All About Metabolic Flexibility and Intuitive Fasting

Over the last few centuries, our bodies have been led astray. They haven’t been subjected to periods of time without food; we’ve been snacking up a storm, and the mechanisms that are supposed to jump into gear during times of fasting have become rusty.

Our bodies have suffered as a result - but so have our intuitions. Thanks to this lack of fasting time, many of us have lost our “metabolic flexibility,” a term that will come up throughout the course of this book. Metabolic inflexibility is the death of intuitive eating.

So what does having a flexible metabolism really mean? Metabolic flexibility is the body’s ability to adapt and use whatever fuel is available to it. If you’ve eaten recently, that fuel is glucose (the sugar that’s in your blood). If it’s been a while since your last meal (or all your blood glucose has been used up), that fuel is stored fat. If you’ve ever entered the “fat-burning” stage of a workout, it means you’ve used up all your glucose and your body is now burning fat for fuel.

You might think of sugar as the ultimate hit of energy, but, as it turns out, fat is a much more efficient fuel source for your metabolism. Think of it like this: your body is a fire in the fireplace, and the kindling is sugar. This type of fuel provides short, effective bursts of flames to get the fire going, but kindling is quick to burn, doesn’t last long, and you have to constantly replenish it to keep the fire burning. The same is true for sugar; you get a temporary energy high and then you crash not long after. If you’ve ever eaten a bunch of sweets and then crashed hard, feeling like you needed a nap a few hours later, you’ve experienced this phenomenon firsthand.

In contrast, fat is like a log of firewood. You can put a log in the fire and know that for hours you’ll have a slow and steady fire burning. Fat is the same: it provides long-term sustainable, stable energy for your body. Being able to rely on either kindling (sugar) or firewood (fat) to keep your fire going is the definition of metabolic flexibility.

Unfortunately, with our high-sugar, high-carb, constant-snacking culture, it’s been a long time since many of us have burned any logs of firewood. In fact, many of us currently rely almost entirely on kindling (glucose) to fuel us and, as a result our cells have lost their ability to quickly and efficiently switch from using sugar to using fat for fuel. In other words, we’ve lost our metabolic flexibility and, therefore, our ability to maintain consistent energy levels, brainpower, and appetites.

This can be a difficult concept to grasp, so it’s helpful to know where in the body all this is occurring. The answer is the mitochondria. If you’re not familiar with this term, the mitochondria are the small energy centers in our cells. They have a few important functions, but above all else, they’re responsible for converting oxygen and nutrients into adenosine triphosphate (ATP), which is the main energy currency in the body. When you lose metabolic flexibility, your mitochondria lose their innate ability to efficiently switch between glucose and fat burning to maintain consistent energy levels - a phenomenon that’s been coined “mitochondrial indecision,” a sort of metabolic purgatory.

The loss of this adaptation can leave us shaky, ravenous, and hangry (have you met her? Hungry and angry’s evil spawn), and - considering the job of the mitochondria is to create cellular energy - extremely fatigued and groggy. We will always be looking for our next hit of sugar, which temporarily pulls out of our cranky, foggy-brained stupor.

As you can guess, having trillions of cells inside us that behave like mini sugar addicts can make it impossible to connect to our body’s intuitive eating patterns. When your body is desperate for sugar because it can’t rely on burning fat for fuel, you’ll be hungry and craving something sweet, carby (or both!) every few hours no matter how much you eat. Those cravings will be so strong that they’ll crowd out your intuition.

If this is you, it’s not your fault. We haven’t given our bodies adequate rest from eating to get in touch with what they really want and need. So, what’s the key to accessing intuitive eating? The answer is regaining metabolic flexibility, which we will do by reintroducing our bodies to times of fasting.

Dr Will Cole’s new book, Intuitive Fasting, is available everywhere books are sold. Order your copy now.

Excerpt published with permission from Penguin Random House