If you think about it, losing weight is actually kind of a mind-boggling thing. One minute you’re carrying the extra pounds, but after a stint of healthy eating and regular exercise, you’re physically smaller. So where does all the fat go?

You might imagine the globules of fat just sort of burst and cease to exist, but POPSUGAR looked into it and it turns out that’s very much not the case. I mean, there’s clearly a reason I don’t have a degree in anything scientific.

“The correct answer is that most of the mass is breathed out as carbon dioxide. It goes into thin air,” physicist Ruben Meerman, MD told POPSUGAR. Yeah, you read that correctly: you breathe out fat. This is unbelievable.

*Starts breathing quicker in an attempt to get rid of more*


Apparently, it’s a rough 80/20 split between fat predominantly being expelled from your body via your lungs, with the remainder being evacuated via your urine, faeces, sweating, and tears.

If you’re feeling a bit dim now for never having known this life-altering information, don’t worry, you’re not the only one. According to the head of the UNSW School of Biotechnology and Biomolecular Sciences, Professor Andrew Brown, there are plenty of doctors and dietitians who were convinced fat was converted into muscle, or that it turned into either energy or heat and was ‘burnt’ in that way.

Sadly, if you’re now thinking that instead of your next work-out at the gym you’ll just cry buckets while breathing quickly – if that’s how fat exits the body, why not give it more opportunity to go, right? – you’ll be disappointed. Fat has to go through a metabolic process before it’s actually lost – but having said that, there are plenty of ways to boost your metabolism, and that will speed up your rate of fat loss.

You can read all about that here:

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Catriona Harvey-Jenner Digital Features Editor Cat is Cosmopolitan UK’s features editor covering women’s issues, health and current affairs.

What Happens to Fat When You Lose Weight?

We all know that adhering to a caloric deficit will result in weight loss. “Eat less, move more,” and your weight will eventually drop.

But have you ever wondered what actually happens to fat when you lose weight? Many people think fat leaves the body as energy or heat, while others think it turns to muscle or gets pooped out1.

As you might know, energy is stored as triglycerides (i.e. fat) in our fat cells. These cells that specialize in fat storage are called adipocytes. Research suggests that the number of fat cells that we have is largely set during childhood2.

If we gain weight, we can increase the number of fat cells throughout adulthood. However, we can’t get rid of the fat cells we already have. (The only way to get rid of fat cells is to have them removed via a procedure such as liposuction.)

So, if the fat cells don’t disappear when we “lose fat,” then what happens to them?

What Happens to the Fat Cells?

When we don’t use energy from the food that we eat, the body converts the energy to a form of fat called triglycerides. A triglyceride is an ester made up of glycerol and three fatty acid groups. These triglycerides are stored in fat cells until hormones release them for energy later on.

Our fat cells make up loose, connective tissue – called adipose tissue – that helps to insulate our body. During exercise (or fasting), energy stored as fat in our adipose tissue can be broken down for muscles and other tissues to use. This process, called lipolysis, is induced by a variety of hormones, including cortisol3.

Related: 5 Lifestyle Hacks for a Shredded Physique

Through lipolysis, fat in triglyceride form is broken down (hydrolyzed) into fatty acids and glycerol, which leave the adipocytes to be used as energy. As a result of this process, fat cells shrink. They don’t dissolve or disappear. Imagine a deflated balloon—that’s a fat cell after the fat is broken down to be used as energy.

What Happens to the Fat?

Where does the fat go after it is broken down to be used as energy? Triglycerides and their fatty acid and glycerol components are made up of carbon, hydrogen, and oxygen. The oxidation of triglyceride results in carbon dioxide, water, and energy. The reaction for the oxidation of a single triglyceride molecule can be summarized as:

C55H104O6 + 78O2 → 55CO2 + 52H2O + energy1

Thus, after fat is broken down, it leaves the body as carbon dioxide and water. Most of it is exhaled as carbon dioxide. The rest leaves the body as water through urine, sweat, or other bodily fluids. Approximately 84% turns into carbon dioxide, and 16% turns into water1. This means that most fat is “lost” through the lungs!

This might sound weird, but if we stop and think about it, carbon is involved in nearly every process that our bodies are involved in—carbon is the basis of all life on Earth!

The food we eat also gets converted to carbon dioxide and water, and carbon dioxide is the reason why we weigh less when we wake up than we did when we fell asleep the night before. We breathe out carbon dioxide for hours while we’re sleeping and lose that mass in the process. (Then we go to the bathroom and lose even more weight via water.)

How to Lose Fat

If you take in 9 lbs of food, water, and oxygen, those 9 lbs need to come back out. But, in order to lose weight, you need to somehow get rid of more than the 9 lbs that you took in. As we discussed, this mostly translates to an increased amount of carbon dioxide leaving the body. In order to exhale more, we need to exercise.

When we inhale, oxygen makes its way into the bloodstream. The muscles produce carbon dioxide, which leaves the blood and crosses the lung membranes to be exhaled. By moving our muscles more, we can increase the amount of carbon dioxide produced by our body. Subsequently, we get rid of more energy than we took in, effectively losing weight.

Fat Loss vs. Weight Loss

In order to lose weight, energy expenditure needs to exceed energy input. But weight is made up of muscle, water, bone, organs, and other tissue in addition to fat.

We can lose more fat and preserve more muscle when we follow a slower weight loss protocol rather than trying to find a quick-fix and lose too much weight at once4. Losing too much weight at once usually means that it’s coming from water and other components rather than fat.

Since we are talking about what happens to fat when we lose weight, we are only concerned with the physical process of losing fat during weight loss. These physical changes can have many side effects.

Related: 7 Habits of Highly Successful & Motivated Gym-Goers

For example, extended periods of weight loss can result in adaptive thermogenesis, a disproportionate reduction in resting metabolic rate (RMR) when weight is lost5. Adaptive thermogenesis largely explains why it seems like metabolism slows down after long periods of dieting.

Weight loss is a complex process influenced by multiple factors. Let’s take a closer look at types of fat to better understand its connection to the weight loss process.

Types of Fat

Fat is located under the skin (subcutaneous fat) and around internal organs (visceral fat). When we lose weight, we generally lose more subcutaneous fat than visceral fat6. Unfortunately, we can’t choose where we lose fat first—or at all. Genetics determines where we lose fat.

White adipose tissue (WAT), or white fat, is what we normally think of when we think of fat. White fat acts as an energy reserve and a cushion for our internal organs. But there’s another type of fat called brown adipose tissue (BAT), or brown fat.

Brown fat helps to protect newborns in cold temperatures, but our brown fat reserves decrease throughout life7. Derived from muscle tissue, brown fat plays a role in energy expenditure and can be generated by exercising and by exposure to cold temperatures7.


A recent report from the Centers of for Disease Control and Prevention (CDC) stated that nearly half of US adults had tried to lose weight in the last 12 months8. “Yo-yo dieting,” or any cycling of gaining and losing weight can be dangerous for many reasons.

Relevant to the topic of this article, weight cycling is risky business because we can’t get rid of fat cells once we have them (unless we get them removed by a surgical or other procedure). It’s important to realize that our number of fat cells can increase when we gain weight but not decrease, even when we lose weight.

Contrary to popular belief, fat does not turn into energy, heat, muscle, or poop when we lose weight. Rather, it leaves the body as carbon dioxide when we breathe and as water through sweat and other bodily fluids. Exercise is the best way to increase this output.

Weight loss is a complex, multifactorial process. Fat is only one component of weight lost during the process. Additionally, fat itself is complex; not all fat is “bad.”

Having an awareness of the role of fat in our body and how it comes and goes can help us to continue to maintain our overall health. Taking care of our bodies starts on a cellular level.

Raise your hand if you’re under the assumption that you poop out fat when you lose weight. It’s a fair guess—and it’s not entirely wrong. But it definitely doesn’t tell the whole story.

Recently, an old study has been circulating that found that a lot of the fat you lose is actually breathed out. Kind of trippy, huh?

In the study, which was originally published in BMJ in 2014, researchers looked at what happened to the atoms in 22 pounds of fat as they were “lost.” About 18.5 of those pounds were exhaled as carbon dioxide through the lungs—and the rest became water, which can be excreted in your pee, poop, sweat, breath, tears, and other bodily fluids, according to the reserach.

When someone starts losing weight, the size of their fat cells actually decreases.

It’s easy to read this and think that you’re breathing out chunks of fat when you lose weight, but that’s not how this works, says registered dietitian Sonya Angelone, a spokeswoman for the Academy of Nutrition and Dietetics. Instead, you’re actually breathing out fat metabolic byproducts as carbon dioxide—not fat cells themselves.

When someone starts losing weight, the size of their fat cells just decreases, says Fatima Cody Stanford, M.D., instructor of medicine and pediatrics at Harvard Medical School and obesity medicine physician at Massachusetts General Hospital, but they don’t actually disappear entirely. They really just shrink.

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Here’s where it gets tricky: When you lose weight, your body actually tries to regain that weight to get back to the status quo. So you might experience decreases in how full you feel and increases in how hungry you feel, says Stanford. WTF, right?

Luckily, exercise can help prevent those fat cells from getting bigger again, which explains why working out helps keep weight regain in check, Stanford says.

So, when you lose weight, keep in mind that it’s really best to do it slowly in order to allow your body to get used to the new normal. Otherwise, it might go into self-sabotage mode. And those fat cells haven’t really gone anywhere—they’ve just lost weight…just like you.

Korin Miller Korin Miller is a freelance writer specializing in general wellness, sexual health and relationships, and lifestyle trends, with work appearing in Men’s Health, Women’s Health, Self, Glamour, and more.

Recently, an old study has been circulating that found that a lot of the fat you lose is actually breathed out. Kind of trippy, huh?

In the study, which was originally published in BMJ in 2014, researchers looked at what happened to the atoms in 10kg of fat as they were “lost.” About 8kg of those pounds were exhaled as carbon dioxide through the lungs—and the rest became water, which can be excreted in your pee, poop, sweat, breath, tears, and other bodily fluids, according to the research.

It’s easy to read this and think that you’re breathing out chunks of fat when you lose weight, but that’s not how this works, says registered dietitian Sonya Angelone, a spokeswoman for the Academy of Nutrition and Dietetics. Instead, you’re actually breathing out fat metabolic byproducts as carbon dioxide—not fat cells themselves.

RELATED: The 15 Best Fat-Burning Foods Of All Time


When someone starts losing weight, the size of their fat cells just decreases, says Fatima Cody Stanford, M.D., instructor of medicine and paediatrics at Harvard Medical School and obesity medicine physician at Massachusetts General Hospital, but they don’t actually disappear entirely. They really just shrink.

Here’s where it gets tricky: When you lose weight, your body actually tries to regain that weight to get back to the status quo. So you might experience decreases in how full you feel and increases in how hungry you feel, says Stanford. WTF, right?

Luckily, exercise can help prevent those fat cells from getting bigger again, which explains why working out helps keep weight regain in check, Stanford says.


So, when you lose weight, keep in mind that it’s really best to do it slowly in order to allow your body to get used to the new normal. Otherwise, it might go into self-sabotage mode. And those fat cells haven’t really gone anywhere—they’ve just lost weight…just like you.

This article originally appeared on Women’s Health US.

RELATED: Why the ‘Square Meal Rule’ Is A Total Game Changer For Weight Loss

Why you shouldn’t exercise to lose weight, explained with 60+ studies

1) An evolutionary clue to how our bodies burn calories

When anthropologist Herman Pontzer set off from Hunter College in New York to Tanzania to study one of the few remaining hunter-gatherer tribes on the planet, he expected to find a group of calorie-burning machines.

Unlike Westerners, who increasingly spend their waking hours glued to chairs, the Hadza are on the move most of the time. Men typically go off and hunt — chasing and killing animals, climbing trees in search of wild honey. Women forage for plants, dig up tubers, and comb bushes for berries. “They’re on the high end of physical activity for any population that’s been looked at ever,” Pontzer said.

By studying the Hadza lifestyle, Pontzer thought he would find evidence to back the conventional wisdom about why obesity has become such a big problem worldwide. Many have argued that one of the reasons we’ve collectively put on so much weight over the past 50 years is that we’re much less active than our ancestors.

Surely, Pontzer thought, the Hadza would be burning lots more calories on average than today’s typical Westerner; surely they’d show how sluggish our bodies have become.

On several trips in 2009 and 2010, he and his colleagues headed into the middle of the savanna, packing up a Land Rover with camping supplies, computers, solar panels, liquid nitrogen to freeze urine samples, and respirometry units to measure respiration.

In the dry, open terrain, they found study subjects among several Hadza families. For 11 days, they tracked the movements and energy burn of 13 men and 17 women ages 18 to 75, using a technique called doubly labeled water — the best known way to measure the carbon dioxide we expel as we burn energy.

When they crunched the numbers, the results were astonishing.

“We were really surprised when the energy expenditure among the Hadza was no higher than it is for people in the US and Europe,” says Pontzer, who published the findings in 2012 in the journal PLOS One. While the hunter-gatherers were physically active and lean, they actually burned the same amount of calories every day as the average American or European, even after the researchers controlled for body size.

Pontzer’s study was preliminary and imperfect. It involved only 30 participants from one small community.

But it raised a tantalizing question: How could the hunting, foraging Hadza possibly burn the same amount of energy as indolent Westerners?

Javier Zarracina/Vox

As Pontzer pondered his findings, he began to piece together an explanation.
First, scientists have shown that energy expenditure — or calories burned every day — includes not only movement but all the energy needed to run the thousands of functions that keep us alive. (Researchers have long known this, but few had considered its significance in the context of the global obesity epidemic.)

Calorie burn also seems to be a trait humans have evolved over time that has little to do with lifestyle. Maybe, Pontzer thought, the Hadza were using the same amount of energy as Westerners because their bodies were conserving energy on other tasks.

Or maybe the Hadza were resting more when they weren’t hunting and gathering to make up for all their physical labor, which would also lower their overall energy expenditure.

This science is still evolving. But it has profound implications for how we think about how deeply hardwired energy expenditure is and the extent to which we can hack it with more exercise.

If the “calories out” variable can’t be controlled very well, what might account for the difference in the Hadza’s weights?

This fundamental concept is part of a growing body of evidence that helps explain a phenomenon researchers have been documenting for years: that it’s extremely difficult for people to lose weight once they’ve gained it by simply exercising more.

2) Exercise is excellent for health

Before we dive into why exercise isn’t that helpful for slimming, let’s make one thing clear: No matter how working out impacts your waistline, it does your body and mind good.

A Cochrane Review of the best available research found that while exercise led to only modest weight loss, study participants who exercised more (even without changing their diets) saw a range of health benefits, including reducing their blood pressure and triglycerides in their blood. Exercise reduces the risk of Type 2 diabetes, stroke, and heart attack.

A number of other studies have also shown that people who exercise are at a lower risk of developing cognitive impairment from Alzheimer’s and dementia. They also score higher on cognitive ability tests — among many, many other benefits.

If you’ve lost weight, exercise can also help weight maintenance when it’s used along with watching calorie intake. In an October 2017 study published in the journal Obesity, researchers examined what happened to 14 of the contestants on the Biggest Loser weight loss reality show, six years after they attempted to slim down for TV. They again found there was no relationship between physical activity and weight loss during the active weight loss of the show.

“So the people who lost the most weight on the show weren’t necessarily the people who did the most exercise — instead, it was the people who ate the least,” said study author and National Institutes of Health mathematician and obesity researcher Kevin Hall. But they also found there was a strong relationship between exercise and keeping weight off. (The study participants who managed to maintain their weight loss after six years got 80 minutes of moderate exercise per day or 35 minutes of daily vigorous exercise.)

“Consistent with previous reports, large and persistent increases in may be required for long-term maintenance of lost weight,” the researchers concluded.

So exercise, in summary, is like a wonder drug for many, many health outcomes.

3) Exercise alone is almost useless for weight loss

The benefits of exercise are real. And stories about people who have lost a tremendous amount of weight by hitting the treadmill abound. But the bulk of the evidence tells a less impressive story.
Consider this review of exercise intervention studies, published in 2001: It found that after 20 weeks, weight loss was less than expected, and that “the amount of exercise energy expenditure had no correlation with weight loss in these longer studies.”

To explore the effects of more exercise on weight, researchers have followed everybody from people training for marathons to sedentary young twins to post-menopausal overweight and obese women who ramp up their physical activity through running, cycling, or personal training sessions. Most people in these studies typically only lost a few pounds at best, even under highly controlled scenarios where their diets were kept constant.

Other meta-analyses, which looked at a bunch of exercise studies, have come to similarly lackluster conclusions about exercise for losing weight. This Cochrane Review of all the best available evidence on exercise for weight loss found that physical activity alone led to only modest reductions. Ditto for another review published in 1999.

University of Alabama obesity researcher David Allison sums up the research this way: Adding physical activity has a very modest effect on weight loss — “a lesser effect than you’d mathematically predict,” he said.

We’ve long thought of weight loss in simple “calories in, calories out” terms. In a much-cited 1958 study, researcher Max Wishnofsky outlined a rule that many organizations — from the Mayo Clinic to Livestrong — still use to predict weight loss: A pound of human fat represents about 3,500 calories; therefore, cutting 500 calories per day, through diet or physical activity, results in about a pound of weight loss per week. Similarly, adding 500 calories a day results in a weight gain of about the same.

Today, researchers view this rule as overly simplistic. They now think of human energy balance as “a dynamic and adaptable system,” as one study describes. When you alter one component — cutting the number of calories you eat in a day to lose weight, doing more exercise than usual — this sets off a cascade of changes in the body that affect how many calories you use up and, in turn, your bodyweight.

4) Exercise accounts for a small portion of daily calorie burn

One very underappreciated fact about exercise is that even when you work out, those extra calories burned only account for a tiny part of your total energy expenditure.

“In reality,” said Alexxai Kravitz, a neuroscientist and obesity researcher at the National Institutes of Health, “it’s only around 10 to 30 percent depending on the person (and excluding professional athletes that workout as a job).”

Javier Zarracina/Vox Components of total energy expenditure for an average young adult woman and man.

There are three main components to energy expenditure, Kravitz explained: 1) basal metabolic rate, or the energy used for basic functioning when the body is at rest; 2) the energy used to break down food; and 3) the energy used in physical activity.

We have very little control over our basal metabolic rate, but it’s our biggest energy hog. “It’s generally accepted that for most people, the basal metabolic rate accounts for 60 to 80 percent of total energy expenditure,” Kravitz said. Digesting food accounts for about 10 percent.

That leaves only 10 to 30 percent for physical activity, of which exercise is only a subset. (You can read more about this concept here and here.)

“It’s not nothing, but it’s not nearly equal to food intake — which accounts for 100 percent of the energy intake of the body,” Kravitz said. “This is why it’s not so surprising that exercise leads to significant, but small, changes in weight.”

5) It’s hard to create a significant calorie deficit through exercise

Using the National Institutes of Health Body Weight Planner — which gives a more realistic estimation for weight loss than the old 3,500-calorie rule —the NIH’s Kevin Hall created this model to show why adding a regular exercise program is unlikely to lead to significant weight loss.

Javier Zarracina/Vox National Institutes of Health Body Weight Planner.

If a hypothetical 200-pound man added 60 minutes of medium-intensity running four days per week while keeping his calorie intake the same, and he did this for 30 days, he’d lose five pounds. “If this person decided to increase food intake or relax more to recover from the added exercise, then even less weight would be lost,” Hall added. (More on these “compensatory mechanisms” later.)

So if one is overweight or obese, and presumably trying to lose dozens of pounds, it would take an incredible amount of time, will, and effort to make a real impact through exercise.

That’s why Hall thinks researchers find again and again that exercise can help maintain weight loss, but it doesn’t help people lose weight. “You need a huge volume of exercise to ,” he said. “But to maintain weight loss does not require a deficit of energy.”

6) Exercise can undermine weight loss in other, subtle ways

Exercise can even undermine weight loss in subtle ways. How much we move is connected to how much we eat. As Hall put it, “I don’t think anybody believes calories in and calories out are independent of each other.” And exercise, of course, has a way of making us hungry — so hungry that we might consume more calories than we burned off.

One 2009 study shows that people seemed to increase their food intake after exercise — either because they thought they burned off a lot of calories or because they were hungrier. Another review of studies from 2012 found people generally overestimated how much energy exercise burned and ate more when they worked out.

“You work hard on that machine for an hour, and that work can be erased with five minutes of eating afterward”

“You work hard on that machine for an hour, and that work can be erased with five minutes of eating afterward,” Hall added. A single slice of pizza, for example, could undo the calories burned in an hour’s workout. So could a cafe mocha or an ice cream cone.

There’s also evidence to suggest that some people simply slow down after a workout, using less energy on their non-gym activities. They might decide to lie down for a rest, fidget less because they’re tired, or take the elevator instead of the stairs.

These changes are usually called “compensatory behaviors,” and they simply refer to adjustments we may unconsciously make after working out to offset the calories burned.

7) Exercise may cause physiological changes that help us conserve energy

The most intriguing theories about why exercise isn’t great for weight loss describe changes in how our bodies regulate energy after exercise.

Researchers have discovered a phenomenon called “metabolic compensation.”

“The more you stress your body, we think there are changes physiologically — compensatory mechanisms that change given the level of exercise you’re pushing yourself at,” said Loyola University exercise physiologist Lara Dugas. In other words, our bodies may actively fight our efforts to lose weight.

This effect has been well documented, though it may not be the same for everyone.

For one fascinating study, published in the journal Obesity Research in 1994, researchers subjected seven pairs of young, sedentary identical twins to a 93-day period of intense exercise. For two hours a day, nearly every day, they’d hit a stationary bike.

The twins were also housed as inpatients in a research lab under 24-hour supervision and fed by watchful nutritionists who measured their every calorie to make sure their energy intake remained constant.

Despite going from being mostly sedentary to spending a couple of hours exercising almost every day, the participants only lost about 11 pounds on average, ranging from as little as 2 pounds to just over 17 pounds, almost all due to fat loss. The participants also burned 22 percent fewer calories through exercise than the researchers calculated prior to the study starting.

By way of explanation, the researchers wrote that either subjects’ basal metabolic rates slowed down or subjects were expending less energy outside of their two-hour daily exercise block.

In a more recent study, published in Obesity in May 2016, Kevin Hall’s group again looked at 14 of the Biggest Loser reality show participants. They took a number of measurements — bodyweight, fat, metabolism, hormones — at the end of the 30-week competition in 2009, and again six years later, in 2015.

Though all the contestants lost dozens of pounds through extreme diets and hours of exercise at the end of the show, by the six-year mark their waistlines had largely rebounded. But the most remarkable finding was that the participants’ metabolisms had vastly slowed down through the study period. They were essentially burning about 500 fewer calories (about a meal’s worth) each day than would be expected given their weight.

This metabolic effect persisted, despite the fact that most participants were slowly regaining the weight they lost.

Dugas calls this phenomenon “part of a survival mechanism”: The body could be conserving energy to try to hang on to stored fat for future energy needs. Again, researchers don’t yet know why this happens or how long the effects persist in people.

“We know with confidence that some metabolic adaptions occur under some circumstances,” said David Allison, “and we know with confidence some behavioral compensations occur under some circumstances. We don’t know how much compensation occurs, under which circumstances, and for whom.”

8) Energy expenditure might have an upper limit

Another hypothesis about why it’s hard to lose weight through exercise alone is that energy expenditure plateaus at a certain point. In another Pontzer paper, published in 2016 in the journal Current Biology, he and his colleagues found evidence of an upper limit.

They cast a wide geographic net, recruiting 332 adults from Ghana, South Africa, Seychelles, Jamaica, and the United States. Tracking the study participants for eight days, they gathered data on physical activity and energy burned using accelerometers. They classified people into three types: the sedentary folks, the moderately active (who exercised two or three times per week), and the super active (who exercised about every day). Importantly, these were people who were already doing a certain amount of activity, not people who were randomized to working out at various levels.

Here, physical activity accounted for only 7 to 9 percent of the variation in calories burned among the groups. Moderately active people burned more energy than people who were sedentary (about 200 calories more each day), but above that, the energy used up seemed to hit a wall.

“After adjusting for body size and composition,” the researchers concluded in the study, “total energy expenditure was positively correlated with physical activity, but the relationship was markedly stronger over the lower range of physical activity.”

In other words, after a certain amount of exercise, you don’t keep burning calories at the same rate: Total energy expenditure may eventually plateau.

In the traditional “additive” or “linear” model of total energy expenditure, how many calories one burns is a simple linear function of physical activity.

“That plateau is really different than the standard way of thinking about energy expenditure,” Pontzer said. “What the World Health Organization and the people who build the Fitbit would tell you is that the more active you are, the more calories you burn per day. Period, full stop.”

In the “constrained” model of total energy expenditure, the body adapts to increased physical activity by reducing energy spent on other physiological activities.

Based on the research, Pontzer has proposed a new model that upends the old “calories in, calories out” approach to exercise, where the body burns more calories with more physical activity in a linear relationship (also known as the “additive” model of energy expenditure).

He calls this the “constrained model” of energy expenditure, which shows that the effect of more physical activity on the human body is not linear. In light of our evolutionary history — when food sources were less reliable — he argues that the body sets a limit on how much energy it is willing to expend, regardless of how active we are.

“The overarching idea,” Pontzer explained, “is that the body is trying to defend a particular energy expenditure level no matter how active you get.”

This is still just a hypothesis. Pontzer and others will need to gather more evidence to validate it, and reconcile contradictory evidence showing that people can burn more energy as they add physical activity. So for now it’s a fascinating possibility, among all the others, that may help explain why joining a gym as a sole strategy to lose weight is often an exercise in futility.

9) The government and the food industry are doling out unscientific advice

Since 1980, the obesity prevalence has doubled worldwide, with about 13 percent of the global population now registering as obese, according to the WHO. In the United States, nearly 70 percent of the population is either overweight or obese.

A lack of exercise and too many calories have been depicted as equal causes of the crisis. But as researchers put it in an article in BMJ, “You cannot outrun a bad diet.”

Since at least the 1950s, Americans have been told that we can. This Public Health Reports paper outlines the dozens of government departments and organizations — from the American Heart Association to the US Department of Agriculture — whose campaigns suggested more physical activity (alone or in addition to diet) to reverse weight gain.

Unfortunately, we are losing the obesity battle because we are eating more than ever. But the exercise myth is still regularly deployed by the food and beverage industry — which are increasingly under fire for selling us too many unhealthy products.

“Physical activity is vital to the health and well-being of consumers,” Coca-Cola says. The company has been aligning itself with exercise since the 1920s, and was recently exposed by the New York Times for funding obesity researchers who emphasize a lack of physical activity as the cause of the epidemic.

Physical activity and diet should never be given equal weight in the obesity debate

Coca-Cola is just one of many food companies that are encouraging us to get more exercise (and keep buying their products while we’re at it): PepsiCo, Cargill, and Mondelez have all emphasized physical activity as a cause of obesity.

The exercise myth for weight loss also still appears in high-profile initiatives, like the former first lady Michelle Obama’s Let’s Move campaign — largely because of the food industry’s lobbying efforts, according to Marion Nestle, a New York University nutrition professor. The White House’s exercise focus to end childhood obesity, Nestle said, was “a strategic decision to make the message positive and doable and, at the same time, keep the food industry off its back.”

But this focus on calories out, or the calories we can potentially burn in exercise, is “an inadequate and a potentially dangerous approach, because it is liable to encourage people to ignore or underestimate the greater impact of energy-in,” an obesity doctor and professor wrote in the journal Public Health Nutrition.

In other words, we can lose sight of the fact that it’s mostly too much food that’s making us fat.

“There are all kinds of reasons to exercise that are good for your health,” says Diana Thomas, a Montclair State University obesity researcher. “However, if you’re trying to lose weight, the biggest problem I see is food. We need to cut back the food we’re eating.”

The evidence is now clear: Exercise is excellent for health, but it’s not important for weight loss. The two things should never be given equal weight in the obesity debate.

10) So what actually works for weight loss?

At the individual level, some very good research on what works for weight loss comes from the National Weight Control Registry, a study that has parsed the traits, habits, and behaviors of adults who have lost at least 30 pounds and kept it off for a minimum of one year. They currently have more than 10,000 members enrolled in the study, and these folks respond to annual questionnaires about how they’ve managed to keep their weight down.

The researchers behind the study found that people who have had success losing weight have a few things in common: They weigh themselves at least once a week. They restrict their calorie intake, stay away from high-fat foods, and watch their portion sizes. They also exercise regularly.

But note: These folks use physical activity in addition to calorie counting and other behavioral changes. Every reliable expert I’ve ever spoken to on weight loss says the most important thing a person can do is limit calories in a way they like and can sustain, and focus on eating healthfully.

In general, diet with exercise can work better than calorie cutting alone, but with only marginal additional weight loss benefits. Consider this chart from a randomized trial that was done on a group of overweight folks: The group that restricted calories lost about the same amount of weight as the group that dieted and exercised, though the exercisers didn’t cut as many calories:

The calorie restriction groups lost more weight than the group that both dieted and exercised.

If you embark on a weight loss journey that involves both adding exercise and cutting calories, Montclair’s Thomas warned not to count those calories burned in physical activity toward extra eating.

“Pretend you didn’t exercise at all,” she said. “You will most likely compensate anyway, so think of exercising just for health improvement but not for weight loss.”

Editor: Eliza Barclay
Visuals: Javier Zarracina
Researcher: Mohsin Ali

Muscle Loss

Muscle requires constant work to maintain—we all know this. But if your attention is on fixing your diet or upping your cardio in order to burn fat, that focus is probably coming at the expense of strength training.

Plus, if you’re more focused on your calorie restriction than working out, you’ll almost assuredly lose muscle tissue weight, Seedman says. Why? Because calorie restriction usually means macronutrient restriction. Without enough protein in your diet, your body can’t rebuild the tissue even if you are strength training.

Not only is it a bummer to forfeit those gains, but muscle directly impacts your basal metabolic rate (BMR)—or the rate at which your body burns calories both while working out as well as at rest. Less muscle means a lower BMR which means a lower calorie burn throughout your day.

Muscle tissue also regulates your insulin sensitivity—the biological process that determines how well your body absorbs nutrients. If you lose muscle tissue from dieting improperly, the nutrients you eat are less likely to be partitioned to your muscle cells and more likely to be turned into fat cells, Seedman adds.

Water Loss

One of the fastest ways to lose weight in the short-term is to cut carbs—that’s because carbohydrates retain some three times as much water as any other type of macronutrient, Seedman explains. When you cut back on carbs, your body isn’t retaining as much water, plain and simple.

But losing water weight is like buffing your car—it makes the exterior look sleeker, but the beautification is short lived and no interior improvements have actually been made. “If you lose intermuscular water, at first it’s not a big deal—it’s like letting a little air out of a balloon,” Seedman explains. But after a few weeks, because muscle is 70 percent water, the tissue adapts to the dehydration and your muscles shrink and start to atrophy. You’re not only compromising the structural integrity of your muscles, but as you lose bulk thanks to the lack of water, you’re also triggering the whole metabolic dysfunction of BMR and insulin sensitivity that comes with losing muscle.

The problem comes when you drop below 50 to 75 grams of carbs on a consistent basis, Seedman adds. You still need to eat some carbs—at least .5 grams per pound of your body weight for a low-carb diet or .75 to 1.5 grams per pound for a more balanced calorie-restricted diet will allow fat loss without losing that intramuscular water.

Fat Loss

Your aim is for maximum lipolysisis—the biological process of breaking down fat lipids and triglycerides in either the food you eat or that are already stored in your body. This mostly happens in the mitochondria of the muscles, which is why the more muscle you have, the more fat you burn, Seedman points out. Exercise has also been shown to upregulate those lipolytic enzymes and improve mitochondria function, which is why working out helps you shed fat.

While that sounds pretty straightforward, it’s actually incredibly hard to predict your potential burn rate. “It’s not just calories in, calories out,” Seedman clarifies. While traditional thinking was that between your BMR and calorie intake, you could calculate how much fat you’ll lose per week. But there are an infinite number of possibilities that can occur among the different enzymes, hormonal response, biochemical reactions, and endocrine function—just to name a few—which can all affect this rate, he adds.

How Do You Know?

If you see more than two pounds disappear in a week, you’re dealing with more than just fat loss. “When guys start upping their workouts and cutting calories, they’ll see fat loss pretty quickly—but never at a rapid rate,” Seedman explains. This two-pounds-a-week is most everybody’s threshold for fat burn. If you drop 10 pounds in a week, the vast majority of that will be water weight and a little bit of muscle loss as well.

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Have you ever wondered “Where does fat go when you lose weight?” You could be mistaken for believing it leaves the body as poop or urine — at least, that’s what many people think is what happens to fat when you lose weight. But the reality is much different. When it comes to the question of where does the fat go when you are losing weight, the answer is that we simply breathe it out. Confused? We’ll explain.

What happens to fat as you lose weight?

A December 2014 study published in BMJ sought to answer the question “Where does fat go once you lose weight?” In the experiment, researchers tracked the atoms in 10 kilograms (about 22 pounds) of fat to see what happened to them when participants lost weight. They found that 8.4 kilograms (about 18.5 pounds) were excreted as carbon dioxide, while the remaining 1.6 kilograms (about 3.5 pounds) left the body as a fluid of some sort: sweat, pee, poop, or other bodily fluids.

So, you lose weight mainly by breathing it out after it’s been converted to carbon dioxide. Does that mean you can just breathe quickly to shed some extra pounds? Not so fast. Speeding up your inhalations and exhalations will only succeed in causing you to hyperventilate, which may lead to dizziness and anxiety. There’s only one way to up your carbon dioxide production: exercise.

Fortunately, exercise is a relatively broad term that can include motions as simple as getting up and walking. The more intense your movements, the faster your breaths will become, and the more carbon dioxide you’ll release. Of course, the key to losing weight is that the amount your body burns must be more than the amount you’re feeding it. You can exercise all you want, but if you continue to consume more calories than you burn, you won’t see your middle shrink at all. Dieting in conjunction with exercise is key.

What happens to fat cells after you lose weight?

It’s important to note that the carbon dioxide you’re breathing out when you’re running isn’t made up of the fat cells themselves. It may be a bit of a bummer to learn that the proper response to the question, “What happens to fat cells when you lose weight?” is not that you lose them but rather that they shrink in size.

From the moment you were born, the number of fat cells in your body increases until you hit your 20s, at which point they plateau. When fat cells die, they’re quickly replaced by more, so the number of fat cells in your body stays relatively even throughout the rest of your life. As for how many fat cells a person has, that figure varies from person to person. You can’t look at someone who is obese and a person who is skinny and say, “The overweight person has more fat cells in his or her body.” It’s what’s inside those fat cells that determines what you see on the outside.

The sad news here is that a person can never “lose” fat cells — they can only increase their fat cell count, according to Kirsty L. Spalding, PhD, a researcher at Karolinska Institutet in Sweden. This fact could explain the body’s urge to gain more weight after you lose it. (No, you’re not wrong in thinking that your body is essentially working to undo all the weight you just lost.) In order to maintain your weight loss, you need to continue to exercise and watch what you eat so that your fat cells stay small.

What happens to fat after liposuction?

We know we just told you that you can’t “lose” fat cells, but what we really meant is that you can’t lose them naturally. If you opt for a procedure like liposuction, then the answer to “What happens to fat cells after liposuction?” is that they’re well and truly gone. Your doctor will suck them out and dispose of the fat (or reinject it in another area of the body that needs plumping).

That said, deciding to have liposuction doesn’t guarantee that you won’t regain the weight you had sucked out. You now must deal with all the other fat cells you didn’t have removed. Those little guys are just as willing to be plumped up as the ones you just got rid of. Even more concerning is that, because you’ve removed fat cells in a troublesome area of your body, they’ll reappear somewhere you may not expect if you tack on a few pounds.

In a July 2011 study published in the journal Obesity, researchers looked at whether women who had liposuction gained back the weight, and if they did, where the weight appeared. It only took a year for the weight that had been suctioned off to reappear, only this time it did not return to the thighs or abdomen where they had the liposuction. Instead, “it was redistributed upstairs,” in the arms, shoulders, and upper abdomen, researcher Robert H. Eckel said. (Researchers aren’t sure if the weight gain was the result of new fat cells forming or old fat cells getting larger.)

Liposuction becomes a cruel game of tag. Once you have the procedure done and “tag” your goal weight, the flab reappears somewhere else. Then, you’re forced to decide whether it’s worth it to have the surgery again for your new problem area or live with your carry-ons.

What happens to fat after CoolSculpting?

OK, so maybe you’re not so keen on the idea of liposuction — but what about CoolSculpting? What happens to fat after CoolSculpting? This relatively new procedure (it was only approved by the Food and Drug Administration in 2010) involves freezing your fat cells gradually using a cooling plate that’s a chilly minus seven degrees Celsius. The low temperatures kill your fat cells, but the machine prevents the rest of your body from getting frostbite. Then, a doctor will usually massage the area and you’re on your way.

CoolSculpting is more convenient than liposuction in a lot of ways: The procedure is less invasive, and there’s a shorter downtime (if any) between treatments. But unlike liposuction, CoolSculpting is marketed as a solution to problem spots, not an all-over treatment.

As for what happens to fat after CoolSculpting, doctors will tell you that you simply pee or poop it out. (Perhaps the answer to the question “When you lose weight, do you poop it out?” is kinda, sorta, maybe.) When cells die, your body will metabolize them, take any nutrients it needs, and excrete them. But as was the case for liposuction, if you don’t maintain your weight loss after CoolSculpting, the chub will return to other parts of the body.

Where does fat go first when you lose weight?

A common concern for women who want to slim down is when you lose weight, where does the fat go first? Ladies who are happy with their breast size worry that their chest is the first area of their body where they’ll lose weight. But the good news (or bad news, depending on how you look at it) is that you can’t target specific sections for weight loss. Sure, you can go to the gym and do a million crunches if you want to tighten your midsection, but there’s no guarantee that you’re going to lose fat there first.

If you know that your chest is usually the first place where you gain weight, then it’s a good guess that it’ll be the first place where you’ll lose weight. This goes for legs, arms, tummy, you name it.

One thing that will make losing weight around the butt, hips, and thigh area especially difficult for women is our own biology. Our bodies want to ensure we’re ready to get pregnant (even if we aren’t!), so that means maintaining a cushion of fat in those aforementioned areas, which are important for childbearing. That’s why it’ll take some time to see these pockets of fat melt off even if you target these areas with specific exercises.

Where does fat go first when you gain weight?

Similar to the question of when you lose weight, the answer to where does fat go when you gain weight is that it’s different for everyone. Female hormones cause fat to settle in the thigh and bum areas. The only way to change that is to alter your hormones. Your problem spots — aka the places where you hold onto excess weight — may be a silent indicators of an underlying health condition.

A January 2008 study published in the Canadian Medical Association Journal conducted by researchers at Harvard and the University of Toronto found a concerning link between a woman’s chest size and her risk for type 2 diabetes. The bigger a female participant’s breasts, the higher her chance of developing the disease. In a 2012 study presented at the American Heart Association’s yearly meeting, researchers at Johns Hopkins noted that people with extra tummy fat were more likely to have restless leg syndrome, a condition that negatively affected their sleep. And the University of Chicago put out a paper in 2010 that noted that women with weight on their hips (known as “pear” body shapes) had poorer brain function than their slimmer peers.

But it’s not all bad. In a June 2010 review in the International Journal of Obesity by researchers at the University of Oxford and Churchill Hospital, the results suggested that women with a bigger posterior were protected from diabetes and heart disease by the extra layer of fat in their tush. A September 2009 study published in BMJ found that women with smaller thighs had an increased risk of heart disease and premature death.

Now that you know what happens to body fat when you lose weight, you can marvel at the miracle that is the human body. Losing weight is hard — especially when your body is basically programmed to work against you — but it’s truly astounding to learn just how efficiently you get rid of fat cells when you burn them. The next time you’re struggling to stay positive about losing weight or are feeling like reducing your belly fat is impossible, take a deep breath and let the weight float away — literally.

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