Evlo Fitness/Education/Body Composition/Metabolic flexibility: Improve energy & body composition while decelerating aging
Shannon Ritchey
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Metabolic flexibility: Improve energy & body composition while decelerating aging

Below is a written version of Fit Body, Happy Joints: Episode 121 “Metabolic flexibility: Improve energy & body composition while decelerating aging”. 

In today’s post, we will talk about metabolic health. We’ll touch on metabolic flexibility, mitochondrial density, and insulin sensitivity. All of these things are linked. And hopefully I can help you understand how exercise affects each and every one of them. 

Metabolic flexibility: The upward spiral

Metabolic flexibility may be something that you’ve heard of. I want to describe what it is, how to improve it, and how improving it can put you on an upward spiral.  

Here’s what that upward spiral is:

You are metabolically healthy. Therefore, your energy levels, focus, and appetite are stable and regulated:

  • This makes it easy to stick to an exercise routine because you have the energy to do so. 
  • It makes it easier to stick to a nutrition plan because your appetite is more regulated and cravings may be more stable.
  • Because you are consistent with your muscle-building routine and nutrition plan, you build muscle, improve endurance, improve metabolic flexibility, and improve body composition.
  • This muscle and improved endurance greatly contributes to even better insulin sensitivity and metabolic flexibility, which further improves appetite regulation and energy levels. 
  • And the upward spiral continues.

This is how you create a lifestyle that is healthy and sustainable. It doesn’t have to be aggressive either. Gentle consistency™️ with your routine can help put you on this upward spiral.

When you’re on this upward spiral, you can easily take a break from exercise/diet. You can go on vacation and come back to your routine way easier because your body is more adaptable. 

My goal for all of you is to help move away from perfectionism about exercise. To move away from “more is better”. I want you to understand how different processes in your body operate so you can apply gentle consistency™️ towards creating a healthier body. 

What is metabolic flexibility?

To put it simply, metabolic flexibility is how efficiently you can switch between fuel sources. Think switching between using fatty acids (fat) or glucose (blood sugar).

All the processes of your body function better when you are metabolically flexible. 

This is why being metabolically flexible can make you feel better, function better, improve your metabolism, and see greater results from training without the grind. Your body can easily access and use the fuel that it has, both from your fat stores and from food, to operate. 

How to know if you’re metabolically flexible

You know you’re metabolically flexible if:

  1. Your energy is stable and you don’t crash mid-day.
  2. Your appetite is regulated. You don’t get “hangry” or crazy cravings.
  3. You have good focus and memory.

You may be metabolically inflexible if you have any of the following symptoms:

  1. Low energy
  2. Weight gain or trouble with weight loss
  3. Insatiable cravings/“hanger”

Many of these symptoms are related to insulin resistance which I will discuss below. 

If you’re struggling with any or all of these things, I want to teach you how to use exercise to improve metabolic flexibility. 

I will teach you how exercise specifically can improve metabolic flexibility, but I also want to reiterate that this is only one piece of the puzzle. Nutrition, stress management, and even morning light exposure will also be major players in this. 

When to seek individual care

If you are experiencing these things, we cannot rely on exercise alone to solve the problems. 

If you are having symptoms that you’re struggling with, invest in a Registered Dietitian. Many times insurance will pay for a RD visits. Additionally, they can help you dial in nutrition components that are necessary for fitness results. They can run tests and give you specific advice based on an in-depth analysis of your body and lifestyle. 

BUT this is a fitness blog. Let’s talk about what metabolic flexibility is and how to train to improve it. 

Mitochondrial health and metabolic flexibility

I don’t think we can talk about this topic without first discussing mitochondria and how exercise influences mitochondrial health.

I want to describe what this means and why it’s important on a very basic level. 

What are mitochondria and what do they do?

Mitochondria are the organelles in certain cells, like muscle cells, that are responsible for producing energy. Without this energy, the cell can’t do its specific function. 

The number of mitochondria in your cells, otherwise known as mitochondrial density, can be increased or decreased. You can also increase the size of each mitochondria as well as its efficiency. 

The more and the larger our mitochondria, the healthier our metabolic health. 

Metabolic health

Metabolic health refers to how well systems like metabolism, hormone regulation, energy production, cell clean up, and inflammation processes operate. 

The more mitochondria we have, the better our metabolic health, the higher our energy, the faster our metabolism. We can reduce signs of aging and the risk of disease by increasing mitochondrial density. 

We can create more mitochondria (mitochondrial biogesis) with exercise. 

Having more mitochondria makes you more metabolically flexible. You can switch between fuel sources easily. This means everything in your body functions better.

Mitochondria produce energy for the cell and use different sources to fuel and operate your cells. The main sources include glucose and fatty acids. 

The more metabolically flexible you are, the easier you can switch from using glucose to fatty acids to ketone bodies.

Cells will use a combination of glucose, fatty acids, or ketones, depending on the type of cell and the activity you’re doing. 

For example, the brain uses ketones and glucose for energy. The liver uses fatty acid oxidation.  Muscle cells use fatty acids, glucose, and amino acids. 

Exercise intensity and mitochondria

When you are exercising, different intensities will generally require different fuel sources.

Light intensity exercise that you can do for a prolonged period, like walking, easy cardio – primarily uses fatty acids.

At higher intensities, your body will switch from using fat to using mainly glucose. 

Low-intensity exercise and fat burning

People hear “Low-intensity exercise burns fat” and then say “That’s what I need to do to lose weight”. Just because low-intensity exercise uses fat as a fuel source doesn’t mean it’s a significant driver for fat loss. 

Let’s use an example. 

A pound of fat takes a deficit of 3,200 calories to burn. If a 150lb person wanted to use exercise alone to burn 1lb of fat, they would have to burn 3,200 calories. On average, this would take about 10 hours of zone 2 cardio or about 7 hours of HIIT.

However, this likely isn’t even accurate. Because this is assuming calorie expenditure is additive. We have recently discovered that it is constrained, not additive. 

Constrained energy expenditure

Therefore, just because you’re burning 3,200 from 10 hours of cardio, doesn’t mean you have necessarily put yourself in a 3,200 deficit. 

This is because your body tends to compensate for increased calorie burn from exercise by borrowing from other processes to keep your energy expenditure stable.

In other words, that 10 hours of zone 2 cardio isn’t accounting for the fact that your body is adapting and constraining your energy deficit. 

How can cardio help metabolic flexibility?

Bottom line: Yes, exercise can use fat as a fuel source. But fat loss has to come primarily from nutritional changes. Separating exercise and fat loss will be a much more effective and sustainable process for you.  

Now, I’m not saying that you shouldn’t do cardio. Cardio can improve your metabolic flexibility, which may indirectly affect how easily your body can use fat as fuel. 

But the mechanism behind how it may contribute to body composition is likely not as simple as “because low-intensity cardio burns fat”.

When you focus on how exercise affects your metabolism and metabolic flexibility, you will focus your efforts on the things that are driving that metabolic flexibility (i.e. muscle building activities).

Muscle mass and metabolic health

Let’s quickly get into muscle mass, and how muscle affects metabolic flexibility. 

When you have more muscle mass, your insulin sensitivity improves. 

Here’s how this works

When you eat, the carbs in your food are broken down into glucose. This glucose travels through your bloodstream. 

For your cells to use that glucose, your pancreas secretes insulin to “let in” that glucose. Any excess glucose that your cells don’t need for energy gets stored as fat. 

When there is a lot of blood glucose, insulin continues to stay elevated, and your cells start to become resistant to insulin. This can impact body composition and increase risk of disease. 

Essentially, your metabolic health and metabolic flexibility suffer as your body struggles to use fuel efficiently. 

Muscle as storage sites

But muscle is a huge storage site for glucose. This means that there are more places for that glucose to go and it’s less likely it gets stored as fat. 

You then have less circulating glucose because there are more places for it to go. Then, insulin production isn’t chronically high, thus improving insulin sensitivity and metabolic flexibility. 

But this requires:

  1. That you build more muscle using a highly structured plan for fitness and nutrition. Random doesn’t not work very well. 
  2. You empty your glucose stores by taking your lifts close to failure.


Many people lift weights, but aren’t taking those lifts close to failure. This means they aren’t challenging the muscle enough to empty the stores of glucose in the muscle cells.

Therefore, those stores are always “topped off”. This makes for less available storage for glucose to go the next time you eat. 

This is why I recommend keeping your lifts simple, taking them close to failure, and fueling properly to encourage muscle growth. 

Mitochondrial biogenesis

Training close to failure also stimulates mitochondrial biogenesis. When you get a stimulus that is challenging past what you’re used to, an energy shortage is created at the muscle cell itself. This increases the stimulus for more mitochondria. More mitochondria improves metabolic flexibility, which will boost energy, improve metabolism, decelerate aging, and decrease risk of disease. 

Therefore, muscle is one way to improve insulin sensitivity and metabolic flexibility, leading to the upward spiral. 

Your appetite is regulated and your energy is high. This further encourages eating and activity behaviors that will be productive for overall fitness and muscle growth. The adaptation of muscle growth further improves insulin sensitivity and metabolic flexibility, further increasing energy levels and metabolism. 

Gentle consistency™️ for muscle mass

This is why I’m such a fan of gentle consistency™️. Slowly working towards building muscle without breaking down your body and overstressing your system will put you on this upward spiral.

I was just thinking last night: I don’t feel like I work that hard in my fitness routine. Meaning that it doesn’t feel like a “grind.” But I’ve slowly built more muscle than I’ve ever had and I feel like I’m in the best shape of my life. 

“The grind” and metabolic flexibility

One of the reasons the “grind” can actually negatively affect metabolic flexibility is because of cortisol’s effect on insulin. 

When you are in a chronic state of fight or flight, cortisol is chronically high in your body. Cortisol pulls glucose out of the liver to power your muscles to fight or run. This increases insulin. 

When this happens on a chronic level, insulin stays high, which means your body isn’t able to use fatty acids as efficiently. You may notice your energy is tanked, your cravings are high, and that it may be harder to maintain a certain body composition.

This is yet another reason to focus on building muscle over just working “hard”. 

Improving metabolic flexibility through exercise

So if we want to improve metabolic flexibility, what are the recommendations as far as exercise?

Number 1: Build muscle

If you have to pick one thing, I recommend it being strength training.

You can get so many of the same metabolic benefits from strength training that you can with endurance or HIIT training. This is because cardio and strength training can’t necessarily be separated. 

Your heart and lungs don’t know the difference between running or lifting weights. They just know that there is an increased demand for blood and oxygen. And you can keep that demand elevated by doing circuit-style training where you aren’t resting between exercises.

This is what we do in Evlo. We work one muscle group at a time. And while that muscle group is resting, we work another. Therefore, your cardiac output stays high because there is constant demand for blood and oxygen. 

Many times you will be in zone 2 cardio during an Evlo class. You’ll be getting similar cardiovascular benefits as if you were to go on an easy run.

I do recommend adding in additional cardio, but if you’re pressed for time, prioritize the strength training workouts. 

Resistance training, both with high loads and low loads, will result in mitochondrial biogenesis as long as you are approaching failure.

Number 2: Zone 2 cardio (your endurance activities)

In general, it seems beneficial to your cells to improve aerobic capacity and endurance. Improving endurance can help improve your body’s ability to use fat as an energy source.

This is because aerobic capacity may improve mitochondrial volume. As mentioned above, this improves metabolic flexibility and your body’s ability to switch between fuel sources.

Therefore, it’s less about the exercise itself burning fat, and more about your body’s ability to improve its metabolic processes.


I recommend 150 minutes/week of light-to-moderate intensity cardio. 

And I’m a big believer in getting in whatever you can. This could mean walking a little faster in the grocery store, taking the stairs, etc. I highly recommend not thinking of Zone 2 cardio as exercise. Weave it into your life and day.

If you have the capacity/time, try including a longer easy cardio session once/week.

Number 3: HIIT

One thing that I don’t think is talked about enough is the cost/benefit analysis of HIIT.

If HIIT is tanking your recovery or tipping your stress bucket, is it really productive? 

Because like I spoke about earlier, chronic stress affects insulin sensitivity which impairs the body’s ability to switch between different fuel sources. 

Benefits of HIIT

HIIT has been shown to improve insulin sensitivity because it depletes glycogen stores. 

This goes back to when your glycogen stores are low in muscle tissue, blood sugar has more places to be stored instead of being stored as fat. 

However, strength training close to failure also does this. So if you aren’t tolerating HIIT, you can still get benefits from an insulin-sensitivity perspective by training close to failure. 

HIIT has also been shown to increase mitochondrial function via mitochondrial biogenesis. But again, lifting close to failure can do this as well. 

Studies have shown similar improvements in mitochondrial function following lower intensity/longer duration cardio and SIT (sprint interval training) or HIIT training. Therefore, if you aren’t tolerating HIIT because of joint health or overall stress, know that lower intensity cardio is still beneficial. 

SIT Training

However, SIT (all-out intervals of 6-30 seconds with 1-5 minutes of rest) specifically has been shown to improve mitochondrial respiration more than other forms of exercise. But I think it’s important to take an individual approach here. Less is more.

Again, it’s a cost/benefit situation. You’re still going to see great improvements in metabolic flexibility and mitochondrial function if you’re just consistent. So although SIT may improve certain markers, it doesn’t mean that gentler forms of exercise are a waste of time or aren’t moving you forward. 

On the other hand, if you don’t have time to do lower intensity cardio, SIT can be a great way to save time. 

In fact, just one session/week of SIT training may be sufficient to drive significant mitochondrial adaptations. 

Things to avoid

I don’t recommend going to 45-60 minute HIIT classes. Iif you are going to add HIIT, track your recovery and stick to 1-2 short HIIT sessions. 

I’m currently not doing HIIT and I’m actually seeing better results and feeling better. For me, this was a cost/benefit situation. I wasn’t recovering well and my body just felt like it needed a break. 

I think there can be a cost/benefit analysis that you do for yourself. Don’t be afraid to add it in or take it out depending on the chapter of your life. 

Remember that improving metabolic flexibility isn’t just exercise. Nutrition is a big piece. If you aren’t eating .75-1g of protein/ideal lb of body weight, you’re going to struggle to build muscle. So make sure to watch the nutrition content if you’re an Evlo member.

Let’s summarize

If you want great energy levels, good body composition, and a body that doesn’t feel broken down, we want to do a variety of exercise. Not only does this help avoid overuse and joint pain, but it also allows for great metabolic flexibility. This correlates to improved energy, improved metabolism, and decelerated aging. 

We aren’t seeing exercise as a tool to burn fat, but more to support a healthy, robust metabolism. This helps you create the lifestyle changes that will improve body composition because your energy and appetite are more stable. Remember: fat loss is going to come primarily through how you’re eating.

Resistance training close to failure can improve metabolic flexibility because of mitochondrial biogenesis. Training close to failure also empties your glycogen stores in the muscle, meaning glucose from food has more places to go. This improves insulin sensitivity and your body’s ability to switch between using fat and sugar as fuel.