Our bodies have several fuels available to produce energy, primarily fat and carbohydrates. What is particularly remarkable is that we do not always use the same fuel, or use it in the same proportion. Our metabolism continually adjusts according to whether we are eating, fasting, resting, exercising, or meeting different physical and cognitive demands, as well as to the intensity and duration of those demands.
This ability to adapt is known as metabolic flexibility.
During rest and fasting, the relative contribution of fat to energy production increases. After eating, particularly when a meal contains carbohydrates, glucose utilization increases. As exercise intensity rises, carbohydrates become increasingly important because they can provide energy at a faster rate.
Metabolic flexibility therefore does not mean burning fat continuously or eliminating carbohydrates from the diet. It means being able to use the most appropriate fuel for the circumstances and to switch efficiently from one to another as physiological demands change.
Although skeletal muscle plays a central role, metabolic flexibility is now understood as a whole-body property. Skeletal muscle, the liver, adipose tissue, heart, nervous system, hormones, and mitochondria all contribute to this finely coordinated process.
A Capacity for Adaptation: The nutrients we consume contain chemical energy. The body converts this energy through complex metabolic pathways into ATP, the principal energy currency of our cells.
ATP makes muscle contraction, nerve signalling, brain activity, and the functioning of our organs possible. Yet both our energy requirements and the rate at which ATP must be produced are constantly changing.
So does the fuel used to produce it.
During the overnight fast, insulin levels fall and the availability of fatty acids released from adipose tissue increases. The body consequently relies more heavily on fat as an energy source.
Glucose, however, does not disappear. Some tissues, including the brain, continue to use it, while the liver helps maintain blood glucose by releasing glucose from glycogen stores and producing new glucose.
After a meal, particularly one containing carbohydrates, insulin secretion increases. This hormone promotes glucose uptake by several tissues, stimulates glycogen storage, and temporarily suppresses the release of fatty acids from adipose tissue.
Glycogen is the body’s storage form of carbohydrate. It is stored mainly in skeletal muscle and the liver. Muscle glycogen is used primarily to meet the energy demands of the muscle itself, whereas liver glycogen helps maintain the supply of glucose available to other tissues.
Fat, by contrast, represents a much larger energy reserve. However, it cannot provide energy as rapidly as carbohydrate when the body suddenly faces very high energy demands.
A simple analogy may help. Imagine a vehicle with different operating modes. At low demand, it can run economically. When demand increases, it needs a more powerful setting. Once the demand falls again, it can change its operating mode.
Metabolic flexibility is the ability to make these changes efficiently and appropriately.
The Fuel Changes with Exercise: Exercise intensity has a profound effect on the proportion of fuels used.
During low- to moderate-intensity activity, such as brisk walking, easy cycling, or running at a comfortable pace, fat can provide a substantial proportion of the energy required.
As intensity increases, the muscles need to produce ATP more rapidly and progressively increase their use of muscle glycogen and blood glucose.
This is why carbohydrates become the predominant fuel during very intense efforts, such as a sprint, a steep climb, or a demanding set of resistance exercise. Fat does not stop being used, but its relative contribution declines because fat oxidation cannot meet such a high energy demand on its own.
Duration also matters. During prolonged exercise at low or moderate intensity, the relative contribution of fat may increase as glycogen availability declines. The response, however, depends on several factors, including exercise intensity, previous training, nutritional status, and individual characteristics.
This is why a metabolically flexible person is not necessarily someone who oxidizes the most fat during a particular test. Rather, it is someone who can appropriately adjust the use of fat and carbohydrate when nutrient availability and energy demands change.
This distinction is important because it prevents one of the most common misunderstandings surrounding the concept. Oxidizing more fat during a workout does not necessarily mean losing more body fat. Changes in body fat depend largely on the energy balance maintained over time, together with other factors such as the preservation of muscle mass.
How Is It Measured?: Metabolic flexibility cannot be determined from subjective sensations or from a single number provided by a consumer device.
One of the methods used in research is indirect calorimetry, which estimates the relative use of fat and carbohydrate from oxygen consumption and carbon dioxide production.
The relationship between these gases provides the basis for calculating the respiratory exchange ratio, or RER, under the appropriate experimental conditions. As a general guide, values close to 0.70 indicate a greater contribution from fat oxidation, whereas values approaching 1.00 indicate a greater contribution from carbohydrate oxidation.
These values should be interpreted with caution. They are not absolute boundaries, and their meaning depends on the experimental conditions and other aspects of metabolism.
For this reason, what matters when studying metabolic flexibility is not an isolated number, but the ability to change substrate utilization when physiological conditions change, for example, between fasting and feeding or at different exercise intensities. RER is useful, but it is not a perfect measure of metabolic flexibility and has important methodological limitations.
When Metabolic Flexibility Declines: Metabolic inflexibility describes a reduced ability to adapt fuel utilization to changes in energy availability or demand.
In general terms, a person with reduced metabolic flexibility may have greater difficulty increasing fat oxidation during certain fasting or low-energy conditions and appropriately increasing glucose utilization when glucose becomes available.
Obesity, insulin resistance, type 2 diabetes, and physical inactivity have all been associated with alterations in this capacity. However, the relationship should not be interpreted too simply.
The direction of causality is not always one-way. Metabolic inflexibility may be part of metabolic dysfunction, but it may also result from it. Insulin resistance, ectopic fat accumulation, abnormalities in adipose tissue, and other metabolic changes can interact and reinforce one another.
Insulin resistance means that certain tissues respond inadequately to a given concentration of insulin. Skeletal muscle, the liver, and adipose tissue all play important roles in this process.
In skeletal muscle, impaired insulin responsiveness reduces the ability to take up and store glucose efficiently. The body may initially compensate by increasing insulin secretion, but this compensatory response can eventually become insufficient. Chronic hyperinsulinemia places sustained demands on pancreatic β-cells, contributing to progressive impairment of their function. It can also disrupt endothelial function and contribute to vascular dysfunction.
This is one reason why exercise is so metabolically interesting.
When muscles contract, glucose uptake increases through mechanisms that do not depend exclusively on insulin. Exercise training also produces adaptations that improve oxidative capacity and the muscle’s ability to handle different fuels.
Exercise does more than expend energy. It also modifies the machinery that enables the body to use it.
How to Improve Metabolic Flexibility: Metabolic flexibility is not developed through a miracle food or an extreme diet. It is supported by a combination of regular physical activity, sufficient muscle mass, a high-quality diet, adequate sleep, and a sensible energy balance.
Move More Throughout the Day: The first step is to reduce sedentary time.
Walking, taking the stairs, travelling on foot, doing household tasks, and getting up regularly from a chair are modest but repeated stimuli.
Everyday movement is often underestimated. A single workout does not turn an otherwise sedentary day into an active one.
For metabolic health, it is preferable to accumulate movement throughout the day rather than concentrate all physical activity into one weekly session.
A simple strategy is to walk every day and interrupt prolonged periods of sitting with a few minutes of movement.
Short walks after some meals may also be useful. Muscle contraction promotes glucose uptake and can reduce the rise in blood glucose that follows a meal.
There is no need to turn every meal into a workout. A few minutes of movement count too.
Build Aerobic Fitness: Aerobic exercise improves the muscle’s oxidative capacity, promotes mitochondrial adaptations, and enhances insulin sensitivity.
Brisk walking, running, cycling, swimming, or using an elliptical trainer are all valid options.
Intensity should be appropriate to the individual’s fitness level. A practical guide is to remain able to hold a conversation during much of the session, although speaking becomes more difficult as intensity rises.
Over time, periods of higher intensity can be incorporated. After an appropriate warm-up, for example, several brief accelerations can be performed with active recovery periods between them.
Interval training can provide additional benefits, but it is not essential for achieving healthy metabolic adaptation. In sedentary or older adults, or in people with diagnosed medical conditions, progression should be gradual and adapted to the individual’s circumstances.
Preserve Muscle: Resistance training is fundamental. Muscle is not simply the tissue that enables us to move. It is also one of the body’s principal organs involved in glucose regulation and energy metabolism.
An adequate amount of functional muscle provides greater capacity to take up glucose, store glycogen, and use different fuels.
A sensible programme may include two or three resistance-training sessions per week targeting the major muscle groups through exercises such as sit-to-stand movements, adapted squats, pushing, pulling, hip-hinge movements, and stability exercises.
The load should allow good technique and a meaningful degree of effort without sacrificing control of the movement.
For older adults, preserving strength and muscle mass becomes particularly important. Sarcopenia, characterized by progressive loss of muscle mass, strength, and physical function, compromises physical capacity and is associated with metabolic disturbances.
This is why, when discussing metabolic flexibility in later life, it is not enough to talk about walking or burning calories. Preserving muscle means preserving an important part of our metabolic capacity.
Eat for Metabolic Health: The diet should provide adequate protein, fibre, vitamins, and minerals and should be based primarily on minimally processed foods.
Legumes, vegetables, whole fruits, nuts, olive oil, whole grains, eggs, fish, dairy products, and unprocessed meats can all form part of a healthy dietary pattern.
Fibre contributes to satiety and can slow the absorption of some carbohydrates. Protein helps preserve muscle mass, particularly during weight loss or when resistance training is performed.
It is also sensible to avoid constant snacking on highly energy-dense, nutrient-poor foods.
Eating frequently does not automatically prevent fat oxidation, but it may facilitate excessive energy intake and prolong the post-meal metabolic state.
The goal is not to go hungry. It is to establish an eating pattern that provides adequate nutrition while limiting the proportion of energy coming from ultra-processed foods.
Carbohydrates Have Their Place: The idea that good metabolic flexibility requires eliminating carbohydrates is mistaken.
Carbohydrates are particularly useful when exercise intensity increases. They are important for high-intensity efforts, endurance activities, and certain forms of resistance training.
The appropriate amount depends on physical activity, goals, body composition, and individual characteristics.
Someone who regularly runs in the mountains or performs demanding resistance training has different carbohydrate requirements from someone who leads a predominantly sedentary lifestyle.
Therefore, good metabolic flexibility does not mean abandoning a particular fuel. It means retaining the ability to use that fuel when it is needed.
Fasting Requires Perspective: Overnight fasting is part of normal human physiology. Avoiding continuous nighttime snacking and allowing a reasonable interval between dinner and breakfast may be a simple strategy for some people.
Intermittent fasting can facilitate a reduction in energy intake and improve some metabolic markers. However, accumulated evidence indicates that its effects on body weight and many cardiometabolic outcomes are generally similar to those achieved with conventional energy restriction when both approaches produce a comparable reduction in energy intake.
This does not mean that fasting is useless. It can be a valid tool when it makes a healthy eating pattern easier to follow. What is not justified is presenting it as a universal strategy or attributing extraordinary metabolic properties to it.
It deserves particular caution in people treated with insulin or certain glucose-lowering medications, those with a history of eating disorders, underweight individuals, people with frailty, and those with certain kidney or liver conditions.
In older adults, preserving muscle mass and ensuring adequate protein intake should take priority over unnecessarily prolonging the fasting period.
Sleep and Recovery Matter: Metabolism does not operate independently of sleep.
Insufficient or irregular sleep can affect appetite regulation, insulin sensitivity, and recovery from exercise. It may also influence spontaneous physical activity on the following day.
Maintaining relatively regular sleep schedules, obtaining sufficient sleep, and avoiding excessive caffeine or screen exposure when these interfere with sleep are simple measures that complement exercise and nutrition.
Metabolic flexibility depends on what we do during exercise and on how well we recover afterwards.
Ideas Worth Avoiding: Metabolic flexibility has become an appealing concept in health communication. Precisely for that reason, it is important to distinguish established physiology from commercial claims.
Oxidizing more fat during exercise does not necessarily mean losing more body fat. The body may temporarily increase fat oxidation without producing a net reduction in body fat if subsequent energy intake compensates for the expenditure.
A ketogenic diet can substantially increase fat oxidation. This does not, by itself, demonstrate greater metabolic flexibility. Predominantly using fat can also coexist with a reduced ability to use carbohydrates when energy demand rises.
Supplements marketed as fat burners do not replace exercise, adequate sleep, a high-quality diet, or the preservation of muscle mass.
Finally, subjective sensations should not be mistaken for metabolic assessment. Having stable energy, tolerating several hours without food, or not feeling sleepy after a meal may be compatible with good metabolic health, but none of these sensations can diagnose metabolic flexibility.
Direct assessment requires specific physiological methods, particularly in research, where different protocols are used to examine how the body responds to changes in nutrient availability and energy demand.
A Simple Strategy: A week designed to support metabolic flexibility can incorporate a few straightforward principles.
- Move every day and take frequent breaks from sitting.
- Take short walks after some meals whenever practical.
- Perform resistance training two or three times per week.
- Include two or three sessions of low- to moderate-intensity aerobic exercise.
- Add occasional higher-intensity work once an adequate fitness base has been established and there are no contraindications.
- Base meals primarily on minimally processed foods.
- Ensure adequate protein and fibre intake.
- Adjust carbohydrate intake to physical activity rather than eliminating carbohydrates as a matter of principle.
- Avoid continuous nighttime snacking without turning fasting into a competition.
- Maintain regular sleep and allow sufficient recovery.
Progress should be gradual. A sedentary person can begin with ten- or fifteen-minute walks and very simple resistance exercises. Duration, frequency, or training load can then be increased progressively.
Consistency produces adaptations that occasional intensity cannot.
A Capacity, Not a Promise: Metabolic flexibility is, at its core, the body’s ability to adapt its use of different fuels to feeding, fasting, rest, and exercise.
It is not a diet, a supplement, a training technique, or a number to be pursued obsessively. It is a physiological property that reflects the body’s capacity to adapt when energy availability and energy demand change.
Muscle plays a central role, but the process is much broader. The liver, adipose tissue, hormones, and mitochondria participate in a complex system of coordination that helps maintain energy balance.
Improving metabolic flexibility therefore does not require pursuing an exceptional metabolic state. It requires fostering an appropriate relationship between food and energy expenditure, activity and rest, effort and recovery.
Regular exercise is one of the most consistent stimuli, particularly when aerobic activity is combined with resistance training. A high-quality diet, preservation of muscle mass, adequate sleep, and a healthy body weight complete the picture.
The goal is not to force the body to burn fat at all times.
True metabolic flexibility means retaining the ability to use the right fuel when circumstances change.
And that ability to adapt is one of the most revealing expressions of metabolic health.
Today’s research is tomorrow’s therapy