In an eye-opening test detailed in Peter Attia’s book, Outlive: The Science & Art of Longevity, sedentary men who were insulin resistant were found to have no ability to tap into fat stores when riding a stationary bicycle at 80% of their maximum heart rate.

The test compared three groups: professional cyclists, moderately active healthy men and sedentary men with insulin resistance. Scientists in the test led by Iñigo San Millán,a former professional cyclist and an assistant professor at the University of Colorado School of Medicine, analyzed how much oxygen the subjects consumed and the amount of CO₂ they exhaled. That information was used to determine how efficiently they produced power and the primary fuels they were using.

“The professional cyclists could zoom along, producing a huge amount of power while still burning primarily fat,” Attia writes. But the subjects who were insulin resistant were unable to access fat stores, “meaning they were metabolically inflexible: able to use only glucose.”

San Millán concluded that while the two groups were vastly dissimilar, in order to improve their health, the sedentary men needed to train in a manner similar to Tour de France cyclists he worked with, athletes who spent around 30 hours per week riding in zone 2 to build a foundation for more intense training.

San Millán defined zone 2 work as the maximum level of effort that can be maintained without accumulating lactate. Riding in the zone trains the body to use fat stores as an efficient fuel source, rather than relying on less-efficient glucose.

“If we are ‘feeling the burn’ in this type of workout, then we are likely going too hard, creating more lactate than we can eliminate,” Attia says.

Big benefits from zone 2 work

The findings are not to suggest that sedentary men should jump into a training program designed for professional cyclists. But it points up the fact that such training is even more important for nonathletes, according to Attia.

“First, it builds a base of endurance for anything else you do in life, whether that is riding your bike in a one-hundred-mile century ride or playing with your grandkids,” Attia says. “The other reason is (San Millán) believes it plays a crucial role in preventing chronic disease by improving the health and efficiency of your mitochondria,” which convert fatty acids to energy.

Attia says he uses a handheld lactate monitor to make sure his pacing is correct while training in zone 2. If lactate levels rise too high, he slows. “It’s all about keeping lactate levels steady in this range, and the effort sustainable,” he says.

Working out in zone 2 is not difficult, even for people who have been sedentary, Attia suggests. For some, a brisk walk might get them to zone 2, while for others it could require walking uphill. Riding a stationary bike, swimming and jogging are other ways to reach the zone.

“The key is to find an activity that fits into your lifestyle, that you enjoy doing, and that enables you to work at a steady pace that meets the zone 2 test: You’re able to talk in full sentences, but just barely,” Attia writes.

Attia offers more details on zone 2 training at peterattiamd.com.

Products related to this article:

Outlive. The Science & Art of Longevity by Peter Attia. Available at Amazon.

THE EDGE Lactate Meter. Available at Amazon.