Key takeaways
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Metabolic rate varies far less between people than commonly believed, and differences are often overstated.
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Lifestyle factors such as muscle mass, movement and sleep have a greater influence on metabolism than genetics alone.
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Metabolism remains adaptable across adulthood, meaning supportive habits can positively influence energy use at any age.
You’ve probably heard someone say, “They can eat anything, they’ve got a fast metabolism.” It sounds convincing, yet here’s a surprising scientific reality. When researchers measure metabolism accurately, most people’s energy expenditure is far more similar than different. So where did the idea of a “fast metabolism” come from, and does it actually exist?
In this blog, you’ll explore what metabolism really is, what science says about individual differences, and how metabolism continues to respond positively to lifestyle habits throughout adulthood.
What metabolism actually means in scientific terms
Before addressing the myth, it helps to clarify what metabolism truly is. Metabolism refers to the sum of all chemical reactions that keep your body alive, from breathing and circulation to digestion and movement.
Resting metabolic rate is the foundation
Approximately 60–75% of daily energy expenditure comes from resting metabolic rate (RMR), the energy required to maintain basic physiological functions at rest. RMR is influenced primarily by body size, lean mass, sex and age.
A large-scale analysis using doubly labelled water found that after adjusting for fat-free mass, differences in RMR between individuals were relatively small ®. This challenges the idea that some people burn dramatically more calories at rest due to an inherently “fast” metabolism.
Genetics play a role, but not the starring role
Genetic factors do influence metabolism, but they account for only a modest proportion of variability. Twin studies show heritability estimates for metabolic rate of around 20–40%, leaving significant room for environmental and behavioural influences (R). This means metabolism is not a fixed trait you’re born with and stuck with forever.
Why the “fast metabolism” idea feels so convincing
If metabolic differences are relatively small, why does the fast metabolism myth persist? The answer lies in how metabolism interacts with behaviour and perception.
Appetite, movement and unconscious activity
People who appear to “eat anything” often compensate without realising it. Studies on non-exercise activity thermogenesis (NEAT) show that spontaneous movement, such as fidgeting or standing, can vary energy expenditure by up to 2,000 calories per day between individuals (R).
This difference is not due to a faster resting metabolism, but greater daily movement. The body subtly adjusts energy output in response to intake.
Energy intake is often misjudged
Research consistently demonstrates that people underestimate their energy intake and overestimate their activity, regardless of body size (R). Observing others’ eating habits without full context can reinforce the belief that metabolism alone explains body differences. In reality, metabolism and behaviour are deeply interconnected.
Does metabolism slow with age, or does behaviour change?
Ageing is often blamed for metabolic slowdown, yet recent research paints a more nuanced picture.
Metabolism across the lifespan
A landmark international study analysing over 6,400 participants found that metabolic rate remains remarkably stable from adulthood through midlife, only declining significantly after around age 60 (R).
Even then, the decline is gradual rather than dramatic. Much of the perceived slowdown earlier in life is linked to reduced muscle mass and movement rather than metabolic failure.
Muscle mass matters more than age
Skeletal muscle is metabolically active tissue. Loss of muscle, rather than ageing itself, contributes to reduced energy expenditure. Research shows that preserving lean mass helps maintain metabolic rate and insulin sensitivity across adulthood (R).
This is encouraging, because muscle mass remains responsive to nutrition and resistance training well into later life.
Adaptive metabolism and why restriction backfires
Another reason the fast metabolism myth persists is confusion around metabolic adaptation.
The body adapts to energy restriction
When calorie intake is chronically reduced, the body compensates by lowering energy expenditure. This phenomenon, known as adaptive thermogenesis, is well documented.
A study following participants from a major weight-loss intervention showed that resting metabolic rate remained suppressed years after weight loss, even when weight was regained (R).
This adaptive response can make it feel as though metabolism is “slow,” when in fact it is responding protectively to perceived energy scarcity.
Why this matters for healthy ageing
Highly restrictive diets can unintentionally reduce metabolic flexibility. Diets that support adequate protein intake, regular meals and muscle maintenance are associated with more stable metabolic function over time. This reframes metabolism not as something broken, but as something intelligent and protective.
What actually influences metabolic health most
If fast metabolisms are largely a myth, what truly supports metabolic health?
Muscle, movement and meal quality
Evidence consistently shows that metabolic rate is most strongly influenced by:
- Lean muscle mass
- Daily movement, including non-exercise activity
- Diet quality, particularly protein and fibre intake
- Sleep and circadian alignment
Each of these factors directly influences energy expenditure and metabolic signalling (R).
Metabolism remains adaptable
Importantly, metabolism is dynamic. Studies show improvements in insulin sensitivity, energy expenditure and metabolic markers in older adults following resistance training and dietary interventions (R). This adaptability supports the idea that metabolic health can be influenced positively at any age.
So, are fast metabolisms a myth?
The most accurate answer is this: Dramatically fast metabolisms are largely a myth, but metabolic differences do exist within a relatively narrow range.
What often looks like a fast metabolism is usually a combination of:
- Higher muscle mass
- Greater daily movement
- Subtle appetite regulation differences
- Supportive lifestyle habits
Understanding this shifts the focus away from comparison and towards factors you can influence.
Metabolism for healthy ageing
Metabolism is not a race you win or lose. It’s a responsive system designed to keep you alive, adaptable and resilient. Healthy ageing is supported by habits that respect this system rather than trying to override it.
Nourishment, movement, rest and consistency allow metabolism to function as intended. When metabolism is framed as a partner rather than a problem, it becomes far easier to support long-term wellbeing.
If you found this exploration of metabolism helpful, you may enjoy our next blog: Do weighted vests help with weight loss?
