Metabolic Flexibility: How to turn your body into a fat burning machine

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Metabolic Flexibility: How to turn your body into a fat burning machine


What is metabolic flexibility?

"Due to possible discontinuities in both the supply and demand for energy, humans need a clear capacity to use lipid and carbohydrate fuels and transition between them.(1)" 

This capacity is a healthy state called metabolic flexibility.

In English, metabolic flexibility is the ability to switch from one fuel source to the next; from fats to carbs and carbs to fat. 

Metabolic inflexibility is just the opposite; the inability (or limited ability) to switch from one fuel source to the other.

How does Insulin impact?

There are many hormones in the body which affect your body's composition and moods. A key to understanding metabolic flexibility is the vital role the hormone insulin plays. 

In a healthy state with normal insulin metabolism, humans can effectively switch from a primarily fat metabolism to a carbohydrate metabolism, and vice versa.
For instance, having more muscle mass mean that more energy is exerted at rest to maintain the same amount of mass than fat. Furthermore, both storage capacity of glycogen and lipids and your level of insulin sensitivity are affected. 


Key Information to Know about Insulin

  • All foods result in the release of insulin.

  • Insulin is the fuel selector switch.

  • When levels of insulin are low, the body is in fat burning mode.

  • When insulin is high, the body is shifted towards carbohydrate metabolism (and fat storing mode).

Throughout the day, our insulin levels go up and down in peaks and troves until we go to sleep. During the hours of the night, our insulin levels are at their lowest which makes it the time when we are most likely to burn fat.

How to know and test if you’re metabolically flexible?

Here are the tests recommended by Mike T Nelson, Ph.D. (

1 – High Insulin

Taking in a whey protein drink [or a high Gi non-dairy protein shake] with some simple carbs on a relatively empty stomach is an effective way to jack insulin levels up. I recommend people try this as their breakfast a few times during the week and monitor their reaction.
  • If you feel like collapsing face-down in your drink and spilling it all over the table, you probably have an issue with high levels of insulin and the resulting crash from it.

  • If you feel great, no worries.

2 – Low Insulin
For this test, I recommend you do some fasting.
In a fasting condition, there's no food coming in to stimulate any increase insulin. If you find that you can't make it more than two hours, you're probably inflexible to burning body fat. Not a good state to be in, my large friend. If you can easily go for 5-6 hours, that's much better.
I have advanced clients hit a 24-hour fast since there's data showing insulin levels reach their lowest point at around the 24-hour mark. Don't worry about the dastardly effects of catabolism; the amount of catabolic breakdown of muscle proteins is insignificant during a fast and isn't worth worrying about and releasing more cortisol.
If you're really concerned, just include one strength training session since the NET result of a strength session is an anabolic response.”

*These are the opinions of Mike T Nelson. Before doing any changes to your diet, consult a nutritionist first.


How to eat and train to become metabolically flexible?

The key to becoming metabolically flexible are timings and diet/nutrition. Both are as important as each other when it comes to fat burning. 

If you did a high-intensity workout such as sprints and power session, increasing your intake of protein and carbs before and the after session will give you an increased level of insulin. The body is then able to utilize the carbohydrates better, giving you a better output in performance.

If the next day you were to do low-level aerobic training. It may be beneficial to do it fasted. When doing slower, workouts for a longer period of time, the body's best source of energy is fat.

By doing this you are matching your fuel to the intensity and maximizing your fuel sources. The benefits to being very metabolically flexible are when you come to competition, you will be able to maintain performance without flag lining. 

What should your macro split be to be the most efficient?

For an active person, i.e. does high-intensity training 3 - 4 times a week

  • 0.75 grams per lb of body weight (total body mass) to 1.5 max

  • 60 - 80g Fat

  • 20g carbohydrates with 20g protein pre and post workout

  • Adjust Carbohydrates to be losing fat whilst still maintain performance

  • Recommended to increase protein if cutting carbs to lower end and vice versa. However, as noted, having a low carbohydrate level for a high-intensity training have been shown to be less beneficial.

For instance, a 140lb (10st or 65kg) person this would equate to: 
105g - 200g P | 60g - 80g F | 105g - 200g C


How to maximize your Metabolic Flexibility?

Eat more protein

Numerous studies have shown that in a calorie deficit, 0.75g per lb of body weight was the minimum. Body builders have shown to increase protein to up to 1.5lb per body weight. This method should only be used for a short period of time as you want to do the minimal Effective Amount (mEA). Work by Devkota S et al. (2) showed that replacing carbs with protein was beneficial in shifting molecular activity in muscle vs. fat; which should improve body composition (more muscle, less fat).



Like most of us (including myself) tend to eat the same things day in and day out.

Having more variety in your life allows you to feed your body with a huge array of micronutrients. These are critical for health, performance and most importantly, prevents OVERexposure to any one particular item or toxin too!  


Add In a Fast

By fasting, exercising on an empty stomach. Fasted cardio has been shown to burn more fat during the session. It has also been shown, that during a fasting condition, insulin levels are low, pushing the body towards more direct fat utilization (burning).

The later is the best form to increase metabolic flexibility as studies have shown to be easiest strategy to maintain and have the quickest recovery time compared to more complex methods.


Author: Charleh Dickinson

Founder & MD of Designed2Eat
Food Marketing Management Graduate at Sheffield Hallam University

Digital Marketing Manager & Food Photographer for KUB Ltd 

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Disclaimer: Please note that we are not a professionally qualified Doctor, Dietician or Nutritionist. The information on our blogs are based only on my own experiences and the research we have found in academia.
If you are interested in changing your diet we would highly recommend doing so under the care of a qualified Health Professional. We firmly believe that there is a no diet that suits everybody. You have to go out there and discover it for yourself and enjoy the journey along the way.

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