Achieving Metabolic Flexibility
Sometimes the best way to elucidate a concept is to compare and contrast two ends of a spectrum. Here we will use the examples of an elite cyclist and a type 2 diabetic to highlight the concept of metabolic flexibility and its importance for your health and fitness.
An elite cyclist and a type 2 diabetic both have bodies that require movement, and their muscles make that movement possible. However, the health of their muscles are what make them so different in their physical abilities and overall health trajectory. In order to understand this difference, we must examine the intricacies of how muscles function.
Muscles are made up of very long cells, or muscle fibers, and there are different kinds of muscle fibers for different levels of performance. There are muscle fibers that are best for a slow, long-duration activity, such as walking, and these are called type 1 (traditionally “slow twitch”) muscle fibers. And then there are muscle fibers that are best for fast, more forceful movements, such as sprinting, and these are called type 2 (traditionally “fast twitch”) muscle fibers.
Each muscle fiber type specializes in different ways of using fuel. The type 1 fibers, for example, have a lot of mitochondria, allowing them to be proficient in using fat stores as a fuel source. Mitochondria are known as the “powerhouse” of a muscle fiber, and the more mitochondria a muscle fiber has (known as its “mitochondrial density”), the healthier it is and better it is able to perform.
Type 2 muscle fibers, on the other hand, have less mitochondrial density because they can bypass the mitochondria and generate energy much more quickly by a process called glycolysis, which breaks down stored glucose (glycogen) as a fuel source. There is even a subset of type 2 muscle fibers, called type 2x, that have virtually no mitochondria and simply use pure stored energy (ATP or creatine phosphate) for highly powerful, rapid movements.
Why does this matter?
A type 2 diabetic has unhealthy muscle. Their muscles do a poor job at turning the fuel sources described above (stored fat and glucose) into usable energy. The mitochondria within the muscles of a type 2 diabetic have compromised function. As a result, a type 2 diabetic will generally have low energy despite a surplus of fuel availability in the form of high blood sugars and excess body weight.
In contrast, the muscle of an elite cyclist is optimal in its ability to turn fuel sources into energy. Their muscles will have a high mitochondrial density, meaning they can use stored fat as a fuel source with ease, and they are able to take more glucose from the blood and store it as fuel (glycogen). In essence, the muscles of an elite cyclist are healthier because they are more “metabolically flexible”, meaning they can more efficiently switch between different fuel sources for energy depending on fuel availability and physical demands.
Where do you fall on the spectrum?
Our criteria for diagnosing diabetes gives the impression that our metabolic health is a binary system – we either have a healthy metabolism (i.e., healthy weight, normal blood sugars), or we don’t (i.e., diabetes). However, everyone’s metabolic health falls on the spectrum between optimal metabolism (i.e., the elite cyclist) and dysfunctional metabolism (i.e., the diabetic), and it is crucial to understand where you are on that spectrum and the levers you can pull to shift your metabolic health.
The Big Lever: Zone 2 Training
One of the most impactful ways you can better your metabolic health is through zone 2 training. Zone 2 training describes a category of exercise where type 1 (“slow twitch”) muscle fibers are activated and mitochondria go to work to use stored fat as fuel. There are 5 zones within this zone-based training model that correlates physiological responses with exercise intensity. Specifically, the production of lactate, which is a byproduct of glucose metabolism, is measured as a blood marker to identify each zone (i.e., in zone 2 training, lactate levels are typically between 1.5 - 2 millimole, indicating a steady state where lactate production is balanced by lactate clearance. There is no buildup.).
However, as most people don’t want to measure their lactate levels during exercise, a good proxy is heart rate. Zone 2 training is typically when your heart rate holds at about 60-70% of your maximum heart rate (MHR) during exercise. The most common way to measure maximum heart rate is the following calculation: MHR = 220 - your age. Therefore, if you are 35 years old, your estimated MHR would be 185 beats per minute, and your zone 2 training would fall within 111 - 130 beats per minute. This is a much more accessible way to understand whether you are exercising in zone 2, since many people now have wearable technology that relays heart rate data; however, if you do not have heart rate data accessible, another proxy to knowing if you are training in zone 2 is the “talking test”. When exercising in zone 2, you should be breathy, but you should be able to carry on a conversation in short sentences. Essentially, zone 2 is that stage where you are definitely exercising, but it’s comfortable.
Training in zone 2 is understood to be the single best tool for increasing mitochondrial function and improving metabolic health. What’s more, training in zone 2 is also very effective for acutely reducing blood glucose, since it allows muscles to take–in blood glucose for energy without requiring insulin.
Where does diet fit in?
Did you know that many elite athletes, such as the cyclist we have been referring to through this article, consume thousands of calories in pure sugar per day during their training, above and beyond their regular balanced meals? And yet they are of a healthy body weight and are regarded as the gold standard of metabolic health. So let’s squash a misconception right up front: carbs don’t kill (or make you fat). The best athletes in the world are eating as much as 800 grams, or 3200 calories, of pure carbohydrate per day during their intense training sessions. Aside from the fact that they are burning many more calories than a sedentary individual, the difference here is muscle health.
Did you also know that most elite endurance athletes, such as runners, cyclists and swimmers, spend 80% of their training in zone 2?
The bottom line here is that muscle health comes first, and diet comes in second to support muscle health. When looking to improve their metabolic health (i.e., blood sugars, body weight), most people immediately look to dietary measures, often through carbohydrate restriction or too severe of a calorie reduction. However, diet should support exercise, and dietary restriction done wrong can lead to low energy, loss of muscle mass, and a lack of motivation to be physically active.
Reducing calories or carbohydrates can be effective for losing excess body weight or lowering blood sugars; however, this is symptom management. Shifting away from managing symptoms and focusing instead on root causes of metabolic function (i.e, muscle health) is what will slide you more towards the “elite cyclist” on the spectrum rather than the “diabetic”.
Instead, look to dietary measures that support muscle health. These would include: 1) eating an appropriate amount of calories for muscle fuel and, 2) eating enough protein to support muscle growth and repair.
If you are choosing extreme dietary measures at the expense of muscle health, you are essentially “missing the forest for the trees”.
If you want to learn more about how you can benefit your own metabolic health and improve your athletic training, schedule a FREE Discovery Call here to see if we are a good fit in working together.
this was very helpful!