Illustration of a mitochondria

Understanding Zone 2 Training: A Concise Scientific Overview

Let’s keep this one short. In 800 words, we will introduce the cardiometabolic functions and physiological underpinnings that support the argument for Zone 2 training.

Let’s begin by examining the lactate threshold results of a typical sixty-year-old blogger:

Chart of fat and carbohydrate calories utilized at different heart rate levels

Note the crossover from burning fats to burning calories as the exercise intensity increases. At low intensities, the mechanism is primarily aerobic, while at high intensities, the energy systems become increasingly reliant on the glycolytic pathway. The levels of exertion correlate to a corresponding level of fat versus carbohydrate utilization for energy.

Dr. Iñigo San-Millán is a top researcher in cellular metabolism and personal coach of two-time Tour de France champion Tadej Pogačar. He is also a favorite podcast guest of Dr. Peter Attia. His Basic Exercise Biogenetics are:

  • The purpose of each training zone is to elicit specific physiological and metabolic adaptations in order to improve performance.
  • The capacity of an athlete to exercise ultimately depends on the ability to transform chemical energy into mechanical energy. Skeletal muscle needs to synthesize ATP for muscle contraction.
  • ATP generation is achieved by two mechanisms—anaerobic and aerobic metabolism.
  • The exercise intensity or metabolic and physiological stress, as well as [the sequential] muscle fiber recruitment pattern, will dictate the energy system and substrate that is activated, which will then correlate with different training zones.

Thus the lower heart rate exertion zones are utilizing fat for energy purposes during the recruitment of Type 1 (slow twitch) muscle fibers. Then, as muscle contractile demands increase, Type 2 (fast twitch) fibers are engaged along with their higher capacity to utilize glucose.

Increasing Lactate Clearance Capacity

Type 1 muscle fibers not only have the highest mitochondrial density and capacity, they are also responsible for lactate clearance. Lactate is mainly produced by the fast-twitch muscle fibers but it is the slow-twitch muscle fibers that play the key role in lactate clearance. Isn’t that an intriguing combo? This is because:

Type 1 muscle fibers contain a transporter called MCT-1 which is in charge of taking up lactate and transporting it to the mitochondria, where it is reused as energy. Zone 2 training increases mitochondrial density as well as MCT-1 transporters.
Cartoon showing a microscope and in close up a mitochondria smiling and saying "I feel so seen right now."

Why is lactate clearance important? Let’s begin with looking at where the inability to clear lactate has some important medical repercussions:

  • Approximately 50% of the US population has diabetes Type II (10%) or is pre-diabetic (40%). “A typical characteristic that we know of people with pre-type 2, or type 2 diabetes, is that they have a poor metabolic flexibility that is also a poor capacity to oxidize fuels.” This results in the buildup of mediators that are “…not only involved with insulin resistance, but maybe with cardiovascular disease or atherosclerosis. They cannot be oxidized in the mitochondria, so they build up outside.”
  • “80% of people with type 2 diabetes also have cardiovascular disease (and vice versa).”
  • Higher blood lactate is a commonality among cancer patients.

If all we want to know is where we’re going to die, so we don’t go there, cancer and diabetes are pretty much at the top of everyone’s avoidance list. Regarding cancer, let’s listen in as Dr. Peter Attia and Dr. Iñigo San Millán discuss the role of mitochondria and lactate signaling in cancer:

  • Cancer is a metabolic disease caused by an injury of the cellular respiration system (i.e., mitochondria).
  • Cancer cells have very high lactate production. The inability to clear lactate is going to drive cell growth and proliferation.
  • In cancer, chronic lactate accumulation is responsible for the acidic microenvironment of the tumor. The more acidic the tumor the more metastatic and aggressive it is.

Cancer and diabetes not enough for you? Lactate measurements on Long Covid patients also point towards mitochondrial dysfunction.

Illustration of cell and parts of cell

Looking for more? The least dumbed-down information comes from Dr. Peter Attia’s podcast and website “The Drive”. The discussion points above are from Deep Dive Back into Zone 2. Definitely subscribe to the free podcast and consider the $149 annual membership to receive comprehensive show notes along with plenty of other exclusive content and features.

Protocols for Zone 2 Training

The basic protocol for Zone 2 training is to first calculate your Zone 2 training heart rate. Most people can use 180 minus their age. See part one of this series for further elaboration.

How frequently? Dr. Attia aims for four one-hour sessions each week, while Dr. San-Millán favors four to five ninety minute Zone 2 workouts that conclude with five minutes of high intensity effort.

It is widely recommended to use a treadmill or stationary exercise equipment. The maximum effects are achieved by maintaining a consistent heart rate.

There are admittedly some competing protocol recommendations out there, especially around volume and the incorporation of higher intensity efforts. Feel free to join the discussion.

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