Is ‘Zone 2’ the Magic Effort Level for Exercise?

When the podcaster and physician Peter Attia published his blockbuster longevity book, “Outlive,” in 2023, he introduced a previously niche workout concept to a much wider audience. Dr. Attia claims that “Zone 2” training, which refers to long and relatively easy bouts of exercise, is crucial to metabolic health — an idea that runs counter to more than a decade of enthusiasm for high-intensity interval training.

Going slow and steady isn’t a new concept, but Dr. Attia and other Zone 2 boosters bring a high-tech biohacking ethos to the topic. It’s not just about going for, say, a relaxed bike ride; it’s about maintaining a very specific level of intensity that’s neither too easy nor too hard.

Zone 2 enthusiasts believe that exercising at that effort level for at least a few hours per week can help ward off chronic conditions such as type 2 diabetes and heart disease. But some scientists who study metabolic health aren’t convinced that this narrow focus is really necessary.

What is Zone 2 training?

The buzz around Zone 2 is based on the work of Iñigo San Millan, a professor at the University of Colorado School of Medicine. A former professional cyclist himself, Dr. San Millan has long helped train cyclists — including three-time Tour de France winner Tadej Pogacar — in addition to his academic research.

His work with cyclists led him to classify exercise into six different training zones, based in part on what type of fuel your muscles are burning, to explain how the body responds to different workout intensities. In the easiest zone, which for an average person might be a brisk walk, you’re burning mostly fat.

As you push harder, you burn more fat — but only up to a point. Beyond a certain level of effort, your body starts relying more on carbohydrates and ramps down fat-burning. Lactate levels in your blood also begin to creep up, a sign that your muscles are working harder. From a metabolic perspective, Dr. San Millan said, “something funky happens” when you cross this threshold.

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