Metabolic Health

Metabolic Flexibility: The Key to Optimal Energy and Well-Being

5 minutes read

Have you ever experienced feelings of shakiness or weakness after a few hours without eating? Maybe you've hit a wall during a run when your fuel runs low. What about that stubborn belly fat that just won’t budge despite being in a calorie deficit? Or perhaps you've encountered mood swings, cravings, or "hangriness" after skipping breakfast.

If any of these situations ring a bell, discovering the power of metabolic flexibility could help alleviate some of these frustrating symptoms, and profoundly impact your overall health, well-being, and performance– both physically and mentally.

What is metabolic flexibility?

Metabolic flexibility refers to the ability of our body to switch between using different fuel sources for energy production [1]. Specifically, it is the transition between burning carbohydrates (glucose) and burning fats (fatty acids) for fuel.

In other words, it’s the ease with which your body can flick a biological switch allowing you to seamlessly convert to burning fatty acids for energy production when your glucose stores have run out.

In normal conditions, after a balanced meal, carbohydrates are the preferred source of fuel because they can be rapidly metabolized, providing a quick and easily accessible source of energy for the body's immediate needs [2].

However, fats are a more sustainable fuel source. Although burning fat requires more complex metabolic processes, in terms of energy density, fat contains more than double the amount of energy per gram compared to carbohydrates [3]. This means fat has the potential to provide a greater amount of energy for the body when fully metabolized.

Being metabolically flexible, or “fat-adapted,” allows you to tap into your own body fat as a source of energy that is almost always indefinite [4].

Think of your body's fuel utilization as an energy grid, and you’re either "on the grid" or "off the grid." When you're "on the grid," your body relies on a continuous supply of carbohydrates to obtain glucose for energy production [5]. When you're "off the grid," your body can use its own fat reserves and burn fat as a fuel source for energy. This can help you achieve a healthier body composition and a constant sustainable supply of energy without crashing [6].

Defining metabolic flexibility, metabolism, and metabolic health

Navigating the science of health and wellness can be overwhelming at times, especially with the influx of vague and confusing information often found on social media and in popular science. However, understanding nuance and grasping distinctions between complex terminologies and how they relate to each other allows you to make informed choices and implement impactful lifestyle changes.

Metabolic flexibility and dispelling myths

We know that metabolic flexibility refers to the body’s capacity to switch between burning carbohydrates (glucose) and fats (fatty acids and ketones) for energy production. Understanding what metabolic flexibility is not, and debunking common misconceptions is equally important, especially if you feel that achieving optimal performance is currently beyond your reach. 

  • Myth 1: Metabolic flexibility is solely determined by genetics and cannot be improved.
  • Myth 2: Metabolic flexibility is only relevant for athletes or fitness enthusiasts.
  • Myth 3: Metabolic flexibility is a quick fix for weight loss.
  • Myth 4: Achieving metabolic flexibility requires constant carbohydrate restriction or strict ketosis.
  • Myth 5: Developing metabolic flexibility requires extreme and unattainable dietary patterns.

In reality, metabolic flexibility is a trainable skill for individuals of all fitness levels. It is not solely determined by genetics and does not require extreme dietary restrictions. By making small yet meaningful changes to your diet, exercise, and lifestyle, you can enhance your body's ability to switch between fuel sources.

Improving metabolic flexibility brings numerous health benefits, including weight control, body composition, brain health, physical performance, emotional stability, and metabolic well-being. Honing the body's fuel-switching capabilities will pave the way for better overall health outcomes [7].


Metabolism encompasses all processes involved in producing and utilizing energy, occurring in cells, tissues, and organs. This complex network of biochemical reactions, including those in the mitochondria (the "powerhouses" of the cells), breaks down nutrients, generates energy, and eliminates waste to maintain overall bodily function and balance. 

In other words, metabolism is how we produce energy. 

Carbohydrate metabolism converts carbohydrates into glucose, transported into cells by insulin for immediate energy.

Fat metabolism, on the other hand, is a different set of reactions that break down fats into fatty acids. Under specific conditions like prolonged periods of carbohydrate deprivation and low insulin levels, the body enters a state known as ketosis. During ketosis, the liver converts fatty acids into ketones, which can serve as an alternative energy source for the body and become a vital fuel for the brain when glucose is scarce [8].

Metabolic health

Metabolic health focuses on the effectiveness, efficiency, and smoothness of your metabolism. Good metabolic health is characterized by stable blood sugar levels, optimal energy production, appropriate fat utilization, and effective metabolic signaling. These factors also help to reduce disease risk and contribute to a prolonged healthspan, allowing you to live more years free from illness and disability.

The hazards of metabolic inflexibility

Metabolic flexibility is a key indicator of physiological adaptability. 

When we are metabolically inflexible, our bodies struggle to efficiently make use of fatty acids and ketones as alternative fuel sources when glucose is depleted. Our ancestor’s physiology evolved to cope with dramatic fluctuations in energy supply and demand during times of feeding and famine. 

However, in today's world, where carbohydrates dominate the standard American diet and ultra-processed grains and sugars are ubiquitous, our bodies rarely have the opportunity to enter "fat-burning mode” [9]. 

The issue lies not in the consumption of carbs per se, but rather in the imbalance between fats and carbs in our diets.

Excessive carbohydrate consumption, frequent snacking, and fewer meals that balance complex carbs, healthy fats, and lean proteins, accompanied by shorter fasting periods, perpetuate a state of constant insulin elevation. These dietary patterns have eroded our body's ability to efficiently utilize fats for energy, and where the expression "use it or lose it" holds true.

Consequently, this constant reliance on glucose keeps our metabolisms hooked "onto the grid," or constantly relying on carbs for energy. This contributes to declining metabolic health, rising obesity rates, and symptoms like brain fog, irritability, lack of focus, and mood swings affecting a significant portion of the population [10]. Metabolic inflexibility is also a hallmark of many age-related metabolic diseases and may play a key role in diseases such as cancer [11].

How to improve metabolic flexibility

So, how can you begin training and enhancing your metabolic flexibility today? There are several evidence-based approaches to consider. 

Thinking of your body as a car can help you to understand how these approaches work; they primarily involve dietary protocols (adjusting the type and timing of fuel intake) and exercise regimes (optimizing the engine) — without the need for restrictive or burdensome changes.

5 metabolic flexibility hacks to try today

  1. Prioritize healthy fats and protein for breakfast to train your body to sustain itself on non-carbohydrate energy.
  2. Eat complex carbs throughout the day, and pair them with healthy fats and lean protein to minimize glucose spikes, prolong fat-burning mode, and to avoid crashes in energy levels.
  3. Extend your fasting window by delaying breakfast to tap into your body's fat stores for energy "off the grid."
  4. Boost your exercise routine with resistance and high-intensity interval training to increase fuel demand and promote fat metabolism.
  5. Incorporate fasted workouts to maximize fat utilization during a state of running "off the grid."

Metabolic flexibility offers benefits beyond weight loss, including improved insulin sensitivity, reduced blood sugar levels, and decreased risk of cardiometabolic disease. Enhanced energy levels, endurance, and mental clarity accompany metabolic flexibility [12]. As your body becomes skilled at switching between fuel sources, you'll notice reduced sluggishness and blood glucose spikes even after a carb-heavy breakfast.

Embracing metabolic flexibility unlocks a realm of vitality and well-being, allowing you to optimize your overall health.

Key takeaways

  1. Metabolic flexibility allows your body to seamlessly switch between carbohydrates and fats as fuel sources.
  2. It's not solely determined by genetics and benefits individuals of all fitness levels.
  3. It's not a quick weight loss fix and does not require extreme dietary restrictions like constant carb restriction or strict ketosis.
  4. Small changes to diet, exercise, and lifestyle can enhance metabolic flexibility.
  5. Improving metabolic flexibility benefits weight control, body composition, brain health, physical performance, emotional stability, and metabolic well-being.



Written by: Natalie Falshaw, MSc
Reviewed by: Emily Johnson, MSc RD

Table of Contents

  • What is metabolic flexibility?
  • Defining metabolic flexibility, metabolism, and metabolic health
  • The hazards of metabolic inflexibility
  • How to improve metabolic flexibility
  • Key takeaways


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