce adenosine triphosphate (ATP), the energy currency of cells.
    • Moderate to High Intensity (Zone 3): As exercise intensity increases, there is a greater demand for ATP production. Mitochondria continue to oxidize fatty acids but also rely more on glucose and glycogen stored in muscle tissue for fuel. At higher intensities, anaerobic metabolism contributes to ATP production, leading to lactate accumulation.
  2. Mitochondria Density: Mitochondria density refers to the abundance of mitochondria within cells, particularly muscle cells. Higher mitochondria density means there are more mitochondria available to produce ATP, enhancing the cell’s capacity for aerobic metabolism. This is especially important for endurance athletes who rely heavily on aerobic energy production.
  3. Mitochondria Capacity: Mitochondria capacity refers to the ability of mitochondria to produce ATP through oxidative phosphorylation, the process by which ATP is generated using oxygen. This capacity is influenced by factors such as mitochondrial enzyme activity, respiratory chain efficiency, and substrate availability (e.g., fatty acids, glucose).

Improving Mitochondrial Markers:

  1. Endurance Training: Endurance exercise, such as long-distance running or cycling, is known to stimulate mitochondrial biogenesis, increasing both mitochondria density and capacity. Consistent aerobic training sessions, particularly at moderate intensities, can lead to significant improvements in mitochondrial function over time.
  2. High-Intensity Interval Training (HIIT): HIIT involves alternating between short bursts of high-intensity exercise and periods of rest or low-intensity exercise. HIIT has been shown to increase mitochondrial density and capacity, potentially through the activation of signaling pathways that regulate mitochondrial biogenesis.
  3. Resistance Training: While traditionally associated with strength gains, resistance training can also have beneficial effects on mitochondrial function. High-intensity resistance exercises can increase mitochondrial density and capacity in muscle tissue, contributing to improved overall metabolic health.
  4. Nutritional Support: Certain dietary factors, such as consuming adequate protein and essential nutrients like iron and B vitamins, can support mitochondrial function. Additionally, nutritional strategies that promote metabolic flexibility, such as a balanced diet with sufficient carbohydrate and fat intake, can help optimize mitochondrial metabolism during exercise.

By incorporating these strategies into a training program, individuals can enhance their mitochondrial function, leading to improved endurance, performance, and overall metabolic health.

3d rendering of a Mitochondrium – microbiology illustration

The relationship between mitochondrial function and various health and performance outcomes, including fat loss, athletic performance, and longevity markers like VO2max, is complex and interconnected:

  1. Fat Loss:
    • Improved mitochondrial function, particularly increased mitochondria density and capacity, can enhance the body’s ability to oxidize fat for energy during exercise and at rest.
    • Aerobic exercise, which stimulates mitochondrial biogenesis and enhances metabolic efficiency, is often recommended for fat loss due to its ability to increase energy expenditure and promote fat oxidation.
    • Higher levels of aerobic fitness, associated with improved mitochondrial function, have been linked to better long-term fat loss maintenance.
  2. Athletic Performance:
    • Mitochondrial function is crucial for supporting energy production during exercise, especially during prolonged or high-intensity activities.
    • Endurance athletes, who typically have higher levels of aerobic fitness and greater mitochondrial density and capacity, demonstrate superior performance in activities requiring sustained effort, such as distance running or cycling.
    • Mitochondrial adaptations induced by endurance training, including increased oxidative capacity and improved metabolic efficiency, contribute to enhanced athletic performance.
  3. Longevity Markers such as VO2max:
    • VO2max, or maximal oxygen consumption, is a measure of aerobic fitness and reflects the body’s ability to utilize oxygen during maximal exertion.
    • VO2max is closely related to mitochondrial function, as mitochondria play a central role in aerobic energy production.
    • Higher VO2max levels are associated with reduced risk of cardiovascular disease, improved metabolic health, and enhanced longevity.
    • Endurance exercise and other interventions that improve mitochondrial function, such as HIIT and resistance training, can lead to increases in VO2max and improvements in longevity markers.

Optimizing mitochondrial function through exercise and other lifestyle interventions can have profound effects on fat loss, athletic performance, and longevity.

By improving mitochondrial density, capacity, and efficiency, individuals can enhance their ability to utilize fat for energy, improve athletic performance, and promote overall health and longevity.

VO2max serves as a valuable marker of aerobic fitness and is closely related to mitochondrial function and long-term health outcomes.The idea behind Zone 2 training is to spend prolonged periods exercising at an intensity that predominantly activates Type 1 muscle fibers. By doing so, several physiological adaptations occur:

  1. Increase in Mitochondrial Density: Zone 2 training stimulates mitochondrial biogenesis, the process of creating new mitochondria within muscle cells. With increased training volume and duration in Zone 2, there is a corresponding increase in mitochondrial density within Type 1 fibers. This allows for greater capacity to produce ATP through aerobic metabolism, enhancing endurance performance.
  2. Enhancement of Respiratory Rates: Mitochondria within Type 1 fibers exhibit higher respiratory rates, meaning they can efficiently utilize oxygen to convert fatty acids into ATP through oxidative phosphorylation. Zone 2 training promotes the upregulation of respiratory enzymes and transport proteins involved in mitochondrial respiration, leading to improved efficiency in energy production.
  3. Increased Transport Proteins: Zone 2 training also stimulates the expression of transport proteins, such as MCT-1 (Monocarboxylate Transporter 1), which facilitate the movement of lactate out of muscle cells for clearance. While lactate is primarily produced by Type 2 muscle fibers during exercise, it is primarily cleared by Type 1 fibers. By enhancing MCT-1 expression, Zone 2 training helps improve the clearance of lactate, reducing its accumulation and delaying fatigue during prolonged exercise.

In summary, Zone 2 training targets Type 1 muscle fibers to maximize fat utilization, promote mitochondrial adaptations, and enhance lactate clearance. These adaptations contribute to improved endurance, metabolic efficiency, and overall performance in activities requiring sustained effort.


Zone 2 training is a method of exercise training that involves working out at a specific intensity level, typically corresponding to a moderate level of effort, for an extended period. The purpose of zone 2 training is to improve aerobic capacity, endurance, and metabolic efficiency.

Here’s a breakdown of the key aspects of zone 2 training:

  1. Purpose: Zone 2 training aims to develop the body’s ability to efficiently utilize fat as a fuel source, improve mitochondrial density, enhance cardiovascular endurance, and increase metabolic flexibility.
  2. Benefits:
    • Improved fat metabolism: Zone 2 training encourages the body to use fat as a primary energy source, which can help with weight management and long-duration activities.
    • Increased mitochondrial density: Regular zone 2 training can lead to greater mitochondrial biogenesis, improving the efficiency of energy production within cells.
    • Enhanced endurance: By training at a moderate intensity for extended periods, athletes can improve their cardiovascular endurance and stamina.
    • Improved metabolic flexibility: Zone 2 training helps the body adapt to using both fat and carbohydrates efficiently, allowing for better performance across a range of intensities.
  3. Differences for Men vs. Women:
    • Women tend to have more oxidative (Type 1) muscle fibers, greater mitochondrial density, and a higher reliance on lipid metabolism compared to men. This means that women may derive similar benefits from zone 2 training as men but may not need to spend as much time in this zone to achieve them.
    • Research suggests that women may benefit more from high-intensity interval training (HIIT) and sprint interval training (SIT) compared to men when it comes to improving metabolic function and endurance.
  4. Measurement in PNOE: PNOE is a metabolic analyzer device that measures various parameters related to metabolism and respiratory function. To determine zone 2 training intensity with PNOE, individuals typically undergo a metabolic assessment where their oxygen consumption (VO2) and heart rate data are collected during exercise. Based on these measurements, trainers or coaches can identify the specific heart rate range corresponding to zone 2 for each individual.

In summary, zone 2 training is a valuable tool for improving aerobic capacity, endurance, and metabolic efficiency, with potential differences in effectiveness between men and women. PNOE can be used to measure and prescribe zone 2 training intensity based on individual metabolic responses.

Adjusted Fat Max and Metabolic Crossover Point are physiological markers used to identify Zone 2 training intensity, particularly in the context of metabolic testing.

  1. Adjusted Fat Max: This refers to the intensity at which fat oxidation rates reach their maximum during exercise, adjusted for body weight. It represents the highest rate at which the body can utilize fat as a fuel source before relying more on carbohydrates for energy.
  2. Metabolic Crossover Point (MCP): This is the point during exercise where the body shifts from predominantly using fat to predominantly using carbohydrates as the primary fuel source. It’s often observed as a change in the ratio of fat to carbohydrate oxidation rates.

These markers are used to determine the upper limit of Zone 2 training intensity, where individuals can maximize fat oxidation while still maintaining a sustainable effort level.

Lactate Threshold Testing vs. Zone 2 Training:

  1. Lactate Threshold Testing: Lactate threshold testing involves measuring the point at which blood lactate levels begin to rise exponentially during incremental exercise. This threshold is often associated with the onset of fatigue and the transition from aerobic to anaerobic metabolism. Lactate threshold testing is used to determine exercise intensities for improving endurance performance and setting training zones.
  2. Relation to Peak Fat Burning Zone (Zone 2): Lactate threshold testing and Zone 2 training are related but target different physiological markers. While lactate threshold testing focuses on identifying the intensity at which lactate accumulates in the blood, Zone 2 training aims to optimize fat metabolism and improve aerobic capacity.

The relationship between lactate threshold and Zone 2 training lies in their impact on endurance performance. Training at or slightly below lactate threshold intensity can improve the body’s ability to sustain higher workloads before reaching fatigue. Zone 2 training complements this by improving fat oxidation rates and metabolic efficiency, enhancing overall endurance capacity.

While lactate threshold testing helps identify intensity thresholds for endurance training, Zone 2 training focuses on optimizing fat metabolism and aerobic capacity within a specific training zone. Both approaches contribute to improved endurance performance but target different physiological mechanisms.

In Zone 2 training, the primary fuel source is fat. This zone corresponds to moderate-intensity exercise, typically around 60-70% of maximum heart rate. At this intensity, the body relies predominantly on aerobic metabolism, where fat oxidation is the primary source of energy. Lactate production is minimal in Zone 2 training because the exercise intensity is below the lactate threshold, meaning that the body can effectively clear lactate as it is produced.

In Zone 3 training, which is higher in intensity, the body starts to rely more on carbohydrates as a fuel source. This zone typically corresponds to 70-80% of maximum heart rate. As exercise intensity increases, the reliance on aerobic metabolism continues, but there is also an increase in anaerobic metabolism, leading to the production of lactate. The body can still clear lactate efficiently at this intensity, but the rate of production may exceed the rate of clearance, leading to a gradual increase in blood lactate levels.

While lactate is produced to some extent in both Zone 2 and Zone 3 training, it is more predominant in Zone 3 due to the higher intensity of exercise.

In Zone 2 training, the focus is on maximizing fat oxidation, while in Zone 3, there is a greater reliance on carbohydrates for energy production, leading to increased lactate production.

  • Zone 2 training is a popular method involving low-intensity, long-duration workouts aimed at improving metabolic flexibility and endurance.
  • It targets Type 1 muscle fibers, known for their efficiency in utilizing fat as fuel, and aims to increase mitochondrial density and respiratory rates.
  • However, for active women, the benefits of zone 2 training are being overstated.
  • Women tend to have more oxidative muscle fibers, greater mitochondrial capacity, and better metabolic flexibility compared to men.
  • Women also have a higher reliance on lipid metabolism and are more efficient at using fat as a fuel during exercise.
  • Research suggests that high-intensity interval training (HIIT) and sprint interval training (SIT) are more effective for women in improving lactate production, clearance, and glycolytic capacity.
  • HIIT and SIT also stimulate the growth of fast-twitch muscle fibers, which are less prevalent in women compared to slow-twitch fibers.
  • While long, slow training is still important for endurance athletes, it may not provide the same benefits in terms of mitochondrial respiration for women as it does for men.
    • Mitochondrial respiration refers to the process by which mitochondria, the energy-producing organelles within cells, generate adenosine triphosphate (ATP), which is the primary energy currency of cells.
    • This process occurs through a series of biochemical reactions that involve the transfer of electrons along the electron transport chain located in the inner mitochondrial membrane.
    • As electrons move through the chain, they release energy, which is used to pump protons across the inner mitochondrial membrane, creating an electrochemical gradient.
    • This gradient drives the synthesis of ATP through the enzyme ATP synthase.
    • Mitochondrial respiration is essential for providing cells with the energy they need to carry out various biological processes, including muscle contraction, cellular metabolism, and other cellular functions.
  • Women should ensure that when they engage in zone 2 training, it is done at a truly low intensity to avoid counterproductive outcomes.


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