Why You Should Never Diet Again: The Science of Weight Loss

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.


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.

YOU CAN (PARTLY) BLAME BIOLOGY

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.

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.

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