The oxidative energy system (OES) uses fats, glucose, amino acids, and oxygen. It is primarily active during low-intensity, long-duration activities. This system is efficient in producing energy but requires oxygen to function effectively.
Training at “moderate” intensity can develop the oxidative system. Moderate intensity requires an effort that is a 3-4 on a 1-10 scale.
One can also use the “talk test” to gauge training intensity. If you’re exercising at a rate that allows you to carry a conversation with some effort, then you’re likely using the OES. If you must pause to catch your breath, you may be training too hard.
Heart rate and blood lactate can also be helpful guides for training intensity. The upper limit of the oxidative pathway is around 70-80% of your maximum heart rate and 2 mmol/L of blood lactate.
Training in the OES is often called aerobic conditioning, Zone 2, base training, or long slow distance (LSD).
For the remainder of this email, I will use Zone 2 and base training interchangeably.
5 Zones Training Model
The 5 zone training model concept is commonly associated with fitness trainer Joe Friel. Joe Friel is an American endurance sports coach and author who developed the concept of training zones for different intensity levels in athletic training.
Zone 2 in the 5-zone model represents training intensity when blood lactate is less than 2 mmol/L. As the intensity increases, the muscles use less fat and more glucose. One of the byproducts of burning glucose (sugar) is lactate. When blood lactate exceeds 2 mmol/L, many consider this to be the entry into Zone 3.
Zone 2 and Your Mitochondria
One of the main reasons for Zone 2 training is that it can positively affect mitochondria, which are the powerhouse of cells responsible for energy production. Here are some effects of Zone 2 training on mitochondria.
Mitochondrial biogenesis: Zone 2 training stimulates the process of mitochondrial biogenesis, which refers to creating new mitochondria within cells. Regular base training can lead to an increase in the number of mitochondria in your muscles, enhancing their capacity for energy production.
Improved mitochondrial efficiency: Base training promotes adaptations in mitochondria that enhance their efficiency in utilizing oxygen and generating ATP (adenosine triphosphate), the energy currency of cells. This improved efficiency can translate into better endurance and sustained performance.
Enhanced fat metabolism: Zone 2 training helps improve the mitochondria’s ability to oxidize or burn fat for energy. This adaptation benefits endurance athletes by enabling them to rely more on fat stores during long-duration exercises, conserving glycogen and delaying fatigue. Improvements in fat metabolism are also highly beneficial for those struggling with diseases related to metabolic syndrome (e.g., obesity, type-2 diabetes, heart disease)
Assessing Metabolic Flexibility
The relationship between glucose metabolism, fat oxidation, and blood lactate is well documented.
Fat oxidation negatively correlates with blood lactate. As work intensity increases, the muscles use less fat and more glucose (sugar) to make energy.
Glycolysis — the process by which muscles turn glucose into energy — positively correlates with blood lactate. The muscles use more sugar as work intensity increases, producing more lactate.
As work intensity increases, there is a point at which the body begins using more glucose than fat. This point is the “crossover” concept. More commonly known as the anaerobic or lactate threshold.
One can use this crossover point to assess their “metabolic flexibility.” Metabolic flexibility refers to the body’s ability to adjust its metabolism and switch between different fuel sources based on energy demand and nutrient availability.
Zone 2 Improves Metabolic Flexibility
The graph below shows the improvements in the metabolic flexibility of an individual under the coaching of Iñigo San-Millán. San-Millán is an applied physiologist and the Director of the Exercise Physiology Lab at the University of Colorado School of Medicine.
Graph reproduced and modified from (San-Millán, 2021)*
The person in the study trained for three months. They conducted 90% of their training in Zone 2. And the remainder of their training at Zone 4.
The gray circles mark the point of crossover before and after the intervention. As you can see in the graph, this individual’s metabolic flexibility has dramatically improved.
The patient came to San-Milán with metabolic syndrome (overweight, high blood pressure, elevated cholesterol). And with three months of mostly Zone 2 exercise, they improved their metabolic flexibility to that of someone who regularly exercises three times (150 minutes) a week.
Zone 2 Frequency and Duration
Zone 2 training is uniquely qualified to improve mitochondrial density, which can help someone avoid or escape metabolic derangement and support higher levels of athletic performance.
The frequency and duration one should train Zone 2 depends primarily on two things:
- Performance and sports goals
- Current health status
Let me offer three possible conceptual arguments for how often one might include Zone 2 in training.
- An endurance athlete or someone with metabolic syndrome might achieve their best results from 3-5 Zone 2 training sessions a week.
- A strength and power athlete might only need to spend 2-4 times a month in Zone 2.
- Someone interested in general fitness might get everything they want out of 1-2 times a week.
The duration of a Zone 2 training session will also depend on your goals and current level of health. However, most people will benefit from 60 to 90-minute sessions.
* San-Millán, Iñigo. “Assessing Metabolic Flexibility and Mitochondrial Bioenergetics.” Clinical Bioenergetics, Elsevier, 2021, pp. 245–68.
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Diving into performance – My VO2 test