Looking to drop some weight? It’s time to cut carbs, right? After all, that’s what everybody’s doing. Not so fast. Before you point your finger at carbs (and unnecessarily deprive yourself), ask yourself this critical question: How metabolically flexible are you? You see, metabolic inflexibility—not necessarily carbs—may be the enemy, and on the flipside, metabolic flexibility may be the key to looking and feeling your best. What is metabolic flexibility? Why is it so darn important? How do you know if you’re metabolically flexible? What are some ways to boost metabolic flexibility? These are good questions, my friend, and we’ll be answering them (and more) below. Enjoy!

What Is Metabolic Flexibility?

Metabolic flexibility refers to “the capacity for the organism to adapt fuel oxidation to fuel availability.” ¹ Pretty straightforward, right? Okay, in simple terms, metabolic flexibility means your body can efficiently and effectively switch between using carbs and fats for fuel based on availability and need. [Note: Under normal circumstances, most people are burning a combination of fats and carbs to meet the body’s energy demands.]

The efficient ability to switch back and forth between fuel sources makes a great deal of sense from an evolutionary standpoint. A flexible metabolism was a necessity for the human species to adapt its energy needs to various climate, seasonal, and vegetation conditions. To put it more simply, metabolic flexibility safeguarded human survival independent of food availability. ² At a fundamental level, the human metabolism (and metabolic flexibility) has been shaped by a wide range of different food sources (biodiversity), abundant daily exercise often under fasted conditions (foraging behaviors), as well as an unpredictable food supply (intermittent fasting).

In practical terms, Inigo San Millan, director of the Sports Performance Program at the University of Colorado Sports Medicine Center, says, “If you’re not metabolically flexible, you’ll have a hard time burning fats or sugars, and that can set you up for disease. If you are, you can enjoy the pleasures of a wide variety of foods and be healthy.” ⁴

Yes, at the extreme end of unhealthy metabolic flexibility—that is, metabolic inflexibility—lies disease, including metabolic disorders like type 2 diabetes, obesity, cardiovascular disease, hypertension, dyslipidemia (e.g., high triglycerides), Alzheimer’s disease, non-alcoholic fatty liver disease, certain types of cancer, and more. But even before things get that “ugly,” the path to metabolic inflexibility may manifest as:

• Feeling tired and fatigued
• Battling brain fog
• Not having the energy to exercise
• Not having the stamina to complete physically demanding tasks
• Dealing with cravings and hunger
• Difficulty with weight loss
• Struggling to manage blood sugar levels
• And more

Simply put, metabolic flexibility is a key to looking, feeling, and performing your best and living your best life.

With that background info in your pocket, here are two fairly common—albeit very distinct—examples of metabolic flexibility (or, lack thereof).

Two Popular Examples of Metabolic Sensitivity

First, let’s pay homage to the increasingly-popular ketogenic diet, which, with its severe carbohydrate restriction, is a stark contrast from the typical American diet. Along those lines, most people aren’t metabolically flexible enough to make this transition very smoothly, and a hefty chunk of folks who jump on the keto bandwagon go through a rough patch known as the keto flu, which is more likely to strike those who are metabolically inflexible.

Here’s another way to think about the keto flu. In a way, it’s your body withdrawing from a long, heavy reliance on carbohydrates. Your body has been conditioned to be a carb-burner, and with keto, you’re forcing it to learn (or re-learn) how to become a fat-burning machine. In other words, the low-carb flu is basically a manifestation of an adaptation to the ketogenic diet. And like many things in life, change often brings with it some discomfort.

Granted, this is an extreme example, and there may be a constellation of factors contributing to keto adaptation and the keto flu. And that brings us to example number two. Ready?

The epitome of metabolic inflexibility is insulin resistance. In fact, researchers first proposed the concept of metabolic inflexibility as an explanation for metabolic disorders, such as obesity, insulin resistance, and type 2 diabetes.

In a nutshell, researchers found that the flexibility of muscles to switch between burning fats and carbohydrate (sugar) is related to insulin sensitivity, percentage body fat, and fitness. While it can get a little thick in the weeds, folks who are metabolically inflexible have a difficult time ramping up fat burning when they should (such as during periods of fasting) and have trouble using sugar after a meal (when insulin is present). ⁵

This explains, in part, why someone with insulin resistance may have difficulty losing body fat while struggling with high blood sugar. This classic example illustrates the unfortunate reality that the metabolically inflexible individual faces: Lackluster fat burning and inefficient ability to use carbs. The result: A heavy reliance on external sources of fuel. In other words, overeating to satisfy a seemingly insatiable ability to meet energy demands.

Just to sum up what metabolic flexibility should look like…under fasting conditions, when insulin is low, the body should be heavily reliant on fat for fuel. When insulin is present (such as after a meal), the body should switch over to burning more carbohydrate, as insulin suppresses fat burning (and increases fat storage) and promotes the use (and storage) of glucose. During exercise, energy needs ramp up considerably, and in turn, the body increases both sugar and fat use for fuel.

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Are You Metabolically Flexible?

While it seems like obesity may go hand-in-hand with metabolic inflexibility and being “normal” weight may be a good indicator of healthy metabolic flexibility, that does not appear to be the case. For example, you may have heard the expression “fat and fit,” which is slang for folks who are “overweight” or “obese” (in terms of body mass index, BMI) with healthy metabolic flexibility (i.e., absence of insulin resistance and other metabolic disorders). The scientific community calls this “metabolically healthy obese” (MHO). On the flipside, it’s completely possible to be “normal” weight yet be metabolically unhealthy. In simple terms, weight alone is not a sufficient indicator of metabolic flexibility and/or health. ⁶

So, how do you know where you sit on the metabolic flexibility continuum? You may have guessed that markers of blood sugar management and insulin sensitivity are considered surrogate markers of metabolic flexibility. Along those lines, fasting blood sugar, tests for insulin resistance (e.g., HOMA-IR), and oral glucose tolerance tests may all provide an indication of metabolic flexibility (or lack thereof). However, these metabolic snapshots don’t really tell you how metabolically flexible you are.

Keep in mind that the concept of metabolic flexibility is all about the body’s ability to adapt its fuel selection based on availability and need. Along these lines, more sophisticated tests for metabolic flexibility assess the difference in fuel use in fasting versus fed states and during exercise (compared to rest). Researchers can estimate how much fat and carbohydrate are being burned for energy at a given time by measuring what’s referred to as the “respiratory quotient,” or RQ for short. Generally speaking, a higher RQ (closer to 1.0) means you’re burning more sugar, whereas a lower RQ (closer to 0.7) means you’re burning more fat.

In addition, mitochondria may be responsible for determining metabolic flexibility. As you may remember from high school biology class, mitochondria are the “power plants” of all living things, and they are responsible for creating energy from carbs and fats. Researchers believe a healthy mitochondria number and function play a critical role in metabolic flexibility. ⁵

At this stage of the game, the only direct test to assess mitochondria density is to do a muscle biopsy, which is not practical. Dr. San Millan at the University of Colorado Sports Medicine and Performance Center recently developed an indirect exercise test that provides a snapshot of the health of a person’s mitochondria. ⁷

Short of that validated test, Dr. San-Millan says people can take a hard look at their energy levels and weight patterns: Low energy, trouble losing weight regardless of dietary changes, and difficulty managing blood sugar levels can be sure signs of unhealthy mitochondria and metabolic inflexibility.

How to Boost Your Metabolic Flexibility

We don’t have to look too far to find the underpinnings of declining metabolic flexibility. Most scientists boil it down to factors like: ^2,8

• Physical inactivity (i.e., sedentary lifestyle)
• Excess caloric intake
• A heavy reliance on processed foods (containing high glycemic refined grains and sugars)
• A high meal frequency (i.e., eating more often and/or throughout a greater portion of the day

In other words, metabolic inflexibility is regarded as almost exclusively driven by lifestyle factors. That’s good news because it means boosting your metabolic flexibility is very much under your control. To do just that, we really don’t have to look much further than the traditional hunter-gatherer lifestyle.

At this stage in the game, we can distill that healthy metabolic flexibility hinges on two things:

• Insulin sensitivity. In simple terms, insulin sensitivity is the antithesis of insulin resistance, and along those lines, healthy insulin sensitivity is synonymous with metabolic flexibility.
• Mitochondrial density and function. Mitochondria are the tiny furnaces inside cells that burn fat and carbohydrates. Generally speaking, the more mitochondria you have and the bigger and more efficient they are, the greater your metabolic flexibility.

To that end, the very things we need to be doing to boost metabolic flexibility are very effective at improving insulin sensitivity and enhancing mitochondrial number and function.

3 Tools to Boost Metabolic Flexibility

1. Physical Activity and Exercise.
It shouldn’t come as a huge surprise that a physically active lifestyle is a linchpin for healthy metabolic flexibility. After all, a sedentary lifestyle is often cited as the primary culprit of metabolic inflexibility.⁹ Along those lines, exercise is an extremely effective tool for improving insulin sensitivity and promoting an increase in mitochondria content and function. In Dr. San Millan’s words, “The only medication that increases mitochondrial function is exercise.” Traditional cardio, high-intensity interval training, and strength training all have their place because each tends to rely on a different mixture of fuel. So, it’s a good idea to include a variety of types of exercise to optimize metabolic flexibility.

Having said that, exercise alone is unlikely to be the answer to boosting metabolic flexibility. After all, it’s pretty clear that exercise can’t undo the detrimental effects of a sedentary lifestyle. To that end, a better idea is to move toward a hunter-gatherer fitness plan, which features a wide range of physical activity.

2. Intermittent Fasting.
Research shows that intermittent fasting (IF) is quite an effective tool for improving metabolic health. IF has been shown to improve insulin sensitivity and stimulate mitochondrial biogenesis, which is science speak for increasing the size and number of mitochondria. Although not the magic pill some may lead you to believe, IF is also an effective strategy to promote weight loss (via calorie restriction), and calorie-restriction-induced weight loss is also effective at improving insulin sensitivity.⁹ There are several variations of IF, including time-restricted feeding, alternate-day fasting, periodic fasting, and fasting-mimicking diets.

3. Cold Exposure.
For many on the metabolic inflexibility end of the spectrum, you can see how boosting metabolic flexibility may require you to get comfortable being uncomfortable. And strategy number three may be the epitome of that for quite a few people, as it means stepping outside your 24/7 temperature-controlled environment. Cold stress—such as cold water immersion (i.e., ice-cold baths), cold showers, and whole-body cryotherapy—is an effective way to stimulate mitochondrial biogenesis, enhance insulin sensitivity, and ultimately, boost metabolic flexibility.⁸

It seems that one way cold exposure seems to flex its metabolic muscle is by activation of brown adipose tissue. In fact, cold exposure has been investigated by scientists as a potential tool to help combat both obesity and diabetes. ¹⁰ The good news is that you don’t need to run around in your skivvies at the North Pole to reap the rewards—although those types of extreme measures would probably do the trick. Recent studies have shown that exposure to temperatures ranging from 50 – 62˚F for 2 – 6 hours per day for 10 days to 4 weeks can significantly boost activity of brown fat (by 45 – 65%). ^11,12

Of course, how much and what you do (and don’t) put into your mouth also influences metabolic flexibility. On one hand, excessive caloric intake drives metabolic inflexibility. On the other hand, caloric restriction (e.g., intermittent fasting) and/or equilibrium promotes metabolic flexibility. Not surprisingly, the typical Western-style diet and its reliance on processed foods (fortified with high glycemic, refined carbohydrates and poor-quality, inflammatory oils) pushes the body toward metabolic inflexibility. Conversely, a diet founded on whole foods (particularly one rich in plant-based polyphenols and healthy unsaturated fats) that contains minimally processed, low-glycemic carbohydrates in appropriate amounts (for activity levels and body type) is a surefire recipe to boost metabolic flexibility.

How to Boost Metabolic Flexibility: A Recap

Like many things in life, the right amount of discomfort leads to learning and growth, and the same holds true for boosting metabolic flexibility. That is, to improve metabolic flexibility, you have to expose your body to the right types and amounts of metabolic stress, which require the body to adapt in healthy ways. Scientists often refer to this healthy dose of stress as hormesis, or hormetic stress, which usually falls right into the “Goldilocks” zone (neither too comfortable nor too harsh). Simply put, if you want to boost metabolic flexibility—which should be important to virtually everyone—then you may need to step outside your comfort zone (but you don’t have to go too far). After all, that’s where the magic happens.