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.”

What is Metabolic Flexibility?

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.

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: 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.

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The loss of this adaptation can leave us shaky and ravenous 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 us 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.

Why Metabolic Flexibility Is the Key to Intuitive Eating

As a functional medicine practitioner, I frequently hear from my patients that they struggle with hunger signals and cravings that have gone haywire. Unfortunately, feeling like you’re never fully satisfied and always looking for the next hit of sugar makes maintaining a healthy weight a challenge, and it can affect wellness, productivity and quality of life in a major way.

As a general rule, if you’re craving candy, carbs and caffeine, that’s not your intuition talking. In fact, you should take those yearnings as a big sign that some of the signals in your body have gone haywire and are now sending you the wrong messages. Let me be clear: 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 can do by reintroducing our bodies to times of fasting. Reestablishing metabolic flexibility is one of the main goals of my book Intuitive Fasting.

Let me warn you: At first, fasting might not feel intuitive at all; you’ll be fending off cravings and hunger and fatigue. Plus, it will require you to uproot your habits, which can be more difficult than you think. But once you achieve metabolic flexibility by sticking to the fasting plan and eating the right foods, fasting will become effortless and intuitive. You’ll be able to trust your body to function at optimal capacity — whether you’ve eaten 6 minutes ago, 6 hours ago, or 16 hours ago.

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