When scientists offer mice or rats a spread of junk food, they consistently find that only some overeat and puff out into little rodent blimps, while others maintain a normal body size.

A similar thing happens in people. In the US, and around the world, we are now overwhelmed with highly palatable, cheap calories. This has helped obesity rates soar on average. But not everyone overeats and becomes overweight, and not everyone who becomes overweight or obese develops illnesses like diabetes or heart disease. This individual variation — why we have different responses to extra calories and weight — is one of the greatest mysteries of modern medicine.

The best place to find answers is an 11-by-11.5-foot room in suburban Washington, DC. This summer, I spent a day there, one of fewer than 100 patients who will do so this year.

The National Institutes of Health Clinical Center’s airtight “metabolic chamber” is furnished only with an exercise bike, a toilet, and a bed. For 23 hours in June, I was sealed in the chamber, while nurses monitored me constantly through a plexiglass window and video camera in the ceiling.

Like a prisoner in solitary confinement, I ate meals delivered through a small, air-locked opening in the wall. Since researchers were measuring every calorie I used, any leftover scrap had to be sent back through the wall and recorded. A heart monitor and three accelerometers on my wrist, waist, and ankle tracked my every heartbeat and movement.

There are only about 30 metabolic chambers in the world, and the NIH is home to three. These highly sensitive, multimillion-dollar scientific instruments are considered the gold standard for measuring metabolism. They’ve furthered our understanding of obesity, metabolic syndrome, and diabetes — diseases that are now among the greatest threats to health worldwide — by letting researchers carefully track how individual bodies respond to the calories they’re offered.

My participation, as a normal weight “control” subject in an obesity phenotyping study, would be used toward this lofty goal.

But I wasn’t interested in joining the study just for the sake of science; I had selfish motivations too. As kids, my two brothers and many of my friends seemed to be able to binge on junk food without gaining weight. Today, my husband can gulp down mountains of pasta and remain skinny. I, on the other hand, have always noticed the scale creeps up quickly when I’m not careful about my diet. And I’ve harbored a suspicion that a “slow metabolism” might help explain my lifelong struggle to control my weight.

Being a self-imposed NIH prisoner was an exciting and rare opportunity — to see one of the most important scientific tools in obesity research up close and to finally get some answers on this long-simmering question about my body.

But my day in the chamber revealed the depths of my misunderstanding about my metabolism. And that the obsession with metabolism speed is distracting, destructive, and based on a myth about obesity and weight management.

Metabolism, explained

If you’ve surveyed the covers of women’s magazines, watched Dr. Oz’s TV show, or strolled down the supplement aisle at the grocery store, you might think your metabolism is a single thing that can be calibrated with “metabolism boosters” like chili peppers or coffee, or by following special diets.

In reality, metabolism is the thousands of chemical reactions that turn the energy we eat and drink into fuel in every cell of the body. These reactions change in response to our environments and behaviors, and in ways we have little control over. (Eating certain foods and exercising a little more generally shifts our metabolic rate only marginally.)

There are three main ways the body uses calories. There’s the energy needed to keep our hearts, brains, and every cell of our body working, known as the basal metabolism. There’s the energy used to break down food, known as the thermic effect of food. And there’s the energy burned off during physical activity — like walking around, fidgeting, or exercising.

The basal metabolic rate accounts for the largest amount of the total calories a person burns each day (65 to 80 percent for most adults). Physical activity, on the other hand, accounts for a much smaller portion — 10 to 30 percent for most people — despite what many people believe. And digesting food accounts for about 10 percent.

There are several predictors of how fast or slow a person’s metabolic rate will be. These include the amount of lean muscle and fat tissue in the body, age, and genetics. Women tend to burn fewer calories than men. Having a higher metabolic rate means your body uses food for fuel (instead of storing it as fat) more quickly. But you can still gain weight if you consume more calories than your body needs. Counterintuitively, heavier people generally have higher metabolic rates than skinny folks to meet the fuel demands of their larger bodies.

These processes, essential to any living organism, are complex, and scientists had been working to unravel them for centuries before the obesity crisis hit.

In the early 1600s, Santorio Sanctorius, an Italian doctor and “founding father of metabolic balance studies,” ran one of the first controlled experiments of human metabolism. He invented the “static weighing chair,” a device that allowed him to weigh himself before and after meals, sleep, toilet breaks, even sex. He noticed fluctuations in his bodyweight, and concluded these could be explained by “insensible perspiration.”

One hundred years after that, French chemist Antoine Lavoisier used a device called an “ice calorimeter” to gauge the energy burn from animals —like guinea pigs — in cages by watching how quickly ice or snow around the cages melted. This research suggested that the heat and gases respired by animals, including humans, related to the energy they burn.

The “metabolic chamber” I entered evolved from Sanctorius and Lavoisier’s work. Over the years, researchers probing the mysteries of the metabolism figured out that the amount of oxygen we take in, and carbon dioxide we let off, changes depending on how quickly we’re using calories and the type of calories we’re using. Measuring these gases in airtight environments can determine a person’s metabolic rate.

The debunking machine

The metabolic chamber — also known as a whole-room calorimeter — is the most precise tool available to track this gas exchange minute by minute.

NIH’s three chambers opened in 2007 to focus on the growing obesity epidemic. Eighteen researchers now use the rooms to run about 400 studies every year. And they are part of a broader “metabolic unit” dedicated to understanding the weight problems, obesity, and diabetes that currently affect up to a third of the people on earth.

Studying thousands of subjects in the metabolic unit — the chambers plus NIH hospital wings for patients with diabetes and obesity — has helped researchers show how adaptable the metabolism is, and how it works with our appetite, body composition, and physical activity levels to adjust the calories we’re burning at any moment.

For example, by giving people a medication that causes them to lose (through their urine) an extra 360 calories per day, they’ve shown that we unknowingly compensate for those calories lost by eating more.

They’ve found that exposing people to cold temperatures while they sleep causes them to accumulate more brown fat — fat tissue whose main function is heat production — and burn more calories. (These results reversed completely when the study participants slept in warmer temperatures again, revealing how dynamic metabolism is.)

In a remarkable study of Biggest Loser reality TV show participants with obesity, researchers showed that crash dieting can permanently slow a person’s metabolic rate, leading them to hang on to the calories they were eating for longer, though this isn’t true for everybody who loses weight.

The big theme in many of these studies: Our metabolism silently shifts under new conditions and environments in ways we’re not usually aware of.

When it comes to diets, the researchers have also debunked the notion that bodies burn more body fat while on a high-fat and low-carb ketogenic diet, compared to a higher-carb diet, despite all the hype.

“We could have found out that if we cut carbs, we’d lose way more fat because energy expenditure would go up and fat oxidation would go up,” said Kevin Hall, an obesity researcher at NIH and an author on many of these studies. “But the body is really good at adapting to the fuels coming in.” Another related takeaway: There appears to be no silver bullet diet for fat loss, at least not yet.

Many basic metabolism mysteries remain. It’s not fully known why two people with the same size and body composition have different metabolic rates. They also don’t know why people can have different metabolic responses to weight gain (where some people with obesity develop insulin resistance and diabetes, for example, and others don’t). They don’t know why certain ethnic groups — African Americans, South Asians — have a higher risk of developing metabolic disorders like diabetes, and why people with diabetes have a higher cardiovascular disease risk.

They haven’t even figured out how the brain knows what the body weighs and, therefore, the mechanism that controls our metabolic rate.

“If I knew how the brain is aware of how much the body weighs, and how to regulate how many calories it burned off, I could change that setting and help an overweight person burn more calories through an increase in metabolic rate,” NIH metabolism and brown fat researcher Aaron Cypess told me over the phone before my stay.

Cypess is using the chambers to work toward that, and figure out whether there might be a drug that can do what very cold temperatures do: help people burn more calories. These and other studies in the chamber are a gold mine for data on the metabolism’s mysteries — data that could eventually help uncover cures for obesity and diabetes.

The meaning of metabolism

For my part in the research, I’d undergo a battery of physical tests — from blood draws to an EKG — and spend a day and night in the chamber. In addition to watching how much I moved and what I ate, the scientists would get a reading on precisely how many calories I burned and what type (carbohydrates, fat, or protein), every minute of the 23 hours I called the chamber home. I’d also have my metabolic rate checked using two other methods (the “metabolic cart” and “doubly labeled water”; more on these later).

In return, I’d get more granular data about how my body works than I ever could’ve hoped for. And that made me anxious.

At age 34 and 5-foot-9, my weight hovers in the 150s, and my BMI is normal. But even as a child, I was chubby and seemed to enjoy sugary and fatty foods more than other members of my family. During my late teens and 20s, I struggled to manage my weight and was at times overweight — a situation that worsened at the end of high school. I moved to Italy and indulged in all the pizza, ice cream, carpaccio, and mozzarella my little town in Abruzzo had to offer. Like a research mouse, I puffed out and returned to Canada the following year depressed about my body. It took several years to really start the process of slimming down.

I’d long believed these fat years somehow wreaked havoc on my body. Specifically, I thought they slowed down my metabolic rate, and that that made me prone to weight gain. But I was about to learn this idea I’d held on to for so long was wrong.

How the metabolic chamber actually measures metabolism

Halfway through my morning in the metabolic chamber, I had eaten and rested at prescribed intervals, and hit the exercise bike for 30 minutes. I also meticulously recorded all my activities in a log — when I was standing and reading, lying down, on the bike — so that the researchers could compare how they tracked against my calorie burn.

Just before lunch arrived, Kong Chen, a metabolism investigator at NIH’s National Institute of Diabetes and Digestive and Kidney Diseases, turned up on the other side of my plexiglass window to say hello.

“How are you doing in there?” he asked.

I was surprisingly comfortable in the little room, I told him, and asked if he could walk me through precisely how the chamber does the work of measuring the metabolism.

Chen, who has a PhD in biomedical engineering, explained that the room I was standing in was almost airtight, with a fixed volume of oxygen and CO2. Through an array of metal pipes spread across the ceiling, researchers captured and measured the oxygen I consumed and the CO2 I produced at every minute.

The reason these gasses matter for metabolism is simple, Chen said. We get fuel in the form of calories — from carbohydrates, fat, and protein. But to unlock those calories, the body needs oxygen. When we breathe in, oxygen interacts with the food we’ve consumed, breaking down (or oxidizing) chemical bonds where the calories are stored and releasing them for use by our cells. The product of the process is CO2.

When air is sucked out of the chamber through the pipes, two things happen: First, gas analyzers measure everything the person inside respired, Chen said. Then the gas analyzers send the values for oxygen consumption and CO2 production to a computer, where researchers like Chen plug them into equations to calculate calories burned and what type of fuel was oxidized.

The amount of CO2 we’re releasing, and the proportions of CO2 to O2, changes depending on how many calories we’re using and whether those calories came from carbs, fat, or protein.

The reason these minute-to-minute measurements are so important is that they allow the chamber to detect subtle shifts of energy expenditure — as little as a 1.5 to 2 percent change over 24 hours — in a way no other tool can. “If you have an intervention — a drug or diet — that changes a person’s physiology by a small percentage, we can measure that,” Chen said proudly.

The next best metabolism measuring method, called doubly labeled water, involves drinking a sample of water that contains (or is “labeled with”) forms of the elements deuterium and oxygen-18. Since they’re not normally found in the body, researchers can determine a person’s metabolic rate by tracking how quickly they’re expelled through urine sampling. But doubly labeled water can only detect a 5 percent change in metabolic rate over seven to 10 days, which is less than half as precise as the metabolic chamber.

These tiny changes in calorie burn might sound insignificant, but over time, they add up. “Ultimately,” Chen said, “it only takes maybe a 100 calorie-per-day difference between food intake and energy expenditure over a few years to gain 10 pounds.” So an extra cookie a day can mean the difference between fitting in your jeans or not.

I asked Chen whether he’d ever used the chamber himself. He told me he was his own first subject, part of an early validation study. What did he learn, and did it change his behavior?

“I found myself to be fairly normal in terms of metabolic rate, which is good and bad I suppose,” he said. “Good because I’m metabolically normal. But it also means that I’m probably just as at risk to anyone else to gaining weight if I’m not watching it. So I’m not one of those people that can eat all they want and not gain weight.”

After Chen’s visit, the rest of my day in isolation whirred by with several more rest periods, exercise bursts, and meals. I went to bed that night thinking about Chen’s results and wondering what the chamber would reveal about me.

“You’re perfectly normal”

The next morning, I woke up groggy from six hours of light sleep. I was eager to open the heavy steel door and get into fresh air.

But the experiment wasn’t over. A “metabolic cart” — which looked like a computer on rollers connected to a tube and a plastic hood — arrived to measure my resting energy expenditure, or metabolic rate when I’m awake but not physically active, and before eating anything. So I lay in a hospital bed as a technician fitted the clear domed hood over my head while the machine captured the CO2 I respired.

On my way out of the hospital, I said goodbye to Chen and thanked the nurses who had cared for me. They reminded me to collect urine samples every day for a week so they’d get a final measure of my metabolism, using the doubly labeled water method. I’d also continue wearing the three accelerometers. Together, this data would give the researchers a sense of my average daily calorie burn as a “free-living subject,” outside the hospital.

A few weeks later, I called Kevin Hall to go over my results. What most surprised me: There was a pretty wide gap between how healthy I was and how unhealthy I expected I’d be.

“[The results] suggest you’re perfectly normal,” Hall said.

My metabolic rate was what he’d have predicted for someone my age, height, sex, and weight. In other words, I didn’t have a “slow metabolism.” I had burned the equivalent of 2,330 calories per day in the chamber, including during sleep, and most of those calories (more than 1,400) were from my resting energy expenditure. My biomarkers — my heart rate, cholesterol levels, blood pressure — were all excellent, suggesting no heightened disease risk leftover from my overweight years.

There were other revealing takeaways. Staying awake cost my body only a few more calories than sleeping, which didn’t surprise Hall. “We know the sleeping metabolic rate is about 5 percent less than resting metabolic rate when you’re awake,” he explained.

What’s more, the 405 calories I burned during 90 minutes on the exercise bike was both less than is advertised in spinning classes and just 17 percent of the total calories I had used, validating once again that workouts typically account for a relatively minor part of total energy expenditure.

Even during sleep, my body was busy. “This goes into the question of, ‘Does your brain’s energy expenditure go up when you’re doing a hard math problem compared to when you’re zoning out watching TV?’ And everyone who has measured that has said ‘no’ — it’s a fixed amount, and your brain is not inactive at any point in time,” Hall said.

As for the “calories in” part: I consumed about 1,850 calories (including 18 percent protein, 36 percent fat, and 46 percent carbs) of the 2,250 calories provided to me. That means I was in an energy deficit, and if I continued eating that much, I’d lose weight.

I also found out that I’m bad at estimating my calorie consumption. During my chamber stay, I told a nutritionist what I’d eaten the day before and filled in a survey of my food consumption over the past year. Based on that, she’d calculated I was eating only 1,500 to 2,000 calories per day. I thought I was being incredibly thorough and generous in my accounting, but if this was really all I ate, I’d be thinner than I am.

The results of these food surveys made me wonder how many of us blame some aspect of our biology for weight gain when we’re really just underestimating our calorie intake, forgetting all the little extras we eat and drink that can add up to pounds over the years. It seems I had too.

I asked Hall if there were any other potential explanations for why I felt I gained weight so easily. He told me NIH does other studies that could answer that. If he had tracked my metabolism before I had lost weight earlier in life, he’d be able to detect any slowdown in response to slimming. Or if I participated in an “overfeeding study” — where I was deliberately fed more calories than my body required — he might detect no change in my metabolic rate. There are some people whose metabolic rate speeds up when they overeat, using the extra calories as fuel instead of storing them as fat, and it’s possible I’m not one of them.

But we didn’t have that data, and according to what he could see, I was in perfect health.

The metabolism myth

I hung up the phone and reflected on the chamber experience — and my quest to better understand my body.

Spending time at NIH reminded me that our epidemic of weight problems, in addition to damaging our physical health, has left in its wake an epidemic of psychological scars — even in those who, like me, manage to lose weight.

I was genuinely surprised, and somewhat relieved, when nurses and doctors kept referring to my biomarkers as “excellent” and to me as “very fit.” Even though I know my bodyweight is in a healthy range, I still feel like a chubby kid.

And you don’t need a history of weight problems to experience these feelings of inadequacy. Celebrities and big businesses — like Goop and Dr. Oz and many of the supplement, wellness, and exercise companies out there — have minted billions off stoking our anxieties about our physical shortcomings. If we only tried a new exercise, bought a new gizmo, or ate a certain way, they suggest, we’d be slimmer, glowier, healthier.

Yet the truth of the metabolic chamber is that there’s a lot of variation in how people respond to diets and exercises, and so far, no single approach has worked to help everybody. That’s why so much of the one-size-fits-all weight loss advice we’re steeped in is so frustrating and futile for so many.

The chamber has also shown that while some people have a “slow metabolism” relative to others their size and age, this isn’t a major cause of obesity. And despite the focus on “metabolism boosting” for weight loss, there’s nothing money can buy that will speed your metabolism up in way that will lead to substantial slimming.

When I look back at what helped me lose weight, there was never a magic bullet — a special diet, exercise regimen, or supplement — that worked. Through plodding trial and error, I discovered habits and routines I could stick with to help me eat less and move more.

I don’t keep junk food in the house, I avoid eating out a lot, I prioritize sleep, and I try to fill my plates with fruits and vegetables. As for exercise, I build it into my daily life — walking or biking to work, or during lunch breaks. And I’ve found mornings and weekends best for dedicated workouts (yoga, running, swimming, spinning, Pilates, etc.).

These routines are a work in progress, and I know that my ability to maintain them is strongly tied to my socioeconomic status and where I live. If I had more personal or financial stress, or lived in a different neighborhood with a long commute to work, I’d probably sleep less and eat more. I certainly wouldn’t be doing Pilates.

Research from the chamber won’t alleviate these socioeconomic drivers of obesity. But a better understanding of human physiology and metabolism — with the help of the chamber — might level the playing field through the discovery of effective treatments. As Lex Kravitz, an NIH neuroscientist and obesity researcher, told me, “Even if a slow metabolism isn’t the reason people become obese, it may still be a place to intervene for weight loss.” The same goes for the other common illnesses — diabetes, cardiovascular disease — linked to extra weight.

More immediately, science from the chamber should debunk our metabolism myths. It certainly debunked mine.

For more about the metabolism chamber, listen to our Today, Explained podcast episode.

For more information about how to join a study at NIH, check out this link on patient recruitment or contact the NIH Clinical Center Office of Patient Recruitment at 1-800-411-1222 or prpl@mail.cc.nih.gov.

Editor: Eliza Barclay

Copy editor: Tanya Pai