The question that seems to hover over all this diet talk is whether any of the myriad weight loss schemes have worked. If one had, shouldn't it have survived the test of time? And if we've gone this long without a diet that has been shown to work — according to science, not simply the sellers of the fad — will one ever emerge that actually does?

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The short answer is no, according to Traci Mann, who teaches psychology at the University of Minnesota and has been studying eating habits, self-control and dieting for more than 20 years. Over the course of her research, largely conducted at the University of Minnesota's Health and Eating Lab, Mann has repeatedly asked these sorts of questions, and always found the same disappointing answers.

Her findings, chronicled in her newly published book "Secrets from the Eating Lab," offer a fascinating explanation for why dieting over the long term is actually impossible.

I spoke with Mann to learn what exactly it is that people misunderstand about weight, willpower and our relationship with food. The interview has been edited for length and clarity.

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You have worked at the eating lab for some time. What is the eating lab? What does one do at an eating lab? The concept of an eating lab is really cool, but I have to admit that I don't quite know what it is.

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I don't want to make it sound any less cool, but it is a lab. And it is where we do eating studies. I study eating at the lab in two basic ways: I either go out into the world, and study people and the way they eat — the way normal people do their eating — or I bring people into the lab and put them in situations that are carefully controlled so that I can see how their eating responds to those carefully controlled situations. These two types of experiments are really opposite sides to the same coin, but you really need both. You need to see what people do in real life, and then you need some way to find out what causes what. And that's what the lab is for.

But you don't just have a lab, you also have a new book, and it talks a lot about how people eat. Is it fair to call it a culmination of your work at the eating lab?

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Absolutely. You know, people are already asking me if I'm going to write another, and I'm like, "Yeah, after I do another 20 years of research."

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So then what is the culmination of your work at the eating lab? In the book, you talk a lot about dieting, and how it doesn't actually work. What have you found?

If you want an overview kind of culmination, well, let's see: People are too uptight about their weight; people are handling that uptightness in a foolish way that doesn't work (that would be dieting); and the reason diets don't work is not what people think.

It all starts with something that suddenly struck me a while back, and that's that nobody has willpower. Everyone is blaming dieters for regaining weight they lose, and that's just wrong — it's not their fault they regain weight, and it's not about willpower, or any lack thereof.

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All this time, doing studies in the lab, almost every single study, without really meaning to, showed some other thing that made dieters overeat. I have found time and again that it's actually some other thing that causes dieters to lose control of what they're eating.

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But the truth is that everything causes dieters to lose control of what they're eating, because dieting is bound to fail, it is destined to fail.

Well, that's pretty provocative. So dieting doesn't work, and it's not for the reasons people think. What are these reasons we are looking past?

What people tend to think is that if only Joe had self-control then he could succeed on his diet forever. And that's not accurate, as it turns out. That's not true.

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After you diet, so many biological changes happen in your body that it becomes practically impossible to keep the weight off. It's not about someone's self-control or strength of will.

What kind of biological changes?

There are three biological changes that take place that seem most important to me.

The first is neurological. When you are dieting, you actually become more likely to notice food. Basically your brain becomes overly responsive to food, and especially to tasty looking food. But you don't just notice it — it actually begins to look more appetizing and tempting. It has increased reward value. So the thing you're trying to resist becomes harder to resist. So already, if you think about it, it's not fair.

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Then there are hormonal changes, and it's the same kind of thing. As you lose body fat, the amount of different hormones in your body changes. And the hormones that help you feel full, or the level of those rather, decreases. The hormones that make you feel hungry, meanwhile, increases. So you become more likely to feel hungry, and less likely to feel full given the same amount of food. Again, completely unfair.

And the third biological change, which I think people do sort of know about, is that there are metabolic changes. Your metabolism slows down. Your body uses calories in the most efficient way possible. Which sounds like a good thing, and would be good thing if you're starving to death. But it isn't a good thing if you're trying to lose weight, because when your body finds a way to run itself on fewer calories there tends to be more leftover, and those get stored as fat, which is exactly what you don't want to happen.

So calling it unfair doesn't even begin to describe the injustice.

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How could it when you have to fight against all of that? You can do it, potentially, but it's going to take over your life. And that's no way to live.

Dieting is actually a lot like starving, physically. It's living like you're starving. A lot of people do it, but what they're actually doing is living as if they're starving. They're putting their body into that exact same state that it would be in if they were literally starving to death.

But there's an entire industry that profits from convincing people that just the opposite is true. How do you reconcile that?

Well, the first thing is that you can't believe anything that they say. And that's by definition, because their job isn't to tell you the truth — it's to make money. And they're allowed to lie.

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These companies make their money off failure, not success. They need you to fail, so you'll pay them again. One-time customers are not the sort of thing that keep these diet companies in business.

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What would you say to someone who says 'I followed or have been following so and so diet, and I've lost weight, I feel better, it's working'? What are they not understanding?

I would tell them that they're in the honeymoon stage. That early stage is great, but it really is a honeymoon stage, and it's going to get a lot harder soon.

For practically any diet — crazy, or not crazy sounding — in that first 6 to 12 months, people can lose about 10 percent of their starting weight. So a 200 pound person will lose about 20 pounds in the short run. But the short run isn't the whole story. Everyone acts like the short run is the whole story, and that anything that happens later is the dieter's fault and not really part of the diet. People act like the only part that is the diet's fault is the beginning bit. The long-term part, people always say that's not the diet, that's the person. And yet, it's clear that that's not true. It's over the long term that you see all these biological changes take control.

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In your book, you talk a bit about how one of the most glaring problems with dieting is how we define a successful diet. What are we getting wrong?

When people lose weight on a diet, they call it a success. And if the weight comes back on, they don't say that the diet wasn't successful — they say 'I blew it.' But that's not correct. It's all part of the diet.

We've conducted studies where we have brought dieters and non-dieters into the lab, and distracted them a little bit. What we have found is that when distracted, dieters eat more than non-dieters. In fact, distraction only affects how much dieters eat. A simple little thing like that tells you that if you're trying to resist eating, the subtlest things can mess you up. All these little things cause dieters to fail in resisting food that don't really affect people who aren't dieting.

Through the years, I have looked tirelessly for things that help people diet, but all I have ever found are things that trip them up.

What about willpower? People love to talk about willpower as though it's what separates the winners from the losers. Is that fair?

An idea that I want to float, if I might, is that willpower is actually a very different thing when you talk about eating. Willpower can be extremely useful in certain parts of people's lives. But when it comes to eating, it's just not the problem. It's not the fix.

I didn't do this in the book, but let me try to explain why.

Let's say you're in a meeting, and someone brings in a box of doughnuts. If you're dieting, now you need to resist a doughnut. That is going to take many, many acts of self-control. You don't just resist it when it comes into the room — you resist it when you look up and notice it, and that might happen 19 times, or 90 times. But if you eat it on the 20th time, it doesn't matter how good your willpower was. If you end up eating it, you don't get credit for having resisted it all those times. In virtually any other arena, that would be an A+, but in eating that's an F.

So it's for reasons like that that someone's willpower, which is measurable by the way, does not correlate with people's weight. It just doesn't. But, and here's the thing, it does correlate with tons of other stuff, like SAT scores, grade point average, and all kinds of other achievement outcomes. And if you think about it, that makes perfect sense. If you're studying for an exam, and give in to checking Facebook, those 10 minutes that you waste don't erase the studying you did before. You haven't lost anything. Whereas with eating, when you suffer that one moment of weakness, it actually undoes all the successful willpower that came before it.

Would you say that it's pointless then to try to lose weight? Or are we simply doing it wrong?

I don't think people should try to live at a lower weight than their set range. If you try to lose weight so that you're below your set weight range, that I believe is folly, or farce. It's not healthy. It's what sets off all those biological changes that are effectively trying to defend your set range. When your body goes lower than your set range, it makes changes to bump your weight back into it. And what people don't know is that if your weight goes above it, it also makes changes to push it back down into the ideal weight.

I do understand that people are people, and want to look a certain way. I get that, of course. That's why I recommend trying to be at the lower end of that set range. I'm trying to convince people that that's the right place to be. It's a healthy place to be, and an easy place to be. It's the sweet spot.

So people should feel comfortable just as they are? That seems like a pretty tough truth to swallow.

If you think about it, people do drop below their set range and stay there. A small percentage of dieters — something like 5 percent — can do it. And they do do it. But they do it by devoting every minute of their life to staying at that weight. Basically, they spend their entire life living like a starving person, fighting biology, and evolution. And to me that seems wrong.

People who have the means to not be starving to death should not be starving to death. How can we ask that of people? It just seems outrageous to me.

What's really sad to me is that it isn't just society that blames dieters when they gain weight. Dieters blame themselves, and I really think that that's a shame. They're in a situation where food looks more tempting, they're hungrier than they should be, their body is getting by on fewer calories, and everything is just working against them. And yet people are always so quick to say, 'well, it was their hand holding the fork.'