#571: Is Zone 2 Training Actually Best for Health? – Prof. Brendon Gurd

Introduction to this Episode

In recent years “Zone 2 training” has gained immense popularity in fitness and health circles, often being proclaimed the “best” cardiovascular training for metabolic health.  Zone 2 training is sustained, low-intensity exercise below the lactate threshold that is commonly employed by many elite level endurance athletes, with a variety of proposed performance benefits. This led to interest from health-conscious folks about its ability to  improve markers like VO2 max and mitochondrial function, which are associated with better health outcomes and longevity.

Prof. Brendon Gurd, an exercise physiologist at Queenʼs University, is  the co-author of a recent review published in Sports Medicine (Storoschuk et al., 2025 ) that examined some of the claims made around zone 2 training and health in the general population.

This episode looks at whether the evidence truly supports Zone 2 as a uniquely effective training zone or if this hype has outpaced the research. These insights help professionals and individuals make evidence-based recommendations and avoid unintended negative consequences of following certain ideas.

About the Guest

Dr. Brendon Gurd is a Professor in the School of Kinesiology and Health Studies at Queenʼs University, where his research focuses on exercise physiology and skeletal muscle adaptation. His work explores how different intensities and types of exercise influence mitochondrial content and function in muscle tissue, with the goal of understanding how to optimize health and performance through training.

He leads the Queenʼs Muscle Physiology Lab and has published extensively on high-intensity interval training (HIIT), mitochondrial biogenesis, and exercise-induced molecular signaling. Dr. Gurd is widely respected for his evidence-based approach to examining training methodologies and their real-world applications.

Useful Terminology for this Episode

• Zone 2 Training: A low-intensity steady-state exercise zone, typically defined as the highest intensity you can sustain while still primarily using aerobic metabolism (below the first lactate threshold). In practical terms, Zone 2 feels “easy” (you can converse comfortably) and is fueled largely by fat oxidation.
• Lactate Threshold: The exercise intensity at which blood lactate begins to accumulate faster than it can be cleared. Crossing this threshold signifies a shift to more anaerobic energy contribution. It marks the upper boundary of moderate intensity – for instance, Zone 2 is below this point, while intensities above it enter a higher zone where fatigue and lactate build-up increase rapidly.
• Critical Power: A concept in exercise physiology indicating the threshold output (pace or power) separating the heavy and severe intensity domains. Below critical power, exercise (in the heavy domain) can be sustained for extended periods (though with some fatigue); above it (severe domain), fatigue occurs very quickly. Critical power roughly corresponds to the upper limit of Zone 4/5 in a five-zone model – exceeding it means hitting maximal effort (VO₂ max levels) and very short duration capacity.
• VO₂ max: The maximal oxygen uptake – essentially the highest rate at which the body can consume and utilize oxygen during intense exercise. It is measured in mL of O₂ per kg body weight per minute. VO₂ max reflects cardiorespiratory fitness; a higher VO₂ max indicates greater aerobic capacity and endurance. Itʼs one of the strongest predictors of health and longevity (higher VO₂ max is strongly associated with lower mortality risk).
• Mitochondrial Capacity: The ability of muscle cells to produce energy aerobically, largely determined by the content and function of mitochondria (the “powerhouses” of cells). High mitochondrial capacity means muscles can generate more ATP via oxidative metabolism, supporting better endurance and metabolic health.
• Polarized Training: An endurance training strategy characterized by doing most sessions at very low intensity and a few at very high intensity, with little time spent at moderate intensity. A classic polarized model is 80/20: about 80% of training volume is easy (Zones 1–2) and ~20% is very intense (Zone 5 intervals).