Metabolic flexibility: the key to optimal energy and wellbeing

Have you ever felt shaky or weak after a few hours without eating? Maybe you have hit a wall during a run when your fuel ran low. What about stubborn belly fat that will not budge despite a calorie deficit, or mood swings, cravings, and hangriness after skipping breakfast?

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

What is metabolic flexibility?

Metabolic flexibility refers to the ability of the 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.

It is the ease with which your body can flick a biological switch, converting to burning fat for energy when your glucose stores have run out.

After a balanced meal, carbohydrates are the preferred fuel because they can be rapidly metabolised, providing a quick and accessible source of energy [2]. Fats, however, are a more sustainable fuel source. Although burning fat requires more complex metabolic processes, fat contains more than double the energy per gram compared with carbohydrates [3].

Think of your body’s fuel use as an energy grid: you are either on the grid or off the grid. When you are on the grid, your body relies on a continuous supply of carbohydrates for glucose [5]. When you are off the grid, your body burns its own fat reserves, helping you achieve a healthier body composition and a sustainable supply of energy without crashing [6].

Dispelling the myths

Understanding what metabolic flexibility is not, and debunking common misconceptions, is equally important, especially if you feel that optimal performance is currently beyond your reach.

Myth 1: metabolic flexibility is solely determined by genetics and cannot be improved.
Myth 2: it is only relevant for athletes or fitness enthusiasts.
Myth 3: it is a quick fix for weight loss.
Myth 4: achieving it requires constant carbohydrate restriction or strict ketosis.
Myth 5: developing it 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 restriction. By making small, meaningful changes to your diet, exercise, and lifestyle, you can enhance your body’s ability to switch between fuel sources, improving weight control, body composition, brain health, physical performance, emotional stability, and metabolic wellbeing [7].

Metabolism and metabolic health

Metabolism encompasses all the processes involved in producing and using energy. This complex network of biochemical reactions, including those in the mitochondria, breaks down nutrients, generates energy, and eliminates waste. 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 breaks down fats into fatty acids. Under conditions such as prolonged carbohydrate deprivation and low insulin, the body enters a state known as ketosis, where the liver converts fatty acids into ketones, an alternative energy source and a vital fuel for the brain when glucose is scarce [8].

Metabolic health focuses on the effectiveness, efficiency, and smoothness of your metabolism. Good metabolic health is characterised by stable blood sugar, optimal energy production, appropriate fat utilisation, and effective metabolic signalling. These factors reduce disease risk and contribute to a prolonged healthspan.

The hazards of metabolic inflexibility

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

In today’s world, where ultra-processed grains and sugars are ubiquitous, our bodies rarely have the opportunity to enter fat-burning mode.

The issue lies not in the consumption of carbs per se, but in the imbalance between fats and carbs in our diets. This constant reliance on glucose keeps our metabolism hooked onto the grid, contributing to declining metabolic health, rising obesity rates, and symptoms like brain fog, irritability, and mood swings [9, 10]. Metabolic inflexibility is also a hallmark of many age-related metabolic diseases and may play a key role in conditions such as cancer [11].

How to improve metabolic flexibility

There are several evidence-based approaches. They primarily involve dietary protocols, adjusting the type and timing of fuel intake, and exercise, optimising the engine, without the need for restrictive or burdensome changes.

Prioritise healthy fats and protein at breakfast to train your body to sustain itself on non-carbohydrate energy.
Eat complex carbs throughout the day, paired with healthy fats and lean protein, to minimise glucose spikes and avoid energy crashes.
Extend your fasting window by delaying breakfast to tap into your fat stores for energy off the grid.
Add resistance and high-intensity interval training to increase fuel demand and promote fat metabolism.
Incorporate fasted workouts to maximise fat utilisation while running off the grid.

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

Key takeaways