Sixteen people. Same extra food. Same surplus. One gained ten times more fat than another, and the changes in their resting metabolism did not explain it.
One person's everyday movement outside exercise rose by 692 calories a day with extra food. Another person's body responded by moving less than it did before.
Sixteen people ate the same extra food, prepared in the same hospital kitchen, weighed to the gram, for eight weeks straight. Almost every meal was eaten under supervision. Every snack was tracked. The researchers at the Mayo Clinic checked the volunteers' home garbage to make sure nothing was thrown away.
All sixteen were nonobese adults aged 25 to 36. None exercised more than twice a week. None took medication, smoked, or had any chronic illness. Each received exactly 1,000 extra calories per day on top of what their body needed to maintain weight. Same surplus. Same diet composition.
Then the researchers watched who gained fat.
One person gained 0.36 kilograms of body fat. Across 56 days of eating an extra thousand calories, their body stored very little of it as fat.
Another gained 4.23 kilograms. More than ten times as much fat from exactly the same extra food.
The question the researchers asked is the same one you have asked watching your lean friend finish a plate that would put weight on you within the week.
Changes in what you call "fast metabolism" did not account for who gained fat when 16 people ate the same 1,000-calorie surplus. The rise in everyday movement outside exercise predicted who resisted it.
- Resting metabolic rate increased only 79 calories per day during overfeeding, and its changes did not account for who gained more fat.
- The rise in everyday movement outside exercise (NEAT) predicted resistance to fat gain with a correlation of 0.77; the authors call it the principal mediator of resistance to fat gain.
- How much NEAT rose with overfeeding differed widely between people: 692 extra calories per day in one person, far less in others.
- In 2023 the same scientist set out the hypothesis that fidgeting is regulated by the brain, and argued that chair-based environments suppress it.
The Thing Everyone Blames Did Not Explain It
If your lean friend can eat like that and stay lean, the explanation everyone offers is metabolism. Resting metabolic rate. The furnace theory: some bodies burn hot while doing nothing, others do not, and the difference decides who gains weight.
This is what every "boost your metabolism" supplement sells. What every "fast metabolism" reel reinforces. What a billion-dollar industry whispers to the person scrolling through fitness content at midnight: your furnace is just slower than theirs.
The researchers measured resting metabolic rate directly. A technique that measures oxygen consumption to calculate energy output, repeated on two consecutive mornings, volunteers fasting since nine the night before, lying still in a dimly lit room for thirty minutes.
Resting metabolic rate rose by an average of 79 calories per day. A 5% increase. That accounted for roughly 8% of the extra thousand calories each person ate.
And its changes did not account for who gained more fat.
The paper gives no correlation for it, only that the changes in resting metabolism did not account for the variability in fat gain. The system that millions of people call "fast metabolism" did not explain which of the sixteen volunteers gained the most fat.
Postprandial thermogenesis (the energy your body burns digesting food) also showed no correlation with fat gain. The thermic response to a fixed meal was identical before and after overfeeding.
Two suspects interrogated. Both cleared.
What Changed Instead: Everyday Movement
Something else was happening, and it was what the study was designed to measure.
The volunteers did not change their exercise habits. The researchers confirmed this with accelerometers, daily interviews, and structured conversations with the volunteers' friends and families. Exercise was held at constant, low levels, so a change in exercise could not explain the differences.
But outside exercise, some of the sixteen volunteers' bodies started burning far more energy: the energy of everyday activity, such as the activities of daily living, fidgeting and maintaining posture. The study did not watch those movements one by one. It calculated their energy as total daily expenditure, measured with doubly labeled water, minus resting and digestion costs.
Scientists call this nonexercise activity thermogenesis, or NEAT. Your lean friend would call it Tuesday.
The average increase in NEAT across all sixteen volunteers was 328 calories per day, accounting for two-thirds of the entire increase in daily energy expenditure. But the average hides the real story.
The change in NEAT correlated with resistance to fat gain with a coefficient of 0.77. That is a correlation coefficient, not a share or a score out of 100. The changes in resting metabolic rate did not account for who gained more fat.
How far that everyday movement rose was, in the authors' words, the principal mediator of resistance to fat gain when everyone ate the same 1,000-calorie surplus.
One Body Burned 692 Extra Calories. Another Moved Less.
The range tells the real story.
In the volunteer with the largest rise, NEAT went up by 692 calories per day. Not exercise, which was held steady. Everyday activity outside it.
At the other end: one person's body responded to overfeeding by moving less than before. Their NEAT dropped by 98 calories per day. Their body's response to extra food was not to burn it. It was to sit down.
That is a gap of 790 calories per day between the highest and lowest NEAT response. Over 56 days of overfeeding, that gap accounts for more than 44,000 calories, the energy in roughly 5.6 kilograms of pure fat.
This is not a spectrum of slightly different metabolisms. This is two fundamentally different biological responses to the same stimulus. One body treats extra food as fuel for movement. Another body treats it as a signal to conserve.
That biological predisposition extends beyond controlled overfeeding. A compiled database of physical activity measured with isotope-labeled water included a twin study where identical twins — even living apart — moved remarkably similar amounts. Genetics may explain up to 78% of the variation in how much you move each day.
What a 692-Calorie Rise Looks Like
Six hundred and ninety-two calories sounds abstract until you picture it.
The researchers calculated what it would take: roughly 15 more minutes of strolling-equivalent activity for every waking hour. Not brisk walking. Not exercise. The casual pace you would use wandering through a grocery store or getting up to refill a glass of water.
Over a waking day of about sixteen hours, that adds up to about four hours more than before the overfeeding. Not in a gym. Not wearing a heart rate monitor. Everyday activity.
The extra was not training. Across the group, exercise did not change with overfeeding: accelerometer readings, daily interviews and interviews with family and friends all showed it. The paper reports these checks for the group, not for any one volunteer, and it does not say whether the rise in everyday movement was deliberate.
The efficiency of exercise itself did not change either. Walking on a treadmill at 3 miles per hour consumed the same oxygen before and after overfeeding. The machine did not get more efficient. The body simply found a different channel to dissipate energy, one that does not show up on any gym log.
Fidgets, previously thought to be spontaneous, are neurologically regulated and highly ordered (non-random).
Twenty-Four Years Later, the Same Scientist Proposed an Answer
If the rise in everyday movement predicted who resisted fat gain, the next question is obvious: why does it rise more in some people than in others?
In 2023, the same James Levine who ran the 1999 overfeeding study published a paper setting out a hypothesis. The title: "The Fidget Factor and the obesity paradox. How small movements have big impact." [1]
His hypothesis: fidgets are not random. They are regulated by the brain.
One of several brain messengers he names is orexin A, made in the hypothalamus; injected into one region of the hypothalamus in rats, it set off fidgets, more at higher doses. Levine calls the drive the Fidget Factor: "the innate neurological pulse that propels humans and other species to move." [1]
He argues it is not unique to humans: molecular studies in zebrafish show orexin mediating swimming activity and energy balance, and worms fidget too. [1]
But in his account the drive does not settle the matter on its own: "Despite impulses firing through the nervous system, people sit because environmental design overwhelms the biology."
The 1999 study measured the what. The 2023 paper proposes a why; its brain-chemistry evidence comes from rats, mice and zebrafish. Same scientist.
The Part No Listicle Will Give You
If the drive to move is innate, the question follows: can you change how much you move?
Six years after the 1999 study, Levine's group ran a different experiment. They fitted 10 lean and 10 mildly obese sedentary volunteers with sensors that measured body posture and movement every half second for ten days. [2]
The lean volunteers were upright, standing or walking, for 152 more minutes per day than the obese volunteers. Had the obese volunteers had the lean volunteers' posture allocation, the authors calculated, they would have burned about 352 more calories per day. [2]
Then the researchers asked whether sitting more is a cause or a consequence of obesity. Seven of the obese volunteers lost weight, 8 kg on average, and ten lean volunteers gained it, 4 kg on average, each over eight weeks.
Both groups kept their original posture allocation. [2] The volunteers who lost weight did not start spending more time upright; those who gained weight did not start spending less. The authors wrote that posture allocation therefore appears biologically determined, in a pilot study whose results need to be confirmed in larger studies. [2]
They did not read it as destiny.
If a biologically determined posture allocation destined people to obesity, they argued, obesity would have been as common 50 years ago as it is today. They speculated instead that lean and obese people respond differently to the environmental cues that promote sitting, and called for approaches that get people out of their chairs and moving. [2] The 1999 paper ends the same way: efforts to enhance NEAT, "perhaps through behavioral cues, may be a fruitful approach to the prevention of obesity." And the 2023 paper calls behaviour-change strategies critical to help people move and "liberate" their Fidget Factors. [1]
Every fitness site that covers this study gives you a listicle. Stand up every hour. Take the stairs. Fidget at your desk. Those suggestions are close to what Levine's group itself proposes. The 1999 study did not test them.
What the 1999 study does support is simpler: in sixteen overfed adults, the explanation most people reach for was not the one that tracked fat gain. The changes in resting metabolism did not account for who gained more fat; the rise in everyday movement outside exercise did.
Whether you can raise your own NEAT on purpose is a question this study did not test; its authors thought it worth trying.
What Comes Next
If resting metabolism did not explain who gained fat when everyone ate the same food, a harder question follows: what IS metabolism actually doing across a human life? The largest database of metabolic measurements ever assembled, 6,421 people measured with the gold-standard method for tracking daily energy expenditure, found that metabolism holds remarkably steady from age 20 to 60. The decline you blamed on turning 30 was never there.
The next time you see a product claiming to "boost your metabolism," this study's data is worth remembering. In it, the changes in resting metabolic rate, the system that product targets, did not account for who gained more fat.
What the study does support: the energy that tracked fat gain was not gym sessions or structured exercise, which were held steady. It was everyday movement outside exercise, and how far it rose with overfeeding.
This study did not test whether you can raise that movement on purpose. Its authors suggest that efforts to enhance it, perhaps through behavioral cues, may be a fruitful approach to preventing obesity, and the same group's later work calls for getting people out of their chairs. Structured exercise, dietary choices and sleep still matter too. The study confirmed that deliberate exercise was stable and measurable throughout.
The shift is "stop chasing the wrong explanation", and look at how much you move outside the gym.
What other research found
What this means for you
The system you have been told is "slow" — resting metabolic rate — did not account for who gained fat in this study: its changes with overfeeding did not explain the variability. The study did not test whether people with a lower resting metabolism to begin with gain more.
The measure that did track fat gain was the rise in everyday movement outside exercise, something few people ever measure.
This does not mean your body handles food identically to everyone else's. It means the usual explanation did not hold in this study, not that your observation was wrong.
In the volunteer whose everyday movement rose most, the rise came to 692 calories per day, with exercise held steady. The paper does not report that volunteer's exercise separately, or whether the rise was deliberate.
If you are the person who cannot sit through a movie, whose legs bounce under every table, who paces while on the phone, that movement burns real calories: fidgeting, posture and daily activity are all part of NEAT. The 2023 paper proposes that the urge behind it is regulated by the brain, and also that environment and habits can suppress or free it.
The 2005 study from the same lab found that when seven obese volunteers lost about 8 kg, and ten lean volunteers gained about 4 kg, both groups kept their original posture allocation. The authors read this as a sign that posture allocation appears biologically determined, in a pilot study needing confirmation, and still called for getting people out of their chairs.
NEAT includes standing and maintaining posture, so a standing desk works on the same energy this study measured. None of these studies tested whether a standing desk or a NEAT tip changes fat gain or weight.
If the tips did not move your scale, that is not evidence that your movement runs on autopilot.
Before you change anything
Sixteen nonobese adults aged 25 to 36, twelve men and four women, studied at the Mayo Clinic. None exercised more than twice a week. None smoked, took medication, or had any chronic illness.
This study does not directly apply to people with obesity, older adults, children, or anyone with a metabolic condition. The three-to-one male-to-female ratio means the findings are stronger for young men than for women. The 2005 study from the same group measured posture allocation in lean and mildly obese adults, also after weight loss and overfeeding, and the 1999 overfeeding protocol was tested only on nonobese volunteers.
Sixteen people is a small sample, even for a supervised overfeeding study. The trade-off: controlled overfeeding with supervised meals, garbage inspection, and repeated calorimetry is hard to run with hundreds of participants. That precision is why 97 percent of energy was accounted for.
The technology is from 1999. The Caltrac accelerometers that checked exercise date from then; NEAT itself was calculated from doubly labeled water and calorimetry, and the authors note that small measurement errors would, if anything, weaken the association they found.
One diet composition was tested — 40 percent carbohydrate, 40 percent fat, 20 percent protein. Whether a different macronutrient ratio would change the NEAT response is unknown from this study alone.
The core finding is strong. The change in NEAT predicted resistance to fat gain with a correlation of 0.77 in a supervised setting where diet and exercise were controlled. The study accounted for 97 percent of the extra energy, which the authors read as near-complete compliance.
The mechanism story is emerging. The 2023 paper is a hypothesis: it names orexin A as one of several brain messengers involved, with evidence from rats, mice and zebrafish.
The biological-determination claim is moderate. It rests on the 2005 study (20 volunteers), which the authors themselves described as requiring confirmation in larger studies. The finding that posture allocation did not change with weight manipulation comes from a pilot-scale design, and the authors themselves argued against reading it as destiny.
This study revealed what the "fast metabolism" explanation was hiding. But if everyday movement outside exercise — not resting burn rate — predicts who gains fat, what is metabolism actually doing across a lifetime? The largest energy expenditure dataset ever assembled — 6,421 people measured from birth to age 95 — found that between 20 and 60, the answer is: almost nothing changes.
What This Study Found
All findings from this paper, in plain language.
- Of the parts of energy expenditure the study measured, only the rise in everyday movement outside exercise (NEAT) predicted who resisted fat gain; the changes in resting metabolism and in the energy of digestion did not.
- Fat gain varied more than tenfold across 16 people eating the same extra food, from 0.36 kg to over four kilograms.
- Everyday movement outside exercise accounted for two-thirds of the total increase in daily energy expenditure during overfeeding.
- How much movement outside exercise rose differed widely between people: in one person it rose by 692 calories per day, in others far less.
- Resting metabolic rate went up slightly, but its changes did not account for who gained the most fat.
- The energy burned digesting food did not track the difference between people who gained a lot of fat and those who gained very little.
- The researchers accounted for 97 percent of the extra calories, as stored or burned off.
- People whose bodies burned more total energy gained less fat, and the relationship was strong.
- Deliberate exercise did not change during overfeeding — the rise in activity energy was the rise in NEAT, everyday movement outside exercise.
- The four lowest rises in movement outside exercise belonged to the four women in the study, though the sample was too small to draw conclusions about sex differences.
- The highest rise in movement outside exercise would look like 15 more minutes of casual strolling for every waking hour, spread across the entire day.
The core question behind this debate: does extra everyday movement simply add more calories burned on top of everything else your body spends? Or does your body have a total energy budget that adjusts to keep spending roughly the same?
Levine's 1999 data supports the first idea. When the volunteers overate, the ones whose everyday movement rose more burned more total energy. The people who burned the most total calories gained the least fat — and the relationship was strong.
A different researcher, Herman Pontzer, published a competing model in 2016. His team tracked the Hadza, a hunter-gatherer population in Tanzania who walk far more than the average Westerner every day. Despite all that extra movement, their total daily energy expenditure was not proportionally higher. The body, Pontzer argues, reallocates energy internally to keep total spending within a range.
Both perspectives may be simultaneously true at different scales. In eight weeks of supervised overfeeding, the rise in everyday movement clearly predicted who gained fat. Over years of everyday life, total energy spending may partially self-correct. The 1999 study answers the short-term question convincingly. The long-term question remains open.