Our aim when exercising is to reach what I call the mitochondrial threshold. To understand what it is, let me briefly explain the relationship between energy and mitochondrial activity.
Mitochondria is an organelle that is present in most Eukaryotic cells (such as human cells). It’s main function is to generate energy in the form of adenosine triphosphate (ATP). mitochondria use byproducts of glucose breakdown that originates either from the food we eat or our energy stores (or converted from fat and protein), to fuel this process. This is called aerobic respiration because it requires oxygen to work.
Why is ATP important?
Our body requires a constant flow of energy. Even when we are sleeping. Our brain, which never turns off, consumes around 20% of our energy. Glucose and fat are great stores of energy but they are too slow a resource to be used in the chemical reactions that keep our cells alive. This is why we need ATP. ATP is very efficient in providing energy for chemical reactions but is very unstable and is not a good energy storage unit.
Think of ATP as the cash in your wallet that you use for daily exchanges. Glucose is more like a medium term store of value like your current bank account and fat is more of a long term store of value like your savings account or gold investment. That’s why ATP is usually called the main energy currency of our cells.
Back to mitochondria.
Mitochondrial dysfunction has been linked to a myriad of chronic diseases, from heart disease to metabolic disease and diabetes and even ageing and cancer. It’s not controversial to say that healthy functioning mitochondria are a good marker of general health.
How do we increase the efficiency of our mitochondria? We need to talk about the other way our cells generate ATP first, anaerobic respiration.
Anaerobic respiration is when our cells produce ATP from the breakdown products of glucose without requiring oxygen. The problem with this energy pathway is that it is substantially less efficient than aerobic respiration. In fact, aerobic respiration generates 13 times more ATP from the same amount of glucose compared to anaerobic respiration.
When is anaerobic respiration used? When oxygen is limited, and more importantly, when the mitochondria are working at full capacity but our cells are still demanding more energy (i.e. when our mitochondrial capacity can’t keep up with our energy requirements)
When we exercise, we increase our energy demands. The higher the intensity, the more ATP we need available, the more mitochondria are utilized to pump out that ATP to meet our energy requirements. The point at which we reach maximum mitochondrial capacity and start generating ATP through anaerobic respiration is what I call the mitochondrial threshold (in reality, anaerobic respiration starts to kick in before this threshold is reached but this is a useful simplification)
This is the sweet spot we want to be hitting when we exercise. Why? Because reaching this threshold signals to our cells that they need to increase their mitochondrial capacity by building more mitochondria and improving mitochondrial efficiency. This raises our mitochondrial threshold and boosts our metabolic efficiency.
So we know our main goal when exercising is to reach the mitochondrial threshold but how do we know when we’ve hit that spot?
In a lab, we could measure lactic acid (a byproduct of anaerobic respiration). Another useful measure is VO2 which is the rate at which our body consumes oxygen during exercise. The aim here would be to reach around 70% of our maximum oxygen consumption capacity or VO2 max.
But most of us don’t have a moving lab at the gym. A good proxy is to use heart rate zones, which is a way of evaluating the intensity of your training by measuring your heart rate compared to your maximum heart rate.
Each one of us has a resting heart rate and a maximum heart rate. The most accurate way to measure your maximum heart rate is to do an exercise tolerance test (directly measure your heart rate while on a treadmill exercising at maximum intensity) but there are calculators you can find online that can be used to estimate your maximum heart rate. Exercise can be categorised into 5 “heart rate zones” based on the level of intensity in relation to maximum heart rate.
Most experts believe that reaching around 70% of our maximum heart rate (or heart rate zone 2) is enough to reach the mitochondrial threshold. There is also evidence that higher levels of intensity for shorter periods (HIIT) can stimulate mitochondrial function.
In short, one of the most important ways to improve our metabolic health is to exercise in zone 2 regularly and use bouts of high intensity training to boost our mitochondrial efficiency and prevent mitochondrial dysfunction.