TempoLife › Features › Nutrition myths, checked › metabolism-is-broken
My metabolism is broken — that is why I cannot lose weight.
Metabolic adaptation to weight loss is real, measurable and frustrating. It is also far smaller than the gap between what people believe they eat and what they actually eat.
Verdict: Mostly falsebroken metabolismstarvation modemetabolic damageadaptive thermogenesis
The claim, as people state it
“My metabolism is broken — that is why I cannot lose weight.”
What is actually known
Resting energy expenditure falls when you lose weight, for the obvious reason first: a smaller body costs less to run. That component is predictable and is computed below from a standard equation. On top of it sits adaptive thermogenesis — a further reduction beyond what size alone predicts, generally measured in the range of tens to a couple of hundred calories a day. It is real, it varies between people, and it partly persists after weight loss ends.
Non-exercise activity also falls, often without the person noticing: less fidgeting, slower walking, more sitting. That change is frequently larger than the adaptive component and is invisible to the person experiencing it.
Against that sits the best-documented finding in the whole field: people under-report what they eat, by a wide margin, and the under-reporting is largest in exactly the people most convinced their intake is already low. Studies comparing reported intake against measured energy expenditure have found gaps of hundreds of calories a day, in people reporting honestly and believing themselves accurate.
True metabolic disease exists and is diagnosable. Hypothyroidism, Cushing's syndrome and some medications genuinely change energy expenditure or weight. That is a blood test and a doctor, not a self-diagnosis from a stalled scale.
Why the myth persists
Because it is the only explanation available that fits the evidence a person has: they are trying hard, and the scale is not moving. "Broken metabolism" respects the effort. Under-reporting does not, even when it is the accurate answer, which is why the true explanation loses to the false one almost every time.
The arithmetic
BMR = 10 × kg + 6.25 × cm − 5 × age + s (Mifflin-St Jeor; s = +5 male, −161 female)
Take a woman of 40, 165 cm tall, who has gone from 85 kg to 75 kg. The size component of the change is not a mystery — it is in the equation above:
| Body weight | Predicted resting energy (kcal/day) |
|---|---|
| 85 kg | 1 520 |
| 75 kg | 1 420 |
| Difference | -100 |
So a plausible worst case for this person is roughly 242 kcal a day below where she started: about 100 from being smaller, plus a further 71 to 142 of adaptive thermogenesis, which is the upper end of what that has been measured at. Real, unwelcome, and worth planning around.
| Intake under-reported by | Size vs the whole metabolic slowdown | Weight it hides over a year |
|---|---|---|
| 200 kcal/day | 83% | 9.5 kg |
| 400 kcal/day | 165% | 19.0 kg |
| 600 kcal/day | 248% | 28.4 kg |
That is the comparison the myth cannot survive. Under-reporting intake by 400 kcal a day — a routine finding, and not dishonesty — is larger than the entire metabolic slowdown, and would hide 19.0 kg of expected loss over a year on its own.
Every figure in this section was computed when this page loaded, from 5 237 food rows and 31 exercise rows already in this repository. Nothing in it is stored as prose.
The honest nuance
None of this means the difficulty is imagined. Adaptive thermogenesis plus reduced spontaneous movement plus increased hunger hormones is a genuine physiological headwind after weight loss, and it is why maintenance is harder than losing.
If weight has changed sharply without a change in eating, or with fatigue, cold intolerance or hair loss, that is worth a doctor and a thyroid panel rather than a stricter diet.
General information, not a diagnosis and not medical advice. Anything touching a condition you have, a medicine you take or a test result belongs with your own clinician, who has information no web page does.
Frequently asked questions
Does eating too little put you in starvation mode?
Not in the sense the phrase implies. Expenditure falls, but it does not fall below intake, and prolonged severe restriction produces weight loss along with muscle loss and misery.
Can metabolic adaptation be reversed?
Weight regain reverses most of it. Preserving muscle with strength training and adequate protein, and keeping daily movement up, reduces how much of it happens in the first place.
Source: USDA FoodData Central (SR Legacy) via the TempoLife food database; MET values from the 2011 Compendium of Physical Activities · checked 2026-09-02
Work it out for yourself
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Stop guessing at your own numbers
Almost every myth on this site ends at "it depends on your total intake and your total movement". TempoLife records both, so the answer stops being a debate.