You should never diet again: The science and genetics of weight loss

To maintain a new weight, you have to fight evolution. You have to fight biology. And you have to fight your brain

Published April 12, 2015 12:00AM (EDT)

     (<a href=''>Pachanon</a> via <a href=''>Shutterstock</a>/Salon)
(Pachanon via Shutterstock/Salon)

Excerpted from "Secrets From the Eating Lab: The Science of Weight Loss, the Myth of Willpower, and Why You Should Never Diet Again"

I’ve given you the bad news: diets fail in the long run. Now, let’s try to understand why.

In social psychology we often say that if you find that most people behave in the same way, then the explanation for their behavior has very little to do with the kind of people they are. It has to do with the circumstances in which they find themselves. For example, most students in class raise their hands and wait quietly to be called on before speaking. It’s not that they are all timid or overly polite types of people. It’s that the classroom setting is sufficiently powerful that without really thinking about it, nearly everyone ends up following the same unwritten rules. When we think about people who regain weight after dieting, it’s a similar principle. It’s not that they have a weak will or lack discipline, or that they didn’t want it enough, or didn’t care. It’s about the circumstances in which they find themselves, and the automatic behavior that is provoked by those settings. In other words: if you have trouble keeping weight off, it is not a character flaw.

When it comes to keeping weight off, a combination of circumstances conspires against you. Each one on its own makes it difficult, but put them together and you are no longer in a fair fight. One circumstance that makes things hard is our environment of near-constant temptation. Two others are biology and psychology. I realize it may seem odd to you that I am calling these things “circumstances,” but, like a classroom setting and the behavior it produces, we need to acknowledge the context in which you regain weight.

To an important extent, weight regain after a diet is your body’s evolved response to starvation. When you are dieting, it may feel as though you are about to starve to death, but you know that you can open the fridge at any time and find more to eat, if you really wanted to. Your body doesn’t know this, however, and you have no way to tell it that you just want slimmer hips or a flatter stomach. All your body knows is that not enough calories are coming in, so it kicks into survival mode. From an evolutionary perspective, the bodies that were best able to survive in times of scarcity (and then pass their genes on to future generations) were those that could use energy efficiently in order to get by on tiny amounts of food. Another quality that would have helped you survive was psychological: a single-minded pursuit of more fuel—and once you located it, the overwhelming urge to eat lots of every type of food you found.

Together, these biological and psychological forces make regaining lost weight all too easy. Let’s take a closer look at the biological ones first, because they set the stage for everything else.


Your genes play an important role in determining how much you weigh throughout your life. In fact, your genetic code contains the blueprint for your body type and, more or less, the weight range that you can healthily maintain. Your body tends to stay in that range—which I will refer to as your set weight range—most of your adult life. If your weight strays outside it, multiple systems of your body make changes that push you back toward it. While this may seem controversial—aren’t we all in control of our own weight?—the role of your genes in regulating weight is backed up by solid evidence. And we don’t even need to rely on high-tech gene mapping to understand this; we just need to study people who share the same genetics.

One classic study compared the weight of more than 500 adopted children with that of their biological parents and that of their adoptive parents. Obviously, if genes matter more to weight than does environment, the children’s weight should be similar to the weight of their biological parents. If learned eating habits have more of an impact on weight, their weight should be more like their adoptive parents. In fact, researchers found that the children’s weight correlated strongly with the weight of their biological parents and not at all with the weight of their adoptive parents.

That evidence always blows me away, but if that’s not persuasive enough for you, there’s also evidence from studies of twins. Twin studies are commonly used to see how much genes matter in all sorts of human features, from personality traits to psychological disorders to physical diseases. The problem for eating studies is, while identical twins share all of the same genes, they also typically share the same eating environment. So if features are common in both twins, it is possible that they are the result of a shared environment.

To tease apart the effects of genes from the effects of the shared environment, researchers located identical twins that were raised in separate homes without knowing each other. It may seem surprising that there are enough sets of twins that meet this criteria, but there are. This type of twin research was partly pioneered in the very psychology department in which I work, at the University of Minnesota (coincidentally located in the twin cities of Minneapolis and St. Paul). If you go up to the fifth floor, the walls are covered with photographs of identical twins that were separated at the age of five months (on average) and had been apart for about thirty years before being reunited as adults. The visible similarities are remarkable, as are the many documented behavioral similarities.

The crucial twin study of body weight (which comes from the Swedish Adoption/Twin Study of Aging) included 93 pairs of identical twins raised apart (and 154 pairs of identical twins raised together). Sure enough, the weights of identical twins, whether they were raised together or apart, were highly correlated. That study, along with several others, led scientists to conclude that genes account for 70 percent of the variation in people’s weight. Seventy percent! What is truly remarkable is that this is only slightly lower than the role genes play in height (about 80 percent of the variation). Don’t get me wrong. I’m not saying you can’t influence your weight at all, just that the amount of influence you have is fairly limited, and you’ll generally end up within your genetically determined set weight range.

Okay, so maybe you can’t easily influence your weight to achieve long-lasting losses, you might say, but it seems all too easy to influence it in the direction of weight gain, right? Actually, it’s not as easy as you might think. Researchers have studied that side of the equation, too—instead of having people lose weight and then try to maintain the thin weight, they had people gain weight and then try to maintain the fat weight. Staying fat shouldn’t be that difficult, should it? In one set of studies, researchers tried to make people fat by overfeeding them. They didn’t want exercise to get in the way of weight gain, so they did these studies with people they could prevent from doing any exercise at all: prisoners. I’m not wild about using prisoners in research because it is often hard for them to refuse to participate, but the researchers explained their plans fully and got permission from each prisoner.

Several fascinating things happened next. First of all, it was remarkably difficult to make the prisoners fat. The prisoners had to eat enormous quantities of food—some of them over 10,000 calories per day, for four to six months—to gain 20 percent of their starting weight. That’s a lot of extra calories, considering men in the United States tend to average about 2,500 daily calories. Some of the prisoners could not gain that much weight, despite eating huge amounts of food, and the prisoners gained much less weight than the researchers predicted based on the amount of calories they consumed. And most surprising, once the prisoners had gained weight, it was very difficult for them to keep it on. They had to continue eating a large number of calories per day (at least 2,700) just to maintain it; otherwise they would lose the weight. When researchers tried the same study with dedicated student volunteers who were free to walk around and exercise some, they were actually unable to turn them obese. In another study, researchers fed twins an additional 1,000 calories per day over what they would need to eat to maintain their weight. They did this for 100 days. Like the prisoners, these twins were unable to maintain the higher weights.

In addition to showing why it is so difficult to maintain a weight higher or lower than is dictated by our genes, these kinds of studies also offer evidence that our genes control how much weight we gain. Even when study participants were fed the same amount of calories, they gained varying amounts of weight. The pairs of twins that were overfed 1,000 calories per day gained anywhere from 9 to 29 pounds. In other words: the same number of calories led some people to gain three times as much weight as other people. Moreover, each twin gained nearly the same amount as their own twin, even though each pair of twins gained different amounts of weight than the other pairs of twins. All of these studies are evidence that your body is trying to keep you within that genetically determined set weight range. When our weight is within this range, we don’t have to fight to maintain it. It’s easy. We can eat a little more or a little less, exercise a little more or a little less—and it won’t have much of a lasting impact. The hard part is trying to get out of that range, because to do so, you have to battle biology. Your body uses many biological tricks to defend your set range, particularly if you get below it, because this is when your body thinks you are starving to death. To save you, it makes you eat more food, and stores some of the energy you consume in case of emergency.

When you are dieting and hungry, your brain responds differently to tasty-looking food than it does when you are not dieting. The areas of the brain that become unusually active make you more likely to notice food, prompt you to pay more attention to it when you find it, and make it look even more delicious and tempting than usual. These are potent signals to eat. At the same time, activity is reduced in the prefrontal cortex, the “executive function” part of the brain that helps you make decisions and resist impulses. Either one of those responses would make you more likely to indulge, but when you put them together, you don’t stand a chance. Your ability to resist is taking a snooze exactly when you most want its support. To make matters worse, this response has been found to be particularly strong in obese people— and it also gets stronger the longer you diet.

Another way your body defends your set range is through hormonal changes. As you diet and lose weight, you lose body fat. Many of us think of body fat as blubbery stuff that just sits there under our skin and makes us look fat, keeps us warm, and helps us float in the ocean, but body fat (also called adipose tissue) is an active part of the endocrine system. It produces hormones that are involved in the sensations of hunger and fullness, and as you lose body fat, the amount of these hormones circulating in your body changes. The levels of hormones that help you feel full (including leptin, peptide YY, and cholecystokinin) decrease. The levels of hormones that make you feel hungry (including ghrelin, gastric inhibitory polypeptide, and pancreatic polypeptide) increase. Just like with the changes in brain patterns, these hormone changes give you an urge to eat, and to eat a lot. One study found that these changes in hormone levels were still detectable in people a year after they stopped dieting.

While these changes in brain reactivity and hormone production are pushing you to eat more, your metabolism also betrays you. It changes partly because you are thinner, and partly due to the effects of (what it perceives to be) starvation. Whether or not you are dieting, your metabolism is affected by your weight. It takes energy to run all of the metabolic processes in your body every day; the more you weigh, the more energy (calories) your body burns just to keep you alive. When you lose weight, even if starvation has no effect on your metabolism, your body will still burn fewer calories, simply because it is now a smaller body to run. This means that the number of calories you ate to lose weight eventually becomes too many calories to eat if you want to keep losing weight.

On top of that, starvation also has an effect on your metabolism. Because there is not enough food coming in, your metabolism slows down to conserve energy. Unfortunately, this doesn’t make you feel full longer or help you lose weight. Quite the opposite. It uses each calorie in the most efficient manner possible, which allows your body to run on even fewer calories than it would need just based on the size of the body. More calories are left unused and can be stored as fat.

The consequences of these changes are problematic, to say the least. When you aren’t taking in enough calories, your body makes storing those calories as fat the top priority, regardless of the dietary fat content of whatever you ate. That’s right, in certain cases, even non-fat foods can get stored as fat. And more alarmingly, this means that a person who loses weight to reach 150 pounds, for example, is not the same‚ physiologically‚ as a person who normally weighs 150 pounds. To maintain 150 pounds after dieting down to that weight, dieters must eat fewer calories per day than people who were 150 pounds all along (not to mention fewer calories per day than they ate to get to that weight) or else they will gain weight.

You know what I find the most infuriating about this situation? People will blame the weight regain on your self-control, even though you are probably eating less food than they are! To maintain your new weight, you have to fight evolution. You have to fight biology. You have to fight your brain. You have to fight your metabolism. These are the ways your body tries to protect you from starvation, and it is not a fair fight. You have to respect this miracle of being human, but you don’t have to like it.


The other foe in the long-term weight loss battle is psychology. When people are dieting and hungry, psychological changes take place. We learned about a lot of these changes from a groundbreaking semi-starvation study that was conducted in the 1940s by Ancel Keys, a professor in the School of Public Health at the University of Minnesota. In it, thirty-six men volunteered to be starved for six months as a humanitarian act so that researchers could test the best ways to help starving people throughout the world. Although this study is always referred to as a starvation or semi-starvation study, I think of it as a diet study, because the men were allowed nearly 1,600 calories per day.

All sorts of things happened to the men during the study, which I will talk more about later, but the most common psychological response was an obsession with food. Before the study started, the men had many interests. They actively followed current events; they were curious about the new city they were living in; and they wanted to become acquainted with each other. Some of them even signed up to take classes on campus. But when the men were starving, the only thing they wanted to think about, or could think about, was food. They lost interest in their humanitarian mission, stopped attending classes, and even lost interest in sex. Their conversations with each other centered on food, their dreams were about food, and their spare time was occupied with thoughts of wonderful meals they had in the past, or plans for what they would eat someday in the distant future. Several of the volunteers vowed to take up careers in the food industry when the study ended—to open a grocery store or restaurant, become a chef, or work on a farm. Even those who had never cooked before started clipping recipes and reading cookbooks (including one volunteer who collected more than twenty-five cookbooks).

This type of behavior would have been useful for our ancestors during times of starvation. Individuals who focused exclusively on food and how to access it would have been more successful at finding some and, therefore, would be more likely to survive than their peers who were able to distract themselves from thoughts of food. But today, it just means that the less we try to eat, the more obsessed we become with food.

To take a closer look at this phenomenon, a collaborator and I examined what happens when people are denied a particular kind of food. We asked the students in my research methods class to participate, and they roped their friends into helping us, too. We had them record how many times they thought about a particular food, every day for three weeks. One of those weeks they were told they were forbidden from eating that food. Sure enough, they thought about the food more often that week than either of the other weeks. This shouldn’t come as a big shock. One of the first stories in the Bible is of Eve struggling not to eat the forbidden fruit. What is surprising, though, is that unlike Eve’s fixation on that delicious, tantalizing apple, the students thought about an off-limits food more frequently even if it was a food they didn’t like very much.

The problem for diets is that almost by definition, you have to forbid yourself from eating all sorts of foods, and for a period longer than a week. On a diet, you will think more about food in general, because you are hungry, and you will especially think about the very foods you have forbidden yourself from having. This just makes the job of avoiding and resisting those foods even harder.

Excerpted from "Secrets From the Eating Lab: The Science of Weight Loss, the Myth of Willpower, and Why You Should Never Diet Again" by Dr. Traci Mann. Published by HarperWave, a division of HarperCollins Publishers. Copyright 2015 by Dr. Traci Mann. Reprinted with permission of the publisher. All rights reserved.

By Dr. Traci Mann

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