Metabolic flexibility and fat loss

Metabolic flexibility is your body’s ability to switch between carbs and fats for fuel to match the intensity of the activity you are doing.

As you sit there reading this, your body should primarily be using fat as fuel. Activity levels are low, energy production doesn’t need to be jacked up high and therefore, your fuel source of choice should be fat.

If you were to head to the gym, go through your warm-up, gradually increasing intensity, your fuel source gradually moves more toward carbs. Note that you are never using 100% fat or carbs. You will be more toward one end of the spectrum or the other, but never all the way across.

When you are strength training or doing higher intensity conditioning, you’re going to be further towards the carbs end of the fuel pool. ATP, the body’s energy currency, needs to be replenished more quickly, and carbs are the fastest way to get that done. Fat is still in the mix, but at a much lower percentage.

Your ability to use the right fuel at the right time is not only essential for better performance, but also for health and fat loss. After all, you can’t burn more fat if you can’t use it.


Your ability to switch between carbs and fat is determined by the intensity of what you are doing, and the presence of insulin.


As intensity goes up, energy demands increase, and carbs are your friend here. High-intensity efforts need carbs to fuel them, so having some carbs before training can be a massive boost. Since fat usage for ATP replenishment is relatively slow, your ability to switch to carbs and use them appropriately is a huge benefit.

During low-intensity periods, fat is the optimum fuel source. Your ability to switch down to use this fuel source is obviously beneficial for fat loss, but also for long duration performance. If you can’t burn fat, you won’t get leaner, and your ability to keep going, even at lower intensities is impaired unless you can eat more carbs to fuel yourself.


Insulin is your fuel selector switch. As carbs are ingested, the rise in blood sugars triggers insulin to shuttle those sugars into muscles for use (or into fat cells via various mechanisms for storage for later use). The rise in insulin helps you move over to carbs (high %) and fats (low %) as fuel.

When exercise intensities go up, having some carbs before training does 2 things:

  • provides fuel for the work you are doing, and
  • raises insulin levels (the fuel selector) to preferentially use those carbs as fuel.

When training is over, and you’re on your slow walk home/ chilling out after, you should be able to switch back to using fats as your main fuel.

Metabolic inflexibility impairs your body’s ability to easily switch down to using fat.

This inflexibility can lead to long-term issues. If you can only use glucose and glycogen for fuel, you need to keep consuming carbs to maintain energy levels. Carb consumption leads to insulin release and over time your tissues get so used to the presence of elevated insulin that they become less and less sensitive to its ability to do its presence.

More and more insulin is required to deal with each rise in blood sugar. You can effectively wear out the pancreas and your ability to produce insulin grinds to a halt. This is Type II diabetes, cases of this very preventable disease have doubled in the last 20 years through a combination of poor dietary choices and sedentary lifestyles.

How to get your flexibility back.

  • Lower intensity efforts such as walking, low effort biking or rowing, should use more fat as fuel. Performing these in a fasted or semi-fasted state (ie before breakfast, having eaten the previous evening) can improve your ability to use fat more easily.
  • Increasing the times between meals will also lead to lower insulin levels after the rise in blood sugars has been dealt with. Lower insulin = a move to fat as fuel. Thereby improving your ability to switch down to fat use at lower intensities.
  • Performing a 24 hour fast once a week has also been shown to improve your metabolic flexibility via an extended version of point 2.
  • Have most of your carb heavy meals pre-training, to provide your high-intensity fuel. Then have mostly protein, veggies and some fats the rest of the time.

If you have questions on this, or any other training and nutrition topic, please feel free to get in touch.

Stay strong,




Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.