Why Weight Loss Is Tied To Biology When we shed pounds, we trigger biological mechanisms that make it hard to keep the weight off. Understanding these systems might help you be kinder to your body.

Biology's A B*tch: 5 Reasons It's So Hard To Keep Weight Off

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Back in the day, scientist Kevin Hall had a vice.

KEVIN HALL: Yeah. Unfortunately, way back when, I was kind of addicted to reality television. And a friend of mine said, you've got to watch this show; this is, like, right up your alley.


UNIDENTIFIED ANNOUNCER: No matter who they train with, their lives will be changed forever.

HALL: And so I turned it on and saw this kind of horror show of a reality program, where people are getting sort of yelled at on treadmills.




UNIDENTIFIED ANNOUNCER: But only one will become the next biggest loser.

GODOY: Yeah, "The Biggest Loser" - that long-running, heavily criticized TV show. Basically, it showed contestants with obesity competing to see who could lose the most weight, and we're talking, like, 100 pounds or more. They went on really restrictive diets and took up hardcore exercise routines. Now, a lot of people think this show was the epitome of everything that's wrong with diet culture. It showed people pushing themselves to unhealthy limits to lose weight, and then it celebrated those losses as if they were the most important thing they'd ever achieve. And Kevin was horrified, too. But he studies metabolism, so he also had questions about what was going on with their physiology.

HALL: The thing that kind of caught my attention was the fact that these people were losing so much weight so quickly, and that didn't mesh with what I knew about weight loss and how difficult it is. And it kind of struck the question of, you know, what's going on inside the bodies of these people?

GODOY: So Kevin decided to do a formal study looking at contestants from one season of "The Biggest Loser." Many of them had gained most or all of the weight back. And even six years later, everyone's metabolism had slowed down a lot, like, even more than research would have predicted.

OK, so Kevin, I have to tell you that I think no piece of research has personally depressed me more than this from you.


GODOY: After reading it, I basically gave up hope that I could ever lose weight because, what's the point? Is this a common reaction? Did you hear this from a lot of people?

HALL: It is, and it's actually a misinterpretation of the research because even though the folks actually had this slowing of metabolism, what people fail to kind of appreciate - the people who were most successful with weight loss were the ones who had the greatest slowing of metabolism.

GODOY: Now, that may sound counterintuitive, but it's all a part of the complex biology that comes into play when we lose weight. There are factors that are and are not in our control. In this episode of NPR LIFE KIT, we're going to explain the biology of weight loss. And hopefully, it'll make you think a little more kindly about your own body. And trust me, I am not going to tell you to take a "Biggest Loser"-style approach. More in a minute.


GODOY: I'm Maria Godoy. I'm a senior editor with NPR's health and science team. In this episode, we are not telling you that you need to lose weight; we're just going to give you five takeaways about the realities of how your biology works. So let's just get right to it. Think of this as the overarching message of this episode - our biology makes it hard to lose weight. One reason why - and this is your first takeaway - it has to do with how your metabolism changes when you lose weight. So let's start with a basic explanation of how metabolism works, which is why we're turning to Kevin Hall.

HALL: I am a senior scientist at the National Institute of Diabetes, Digestive and Kidney Diseases. I study metabolism, nutrition and obesity.

GODOY: So Kevin, what exactly is metabolism?

HALL: So metabolism is the process by which we take in nutrients from our food, and we convert those nutrients to energy, which drive all of the biological processes inside our bodies.

GODOY: Now, metabolism varies from person to person; scientists aren't totally sure why that is. Part of it has to do with how much lean muscle mass you have versus fat on your body. But a lot of it is very individual. But we do know that as you get older, your metabolism slows, typically in your 50s or 60s. It also varies by gender.

HALL: Generally speaking, men burn more calories than women, and that's...

GODOY: Patriarchy, OK.

HALL: Yeah.


HALL: And yeah, as unfair as that sounds, it's primarily related to our body composition, and in other words, men have more lean tissue than women for the same body weight.

GODOY: But for everyone across the board, metabolic energy burn occurs in three main ways. First, there's your resting metabolism - what your body burns in order to keep your organs pumping and, you know, stay alive. Resting metabolism is basically what you would burn if you laid in bed all day, and you would burn something.

HALL: Yeah, and so that contributes more than 50% of the total calories that people burn.

GODOY: And then there's the kind of metabolism it takes to digest and absorb the calories in our food.

HALL: That's something called the thermic effect of food. And so that tends to be about 10% of the total amount of calories that you eat. So for example, if you ate 3,000 calories over the course of a day, it would cost about 300 or so calories in order to digest, absorb and store those calories in the various storage pools in the body.

GODOY: And the rest, roughly 30% of the calories you burn, it's through physical activity. But for most folks, purposeful exercise - you know, like working out - that accounts for a very small part of this type of metabolic burn. We're really just talking moving.

HALL: You know, walking around, sitting upright, moving from place to place - you know, that can contribute probably something like 30 - 25% to 30% of your total calories, and of course, you can increase that quite a lot if you start engaging in a lot of exercise or purposeful changes to your physical activity patterns.

GODOY: So to recap, you have three basic types of metabolism - one for resting, one for digesting and one that doesn't rhyme, you know, movement. So say someone has lost weight - basically, their metabolism is like, wait, there's less of me to move around, so I don't have to work as hard, woo hoo (ph).

HALL: The trick here is that calorie burn is, in some sense, proportional to your body weight. So the heavier you are, the more calories it costs to move around and the more calories it costs to kind of basically maintain that body at rest.

GODOY: So that's one reason why your metabolism slows down as you lose weight. It's simple physics. The other thing your metabolism is trying to do...

HALL: Your body does persistently fight back and try to make you regain the weight that you've lost.

GODOY: So in that sense, it doesn't matter how much willpower you have. Your biology has other plans. Your body will try to pull you back up to that higher weight because that's what it's used to. Kevin thinks about it in this way.

HALL: Imagine that your weight loss is kind of like a spring. You've got a ball on the end of a spring. And you're trying to lose weight, which means you're trying to do something to your diet and your physical activity to basically pull the ball against the spring. The more that you pull on that ball, the more it pulls back on you.

GODOY: And it's really easy for that spring to snap back in your face. That's why a crash diet or any other type of restrictive eating, it can work at first - you're keeping up that tension in that spring by eating restrictively, exercising a ton - but once you stop doing the changes that helped you lose weight, for whatever reason, because you got sick of the diet or got injured or life got in the way, well, that spring snaps back to its original form, and that's your metabolism taking back control.

HALL: It does pull you backwards. It means that whatever changes you make to your diet or physical activity behaviors has to be kept up permanently in order to avoid your weight regain.

GODOY: And this is our second takeaway - if you choose to try to lose weight, make changes that you can live with for the long haul because you're going to need to stick to them if you want to keep the weight off. In other words, make changes you actually like and aren't just gritting your teeth through. So if you need guidance on what kind of diet you should follow, we do have a whole three-part guide on making healthy eating choices. So check it out. And by the way, when I say diet, I actually mean just what you eat, which is the definition of the word diet. I am not talking restrictive eating.

Now, we've been talking a lot about metabolism, but let's move on to another part of your biology.

HALL: For all of the attention that metabolism gets, I think that the appetite side of things is actually much more important for explaining weight regain and sustainable weight loss.

GODOY: There are lots of different hormones at play when it comes to hunger, but let's focus on one called leptin.

HALL: Leptin is one of those hormones that is sensing an energy deficit, sensing a calorie deficit, that you don't have enough calories on your body or you're not consuming enough calories to be a sustainable, healthy person.

GODOY: Basically, leptin is the messenger to the brain telling it when to eat and when to stop eating. And when you weigh more, you tend to have more leptin in your bloodstream. If you lose weight, your leptin levels drop, too.

HALL: When those levels of leptin drop, it's like a starvation signal, in some sense. When people lose weight, their appetite does go up. They don't just want to return to eating what they were eating before. They seem to want to eat even more than they were eating before to kind of rapidly recover that weight loss.

GODOY: And it's not just that you get hungrier when you lose weight. Food actually becomes tastier - damn hormones.

HALL: So it seems like we become hyperaware of food cues and our taste sensitivity becomes more acutely responsive - you know, food tastes sweeter, food tastes better to you than it did before; this idea of, you know, hunger being the best source, right? The hungrier you are, the more tasty the food is.

GODOY: So that's your third takeaway. And I am sorry to be the bearer of this news, but the hormones in your brain are conspiring to make you hungrier when you lose weight - which is just diabolical, but also, give yourself a break. You are not a glutton. This is not a moral failing - it's just hormones. Hearing all of this, it might feel a little depressing that your body is actively trying to work against your effort. But for me, knowing the science is actually kind of empowering because as someone whose weight has gone up and down a lot over the years, I now understand what is out of my control. So I don't feel like I'm failing.

And science also tells us what factors are in our control. And that brings us to our fourth takeaway - when it comes to weight loss, what you eat is more important than how much you exercise. Why? Because to lose weight, you have to take in fewer calories than you burn. And unless you are working out a lot, you are probably not burning enough to make a real difference. A half hour on the treadmill might burn, say, 350 calories, but then a lot of people tend to think, I just worked out. I deserve a whole pizza.

HALL: Exactly.

GODOY: That pizza almost certainly contains way more calories than you just burned.

HALL: Your body's compensating by maybe eating more calories, or you're compensating for the rest of the day - you know, you go to the gym, you work out, and then you go home and you crash on the couch earlier than you normally would have or something like that.

GODOY: Now, if you're a pro athlete, it's a different story. But for most folks, exercise is a minor player in weight loss. The fact is, it's a lot easier to cut out 600 calories by skipping that Starbucks muffin than it is to burn it by running for an hour or more. And there actually is some evidence that doing too much exercise, like exercising all day long, it might actually slow down your resting metabolism, which, if you recall, is the main source of how we burn calories. And this evidence is really interesting. It comes from a study of a tribe in Tanzania called the Hadza.

HALL: This is a hunter-gatherer tribe in Africa who are just well-known to, as a general result of their lifestyle, be very physically active.

GODOY: They tend to walk about four to seven miles a day between collecting water, hunting down food and other daily activities. There's a researcher at Duke University named Herman Pontzer, and he wanted to see how they're highly active lifestyles impacted their metabolism. So he tracked their movements with high-tech devices.

HALL: It's almost like Fitbits but a research-grade Fitbits to measure how much they're moving around. And indeed, they move around a lot more than the average sort of Westerner who lives in their sort of sedentary environment and office - for example, office-type work. And so the question was, well, obviously, if these folks are moving around so much more, they must be burning many, many more calories than a sedentary person.

GODOY: You would think so. But even though the Hadza were moving way more in their day-to-day than, say, an average American office worker...

HALL: The surprise was that once you accounted for their body composition, they were burning no more calories in total than the sedentary person kind of going about their day-to-day activity as a - in a kind of industrialized Western society.

GODOY: But how can that be? Weren't they walking a whole lot more?

HALL: They were walking a whole lot more, yes, and the only way that can be is if some other aspect of their total calories had shut down, had slowed down in order to compensate for this greatly increased physical activity. And so that led to this notion that energy expenditure is, in some sense, constrained, and so that if you do become very, very physically active, perhaps - and, again, this is just a theory at this point - perhaps your resting metabolic rate might slow down to compensate.

GODOY: Basically, your body is doing some budgeting because you still need to hold onto calories to, you know, stay alive, because your lungs need to keep breathing, your heart needs to keep pumping. So when you boost your physical activity a lot, it may be that your body holds back more calories to maintain these basic biological processes. In other words, your resting metabolism slows down. So don't think you can exercise your way out of a bad diet.

HALL: Don't go running just so you can eat, you know, a piece of chocolate pie or something like that.

GODOY: (Laughter).

HALL: Go running because you like running and because it has independent health benefits, regardless of weight.

GODOY: And it's not just running - exercise in general has benefits like better mood, reduced anxiety and better blood sugar levels, even better sleep, which is a big one. So if you want advice on how to start an exercise habit you'll actually stick with, we've got a whole guide for that.


GODOY: Now, let's talk maintenance. Even people who do manage to lose weight, many of them struggle to keep it off. Trust me, I've been there in the past. But that's not the case for everyone. Some of the best data on how people maintain weight loss over the long haul comes from something called the National Weight Control Registry. It's a database that's been around since the 1990s, and it tracks people across the U.S. who've lost at least 30 pounds and kept it off for at least a year.

HOLLY WYATT: And these are people who've been successful. They're the people that we used to think were very rare but are definitely out there.

GODOY: That's Dr. Holly Wyatt. She's an associate professor of medicine at the University of Colorado Anschutz Medical Campus. She studies these people to figure out why they've been successful. And the No. 1 thing they have in common? They exercise every day for about an hour. And that's your final takeaway - exercise seems to play a really big role in maintaining weight. All right, I know, I just told you that exercise has little effect on weight loss, but it seems to play a very different role when it comes to weight maintenance.

WYATT: When it gets to weight loss maintenance, it's going to switch. Now all you want to do is balance how many calories you're eating with how many calories you're burning. That's very different than weight loss. And we know, from a physiology standpoint, the best thing for that isn't nutrition; it's exercise. Exercise is what helped keep it off. It now flip-flops, and it becomes the most important thing.

GODOY: The reason why has to do with changes in your metabolism and hormones we told you about earlier. When you lose weight, you burn fewer calories. But you're probably feeling hungrier, so you want to eat more calories. And Holly says you're basically facing an energy gap between what you burn and how much you actually want to eat.

WYATT: And the greater the weight loss, the bigger that energy gap.

GODOY: A lot of people think once you've lost weight, you've got to eat a lot less forever to keep it off; even people in the National Weight Control Registry seem to think so. They report eating about 1,400 calories a day on average, which is not that much.

WYATT: But you cannot do that forever. Adherence to any diet at 13- or 1,400 calories, I can almost guarantee, is going to start to fail, and you're going to eat more.

GODOY: And it turns out, the people who Holly studies who do manage to keep weight off long term, they are eating more than that. They probably just don't realize it because research shows that people are notoriously bad at trying to count calories.

Holly and her colleagues at the University of Colorado, Victoria Catenacci and Danielle Ostendorf, they recently published studies that took highly detailed scientific measurements of the metabolisms of weight-maintainers - these people had lost about 58 pounds on average and kept it off for about a decade. And what they found was these folks were burning around 2,500 calories a day, but since their weight was stable, that meant they were also eating 2,500 calories a day. In other words, they didn't have to eat a highly restricted diet to keep the weight off, they were just exercising more. They weren't necessarily hitting the gym really hard, though.

WYATT: They're doing all kinds of activity. They do a lot of walking, but they do resistance training. They do a variety of activity. So it's not specifically one activity that's associated with success. I think, more than anything, it's the volume of activity.

GODOY: It's about integrating exercise into your daily life. Now, there is some evidence that some people who lose a lot of weight, their metabolisms slow down even more than you'd predict, just based on the changes in their weight and muscle and fat tissue. But here's the thing - even though their metabolisms slow down more than expected, that doesn't seem to predict who's going to keep the weight off. In fact, that's what Kevin found when he looked at "The Biggest Loser" contestants.

HALL: It was differences in physical activity that were the best predictor of who was able to successfully maintain weight loss over time.

GODOY: In other words, the people who did the best at keeping weight off were also the ones who exercised the most. But we're not talking torturous exercise like what was depicted on "The Biggest Loser." Kevin says you should choose something you actually like doing because you're going to have to keep up the exercise if you want to keep off the weight.

HALL: You better choose some sort of physical activity that you enjoy doing, that doesn't feel like a chore, that you can enjoy incorporating into your life persistently because your body does persistently fight back and try to make you regain the weight that you've lost. The point is, is that you can win that battle and - as long as you can enjoy and sustain those behavior changes.

GODOY: So you might hear all of this and think, maybe the stress of weight loss isn't worth it, given all these biological hurdles? And we really don't know everything about why some people manage to keep weight off, when so many other people don't. Or you might decide to try to lose weight anyway, knowing the limitations you're likely to face and the effort it's going to take to maintain that loss. And if you do want to try to lose weight, maybe rethink what your end goal is. Instead of trying to look like some unattainable magazine cover, perhaps just focus on changes that make you feel healthier.

HALL: Those types of changes that you make then become rewards in their own, not specifically tied to the weight loss, per se.

GODOY: So some weight loss is possible, but set realistic expectations, and learn to rejoice in the changes that you're making to make yourself healthier.

HALL: Exactly, exactly. Weight is just one indicator of health, and it's a pretty imperfect indicator, at that.


GODOY: All right, let's recap. Takeaway No. 1 - biology makes it hard to lose weight and keep it off.

HALL: It decreases metabolism and increases appetite.

GODOY: Takeaway two - if you do try to lose weight, find healthy habits that you enjoy. You want to make changes that you can stick to over the long haul. Next up, takeaway No. 3 - if you feel hungrier than ever when you're losing weight, it's your hormones, not a lack of willpower. No. 4...

WYATT: Nutrition is doing the heavy lifting when you lose weight.

GODOY: But - and here's our final takeaway...

WYATT: Exercise is really what's important when you want to keep it off.

GODOY: And that doesn't have to mean hardcore exercise.

WYATT: No, you can do moderate intensity of activity - works just as good.


GODOY: For more NPR LIFE KIT, check out the next episode in this guide. It's about developing a saner mindset for weight loss. If you like what you hear, make sure to check out our other Life Kit guides at npr.org/lifekit. And while you're there, subscribe to our newsletter so you don't miss anything. We've got more guides coming every month on all sorts of topics. And here, as always, is a completely random tip, this time from NPR's Camille Smiley.

CAMILLE SMILEY, BYLINE: If you ever accidentally use a permanent marker on a dry-erase board, all you have to do is take a regular dry-erase marker, write directly over the accidental permanent marker writing, and then you can just use an eraser, and it comes right off.

GODOY: If you've got a good tip or want to suggest a topic, email us at lifekit@npr.org. I'm Maria Godoy. Thanks for listening.


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