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Zone 2 Training: Worth the Hype?
Zone 2 training is among the fastest-growing exercise trends in the longevity and performance communities. You'll often hear Zone 2 referred to as the "sweet spot" for creating mitochondrial (energy-producing cell) health and increasing fat-burning capacity. Voices such as Dr. Peter Attia have helped to promote the trend.
However, a recent analysis provides some damaging evidence against Zone 2 training as the optimal intensity for creating mitochondrial capacity or increasing the body's fatty acid oxidation (fat-burning).
Now that we've established this fact, what should you do? Is Zone 2 training something you should build into your daily routine as a non-elite athlete or should you focus on other forms of exercise?
Your answer is somewhat complex. Zone 2 training certainly has value; however, it may receive excessive hype for many non-elite athletes.
What Does Zone 2 Training Really Mean?
Exercise zones refer to the different intensities associated with various forms of exercise. However, Zone 2 does not have a universally accepted definition. The primary definition most commonly referenced by Attia is based on lactate levels. Lactate is a by-product created when muscles utilize energy. When the body exercises at an elevated intensity, lactate production exceeds the body's ability to metabolize it. At that point, fatigue and burning sensations typically begin to occur.
The goal of Zone 2 training is to remain beneath that threshold, thereby maintaining relatively constant lactate levels. Most individuals utilize the "talk test." Zone 2 refers to a pace at which you are exercising yet able to maintain a comfortable conversation.
The Two Large Benefits of Zone 2 Training
Zone 2 training proponents advocate for its unique benefits in two areas:
1. Mitochondrial capacity
This is a general term to define the health and functioning of mitochondria. Mitochondria are crucial for metabolic health, endurance and aging biology. Improved mitochondrial function is generally viewed as beneficial.
2. Fatty acid oxidation capacity
This term defines the body's ability to utilize fat as a fuel source, specifically during physical activity. Enhanced fat oxidation capacity is associated with greater endurance performance and greater metabolic flexibility, defined as the body's reduced dependence on glucose for fuel.
Everyone agrees that the previous objectives are valuable. The disagreement exists as to whether Zone 2 is the most suitable method to achieve those goals.
The Zone 2 Controversy
Elite endurance athletes engage in extensive amounts of Zone 2 training and they possess exceptional mitochondrial function and high fat oxidation rates.
The new analysis identifies two issues with this line of reasoning.
Firstly, elite endurance athletes also complete high-intensity training. Therefore, it is unclear as to which type of training contributes to the development of each area.
Secondly, elite endurance athletes have extremely high volumes of training. Elite endurance athletes can train between 15 and 25 hours per week. The average person struggles to find time for 2 to 3 hours of training. The extreme difference in volume greatly affects what is strategically optimal.
If Zone 2 is only capable of producing strong signals for mitochondrial adaptation after prolonged periods of training, then it may not be the most efficient use of limited training time.
Is Zone 2 Best for Mitochondrial Function?
The new analysis indicates that Zone 2 may create insufficient stress for cellular energy systems to initiate strong mitochondrial adaptation, particularly in non-endurance trained individuals.
An example of this concept is the belief that mitochondrial growth is triggered when cells undergo meaningful energy strain. The authors indicate that Zone 2 generates minimal energy stress unless it extends over a lengthy period of time. Evidence also exists that the majority of significant cellular energy stress may develop after prolonged training sessions, usually exceeding two hours.
Most people would view this as an obstacle. If you have only a few hours of training time per week, you can't rely on extended sessions of Zone 2 training to stimulate mitochondrial adaptation.
The authors also examined results rather than mechanisms. They reference literature indicating that training at intensities above Zone 2, including high-intensity interval training, is more consistently successful in developing important signaling pathways and enhancing mitochondrial function, particularly in non-elite endurance trained populations.
Therefore, the analysis implies that training at higher intensities may be more successful in enhancing mitochondrial function in the typical individual compared to Zone 2.
Is Zone 2 Best for Increasing Fat Oxidation Rates?
The evidence here is weak.
The review article identified extremely limited research comparing Zone 2 training-induced changes in fat oxidation rates. One study demonstrated a year-long program of Zone 2 training resulted in an increase in maximum fat oxidation rates, supporting the premise.
However, there are also studies that demonstrate low-intensity training increases fat oxidation rates more than high-intensity training, while others show the opposite. Additionally, a meta-analysis reviewing numerous trials indicated both moderate and high-intensity training equally improved fat oxidation rates.
Thus, the best summary is: training improves fat oxidation rates, and while Zone 2 has been shown to improve fat oxidation rates, there is no compelling evidence to indicate that Zone 2 is superior.
Real-World Implications of This Research
Time is the largest factor.
For elite endurance athletes with large volumes of training time available, it can be logical to perform a great deal of Zone 2 and incorporate high-intensity training.
For the vast majority of non-elite athletes with limited training time, Zone 2 can displace high-intensity training that produces more adaptations per minute, particularly for mitochondrial function and cardiorespiratory fitness.
Cardiorespiratory fitness is one of the strongest predictors of long-term health. A common measure is VO2 max, the highest amount of oxygen an individual can utilize during maximal exercise. Greater VO2 max is closely correlated with lower mortality risk.
Training at higher intensities is more effectively capable of raising VO2 max than performing moderate-duration work. This broader concept is discussed in exercise snacks and small-effort fitness gains.
A More Pragmatic Methodology
If you are not an elite endurance athlete, and you have limited training time, a more rational approach is to:
- Prioritize high-intensity training performed in a safe manner
- Include resistance training and power-based work when applicable
- Only include Zone 2 training if you have additional time after completing your high-intensity training requirements
A common minimum target for vigorous activity is approximately 75 minutes of vigorous physical activity per week. Combining moderate and vigorous activity is acceptable. If you can comfortably exceed this, Zone 2 can still be a viable addition.
Zone 2 training is not ineffective. It simply appears to be an overhyped method of utilizing limited time when attempting to enhance mitochondrial function and produce the greatest overall health impact.
A Broader Framework
See how to maximize the benefits of exercise and minimize the risks.
The Bottom Line
Zone 2 training has often been promoted as the ideal form of training for mitochondrial function and fatty acid oxidation. However, a recent analysis contests this claim, particularly for non-elite endurance-trained athletes.
If you have limited time, the potential benefits of using higher-intensity training may provide a greater return on investment, particularly regarding mitochondrial function and VO2 max.
If you have sufficient time, incorporating Zone 2 may be a reasonable decision. Your specific plan is likely influenced by your availability, your current fitness level, and your desired fitness goals. It can also be combined with resistance training to facilitate healthy aging.