When it comes to metabolism, most people primarily think of one thing: fast or slow? But this simplistic view is long outdated. Modern metabolic research focuses on a completely different ability: metabolic flexibility. This refers to the body's ability to seamlessly switch between different energy sources as needed. This "forgotten health capability" determines whether we are energetic during the day or experience an afternoon slump, whether we lose weight easily or fight stubborn fat deposits – and even how healthily we age.
What is metabolic flexibility?
Metabolic flexibility is the ability of an organism to adapt its metabolism in response to physical activity or available energy substrates – especially fats and carbohydrates. Simply put: A metabolically flexible body can efficiently switch between burning fat and carbohydrates depending on the situation. .
This ability is anything but self-evident. The body constantly has to decide which energy source it is currently using. :
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At rest and during low exertion, it should primarily burn fat .
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During periods of high exertion and after carbohydrate-rich meals, it should switch to carbohydrates (glucose).
A metabolically flexible organism effortlessly manages this transition. It can quickly absorb and utilize the excess glucose from a meal into its cells – and seamlessly access fat reserves during periods of fasting or prolonged exertion. .
The three dimensions of metabolic flexibility
Metabolic flexibility encompasses several related but distinct concepts, which metabolic researcher Bret Goodpaster and colleagues described in the journal Cell Metabolism in 2017. :
1. Substrate flexibility (fat ↔ carbohydrates)
The ability to efficiently utilize either carbohydrates or fats, depending on availability. This flexibility can be quantified using the respiratory quotient (RQ): an RQ of approximately 0.7 indicates pure fat burning, an RQ of approximately 1.0 pure carbohydrate burning .
2. Efficiency-flexibility (aerobic ↔ anaerobic)
The originally described form of metabolic flexibility refers to the ability to generate energy through either aerobic or anaerobic respiration. At high efficiency, pyruvate is converted aerobically to acetyl-CoA (36 ATP per glucose molecule), while at low efficiency, it is converted anaerobically to lactate (only 2 ATP per glucose molecule). .
3. Insulin reactivity
The ability of muscle to increase glucose oxidation in response to insulin. This definition focuses on the signaling effect of insulin on muscle metabolism. .
Why metabolic flexibility is crucial for health
Protection against metabolic diseases
People with metabolic inflexibility have difficulty switching between burning fat and carbohydrates. This inability is associated with insulin resistance, obesity, type 2 diabetes, and other metabolic disorders. A flexible metabolism, on the other hand, helps to reduce the risk of these diseases. .
The good news: Metabolic flexibility is now considered a trainable trait. Metabolism after age 30 is therefore not a matter of fate, but a trainable system. .
The connection to healthy aging
Metabolic flexibility decreases with age in all three axes. :
A sedentary lifestyle with constant carbohydrate availability leads to a downregulation of the enzymes for fat oxidation – the body "forgets" how to burn fat and becomes dependent on external glucose supply. .
Simultaneously, pyruvate dehydrogenase activity decreases due to mitochondrial dysfunction and oxidative stress. This problem is particularly evident during exertion: glucose is utilized inefficiently (2 instead of 36 ATP per molecule), leading to faster exhaustion. .
Additionally, the ability to respond intracellularly to the insulin signal with increased glucose oxidation decreases. .
Tim Hollstein, a metabolism researcher from Kiel who presented his research in the 3sat documentary "The Metabolism Update", emphasizes: Studies suggest that slim people often have greater metabolic flexibility, which allows them to adapt more quickly to different nutritional and stress situations. .
Impact on energy and performance
The first signs that metabolic flexibility is not ideal are everyday phenomena. :
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Energy dips in the middle of the day
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Drowsiness after eating
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The urge to snack immediately after a meal
In endurance sports, metabolic flexibility is a crucial performance factor. Those who train them can remain efficient for longer, conserve glycogen stores, and reduce the risk of performance slumps. Carbohydrates are only available to the body in limited quantities. If glycogen stores are depleted, performance drops significantly – even if the heart, lungs, and muscles are still functioning optimally. .
How metabolic inflexibility develops
The causes of metabolic inflexibility are manifold. :
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Insulin resistance (e.g., in type 2 diabetes): Fat burning remains inhibited.
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Mitochondrial dysfunction : Reduced capacity for fat burning.
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Chronically high carbohydrate intake : Reduced adaptation to fat burning
In untrained older people, all three deficits can be present simultaneously and reinforce each other. :
"The body can neither efficiently use fat as fuel (downregulated fat oxidation enzymes), nor efficiently utilize glucose (reduced pyruvate dehydrogenase activity), nor respond to the insulin signal with increased glucose and glycogen utilization (reduced insulin reactivity). If insulin resistance is also present, blood glucose reaches the muscle cells only to a limited extent."
Those affected are metabolically "trapped". At rest, the body lacks the capacity to burn fat; during exertion, glucose is inefficiently converted to lactate. This creates a paradoxical dependence on frequent carbohydrate intake despite inefficient utilization – leading to rapid fatigue even during moderate exertion.
Current research: Metabolic flexibility in focus
The Medical University of Vienna announced in July 2025 that Thomas Scherer had taken up a professorship in the field of endocrinology and metabolism. His research lab focuses, among other things, on how the aging process affects metabolic flexibility, "obesity memory," and nutrient distribution in the body. These questions are investigated within the framework of the FWF-funded Cluster of Excellence "Metabolic Control of Aging and Disease – MetAGE." .
The new research consensus from the turn of the year 2025/2026 emphasizes that the problem is not a sluggish metabolism, but rather declining metabolic flexibility. This often decreases from the age of 30 and becomes a key factor in weight problems and energy loss. .
How you can improve your metabolic flexibility
The good news: Metabolic flexibility can be specifically trained. .
Movement and training
Regular aerobic training at low intensity (Zone 2 training) can counteract the problems of metabolic inflexibility. :
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It stimulates mitochondrial biogenesis.
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Reactivates the fat oxidation enzymes
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Improves pyruvate dehydrogenase activity
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Increases insulin reactivity
The basis for this is a high training volume in the low to moderate intensity range with little or no carbohydrate intake. This promotes fat burning and supports structural adaptations in the muscles.
In addition, intensive training stimuli such as interval training (HIIT) or threshold training are necessary. These train the efficient use of carbohydrates and improve the ability to quickly provide energy during changes in exertion. .
Nutrition strategies
The concept of "Train Low – Compete High" has proven successful. :
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Carbohydrate periodization : Complete long, easy sessions with low carbohydrate intake to promote fat metabolism capacity.
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Targeted carbohydrate intake before intense training sessions and competitions for maximum performance
Intermittent fasting (e.g., an 8-10 hour eating window) is no longer considered just a weight loss tool, but an essential tool for metabolic health. The time restriction trains the mitochondria and improves insulin sensitivity.
Sufficient protein intake is also important: For people aged 30 and over, a new guideline of 1.2 to 1.6 grams per kilogram of body weight is emerging .
Conclusion
Metabolic flexibility is far more than a scientific concept – it's the foundation for energy, health, and healthy aging. Those who train their metabolism to efficiently switch between energy sources benefit from stable energy levels, better weight management, and protection against metabolic diseases.
Modern research shows that metabolism after age 30 is not a matter of fate, but a trainable system. With the right combination of exercise, nutrition, and timing, we can maintain and promote our metabolic flexibility throughout our lives.
Official sources & studies:
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Wikipedia: Metabolic flexibility (with numerous scientific sources)
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Medical University of Vienna: Thomas Scherer takes over professorship (July 2025)
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UKSH: Kiel metabolism researchers featured in 3sat documentary (November 2025)
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ad-hoc-news: Nutrition 2026 – Metabolism from age 30 onwards requires a strategy (December 2025)
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Datasport: Metabolic flexibility – the underestimated factor (February 2026)
