The Rise of Zone 2 Training: A New Perspective on Longevity and Metabolic Health

Introduction to Zone 2 Training: A Paradigm Shift in Fitness

In 2023, Dr. Peter Attia, a renowned podcaster and physician, brought attention to a previously niche workout concept called "Zone 2" training through his book Outlive. This approach, which emphasizes long, relatively low-intensity bouts of exercise, challenges the mainstream enthusiasm for high-intensity interval training (HIIT) that has dominated fitness culture for over a decade. While the idea of slow and steady exercise isn’t new, Dr. Attia and other proponents of Zone 2 training infuse it with a modern, high-tech biohacking ethos, advancing a specific and scientifically grounded approach to improving metabolic health. At its core, Zone 2 training isn’t just about leisurely bike rides or casual strolls; it’s about maintaining a precise intensity level that’s challenging enough to drive physiological benefits but not so hard that it leads to burnout or excessive strain.

What is Zone 2 Training?

The concept of Zone 2 training is rooted in the work of Iñigo San Millan, a professor at the University of Colorado School of Medicine and a former professional cyclist. Dr. San Millan has worked with elite athletes, including three-time Tour de France winner Tadej Pogacar, and has developed a system that categorizes exercise into six distinct training zones. These zones are based on the type of fuel the body uses and how it responds to different intensities. For the average person, the easiest zone might involve a brisk walk, during which the body primarily burns fat for energy.

As exercise intensity increases, the body continues to burn fat but only up to a certain point. Beyond this threshold, the body shifts toward relying more on carbohydrates, and lactate levels in the blood begin to rise—a sign of increased muscle effort. Dr. San Millan notes that "something funky happens" metabolically when this threshold is crossed. Over years of working with elite cyclists, he observed that top performers often spend significant training time just below this critical intensity. While they engage in other types of training as well, the time spent in Zone 2 correlates with improved performance over time.

Why Might Zone 2 Be Good for You?

The benefits of aerobic exercise are well-documented, but the question remains: is Zone 2 uniquely beneficial compared to slightly easier or harder workouts? According to Dr. San Millan, Zone 2 represents a "sweet spot" where the body optimizes its use of both fat and carbohydrates. At this intensity, the mitochondria—the cellular "powerhouses" responsible for converting food into energy—are challenged just enough to spur significant improvements. If the intensity is too low (Zone 1), the mitochondria may not be stressed enough to drive meaningful adaptation. If the intensity is too high (Zone 3 or above), the body shifts toward burning carbohydrates, reducing fat utilization and increasing lactate levels.

Dr. San Millan and other scientists believe that the health of the mitochondria is a critical predictor of future metabolic health. Problems in mitochondrial function can signal the early stages of chronic conditions such as type 2 diabetes, heart disease, and even Alzheimer’s disease. By exercising in Zone 2, individuals can encourage their mitochondria to adapt and grow, potentially warding off these conditions.

How Do You Know When You’re in Zone 2?

Determining the exact intensity of Zone 2 requires specialized testing, such as a visit to an exercise physiology lab, which isn’t practical for most people. Heart-rate zones on treadmills or other equipment aren’t reliable because they often use different classification systems than Dr. San Millan’s six-zone model. For the tech-savvy, some enthusiasts use lactate testing—an approach that involves pricking the skin for blood samples during exercise—to monitor their intensity.

For the rest of us, Dr. Attia suggests a simpler method: paying attention to how the exercise feels. Zone 2 corresponds to an effort level that’s moderate but sustainable. As Dr. Attia describes it, you should be moving "slow enough that you can maintain a conversation, but fast enough that the conversation might be a little strained." He recommends starting with two 30-minute sessions per week and gradually increasing to at least three hours of Zone 2 training weekly.

What Does the Science Say?

While the case for Zone 2 training is compelling, it’s important to note that much of the evidence is observational rather than experimental. Dr. San Millan believes that the pace sustainable in Zone 2 reflects mitochondrial health, and he points to the success of elite cyclists who spend significant time in this zone. However, when researchers directly measure mitochondrial density through muscle biopsies, they find that high-intensity exercise above Zone 2 often produces greater improvements in mitochondrial function.

Kristi Storoschuk, a doctoral candidate in muscle physiology, notes that while Zone 2 training has its benefits, it may not be the most efficient way toboost mitochondrial health. Similarly, a recent systematic review by scientists in Norway and Denmark concluded that higher-intensity exercise tends to have a more significant impact on mitochondrial function. This raises an important question: does the hype around Zone 2 training risk convincing people that lower intensity is always better, potentially leading them to exercise at levels too low to drive meaningful benefits?

Balancing Zone 2 Training with Other Workouts: The Practical Takeaway

While the debate over the superiority of Zone 2 training continues, many scientists agree that the best exercise is the one you’re willing to do consistently. Martin MacInnis, a professor at the University of Calgary, suggests that the real value of Zone 2 may lie in its sustainability. Because it’s relatively easy on the body, it allows for quick recovery and repeated sessions, which over time can lead to significant cumulative benefits. This aligns with the observation that top endurance athletes typically spend about 80% of their training time in low-intensity zones and 20% in high-intensity efforts—a balance that seems to optimize both performance and long-term health.

Dr. San Millan himself emphasizes that Zone 2 is just one piece of the puzzle. "Zone 2 is part of it," he says, "but you need to do other intensities as well." The takeaway for most people is clear: incorporating regular Zone 2 training into your routine can be a powerful tool for improving metabolic health and longevity, but it’s most effective when paired with a balanced approach to exercise that includes occasional higher-intensity efforts. In the end, the key is to find a routine you enjoy and can stick with—because consistency, more than any specific intensity, is the ultimate driver of long-term health and fitness.