What was nutritional advice like back in the '30s and '40s?
Well, there are some wonderful posters actually that go back to World War I, where the advice in fact, for very different reasons, was pretty much the advice we're giving now: Eat more whole grains. But it was because we wanted to reduce the inputs of energy into producing food. We want more food to send to the army. If you look at the series of public relations posters that were done in World War I, [it was] very close to what it is now.
What happened then of course in the '40s, we had rationing. People worried about the food supply, had to stand in lines to get butter, which everybody wanted. Margarine was certainly the poor alternative.
Then we came into a time of plenty. The idea was that a full plate of what some people have called the three-lump diet -- meat, potato, and vegetable on a plate -- was the idea of having arrived at what was the consummate healthy diet: healthy, big portion of meat, some starch to go with it, and maybe a little vegetable arrives on the plate.
The idea here is just having enough to eat?
Having enough of everything you eat. Certainly the notion of not being overweight was out there. I don't think anybody ever thought overweight was a desirable thing. It just wasn't such a problem, in large part because not only was the food supply more limited, a little bit less varied, but also people had to walk more. There was enforced physical activity. You walked to school; you walked home at midday, many kids for lunch, walked back to school for the afternoon session. That's four sessions of walking which there was no choice. Parents didn't have two cars [to] drop their kids off at school.
Multiply that small change, the shift from kids walking to school to riding to school, by the hundreds and thousands of other changes in our daily lives, and you get an equation which predicts imbalance in calories and the obesity problem that we're now facing.
This shift: How much is at stake when you start to think of diet in that way -- as a factor in health?
Well, the epidemiologists really can develop models which tell you exactly how many lives a year are likely to be saved, or how much the risk of disease and death is predicted by differences in consumption of fruits and vegetables and grains and saturated fat. Those numbers are really pretty big. It's decreases in cardiovascular disease deaths, decreases in cancer deaths, the big killers, osteoporosis, which is a major killer. And the irony of that is that osteoporosis is really a disease that begins in childhood, and is very well predicted by the amount of calcium that a kid puts down in their bones in the elementary school years up to teenage. It's not one we've thought of. It hasn't got as much publicity recently, but we now know that loading the skeleton with jumping, wild activities that kids really love to do if let loose to do them, giving them adequate intakes of calcium, will predict whether or not they get osteoporosis in later life, and that's a huge killer.
Talk about the change in the story.
The difference between eat to have enough -- and you need energy, you've worked hard in out in the cold, etc., etc. -- is very different than talking to the public about eating five servings of fruits and vegetables a day because there are big associations with decreased cancer risk, for example.
There are multiple problems with the latter message, and that is that surprise, surprise, there's a lot of the population that really doesn't pay a whole lot of attention to the relationship between what they eat and their health, and certainly not to long-term health. Taste swamps health as an indicator of what people choose by orders of magnitude. The cost: For a lot of the population, they simply look at a pound of meat and a pound of broccoli and they say, "They cost in the same ballpark. Why would I buy a pound of broccoli rather than a pound of meat? That's not what's going to keep my family full and happy, besides." So it's a very complicated set of issues. The health message simply does not resonate with a lot of the population.
But getting media exposure to these health messages is relatively easy. People want to know about diet and health, even if they don't act on the advice.
I think the issue of media exposure and media coverage has two sides to it. It has good side and it has a bad side. The media coverage, unfortunately, by and large -- and I'm talking now about news coverage, coverage of the science -- does not really give a story in context. "Eat more carrots as part of a diet which includes fruits, a lot of fruits and vegetables." That's a real snore. It's very hard to get across. And unfortunately, the message tends to get to be "Eat more carrots" one week, "Eat more tomatoes" the next week, often associated with a study that comes out: "Eat more tomatoes because tomatoes are an excellent source of lycopene, and our studies have shown that there's a decreased risk of prostate cancer associated with increased lycopene consumption." So that week it's tomatoes. It doesn't come out as: "Here's what the diet looks like, and here's what you ought to do as sort of the micro units of that diet."
The awareness of the link between saturated fat, cholesterol, heart disease -- why was this such a compelling story?
Well, the origins of the cardiovascular disease prevention possibilities of diet really lit up like a Christmas tree when Ancel Keys did this huge study in which you could show differences in cardiovascular risk associated with differences in diet. In diets which were low in fat, and then more clearly low in saturated fat, people just had less cardiovascular disease. The death rate was a lot lower. And then the studies evolved to look at people who were born in countries where the disease rates were low, and what happened to them when they came to the good old U.S.A. and assumed our diet. The disease risk changed dramatically. Guess what? They look like the rest of Americans within two generations. There was a complete wipeout of differences that were associated with their country of origin.
What makes you so sure it was the diet?
Well, one could say, "What makes us so sure it was the diet?" And I think we've said that. And I think it's not diet alone. It's a whole set of issues about diet and health.
One of the things that always gets lost in this story, I believe, is that, for God's sake, we're doing some things right, and we are living longer, and we are living with decreased risks of certain diseases. To be sure, a lot of that is related to better medications. But in fact, we eat a pretty good diet in terms of safety, for one thing. Many people are eating a perfectly healthy diet. It's not an all-or-none phenomenon. It's not that Americans are doing this or Americans are doing that. It's different segments of the population whose diet needs considerable improvement, and I think probably we have people at both ends of the curve, and most people in the middle of the curve. I think some of the population have made changes, others are way back at what we call the laggard stage, and others are at the innovation stages, where they're doing everything they possibly can to optimize their diet.
Talk about the fat story.
The fat story is really something of a small disaster in its evolution. And I think again there are multiple problems. One of them is the evolution of science. Originally, it was saturated fat, and then we began to understand more about which fatty acids it was. And also [in] the original story, cholesterol was a big player, and dietary cholesterol was associated with blood cholesterol. Well, it turns out that dietary cholesterol occurs in so much smaller amounts, its effect on serum cholesterol is not nearly as powerful as the effect of saturated fat. Well, that story, American consumers understood that cholesterol was bad. And the food manufacturers put "cholesterol-free" on every vegetable product they turned out. And if one did it, the other had to do it, because that's competition.
Then we learned, well, cholesterol wasn't the big player. It was saturated fat that really is the big player. From there we went on to types of fatty acids, and so we produce margarines with vegetable oils and the public begin to shift to margarines with vegetable oils. That's fine, and the whipped margarines have less saturated fat than the solid ones. And some of them have water whipped into them, and they have even less fat.
But then we looked and said, "Oh my gosh. In taking out saturated fat, manufacturers are using trans fats, which behave in the body like saturated fat." So the message that got out in the media was, "Butter is better than margarine." And there are headlines from a few years ago that say that. Many, many headlines. And that was one where the consumers just threw up their hands and said, "Oh my God, again they've pulled a dirty deal." It turns out that they both behave pretty much the same. And "less fat, more vegetable oil" is really still the message, which we started with. It's still the same message.
All baked goods would have trans fat?
A lot of baked goods have trans fats, except those that are made with butter, you know, which have the saturated fatty acids. Manufacturers have tried very hard, and are trying very hard, to get the trans fats out of their products. But I can tell you that what we call organoleptic factors, which is a funny, elaborate word for talking about mouth feel and the sensory qualities, and the cookies made without trans fats, to the very sensitive mouth, don't taste the same. Some people will tell you they do, and those who are really super-sensitive tasters will tell you that they don't.
I think a lot of this issue begs the question of a concept that Americans have a really hard time with. It's that concept of moderation. If we could eat two cookies and move on, we'd need a lot less of what I call this jiggering with the food supply. But we don't. We want to eat a lot of things, and a lot of what we eat, and a lot of variety.
And that's the other part of the fat story, which is that fat does have twice as many calories per gram as carbohydrate. So about 10 or 15 years ago, somehow we got this notion that: Aha. If we take the fat out of foods, we will be able to reduce the total caloric intake, and people will be able to control their weight. And so industry got very busy making low-fat, reduced-fat, fat-free products. And they flooded the marketplace and they flew off the shelves. And guess what? A lot of them weren't very good, and so they died on the vine very quickly. The ones that were good, people ate in excess. They didn't look at the portion size. They didn't look at the calories. They just ate them. There's no free lunch. There were calories. Guess what? People didn't lose weight. They got frustrated and went back to eating the food they liked in the first place.
What did [people think] when they saw "low fat, fat free"?
"Low-fat, fat-free, good for me. These have got to be good for me if they're marked low fat." Of course in many of the foods, if you looked at the label of the regular and the low fat -- and you can still do this -- the caloric difference isn't all that much. In some cases it's more significant than in others, but the issue is that if you take out fat, you got to put something in. That something, surprise surprise, has calories. It's most likely carbohydrate, a little bit of protein, and those both have calories. So you can't continue to take everything out and have some flavor and texture left, and the calories in many of those foods were not very different.
Some argue the simplification of advice [caused public confusion] and some argue nutritionists should have seen this coming. Where do you cast the blame?
I don't really want to cast blame on who's to blame for the confusion of the American public. I think we're probably all to blame. I think one of the problems, speaking for myself as a nutrition professional, is our tendency is to want to tell people the whole story, all at once, and change this to this, and this to that, and that to that -- a whole long menu of things that you should do. And the poor person on the receiving end of that, it's just overwhelming. It's just too much. So we've tried to throw the book at people a lot of the time.
I think another thing is that the public sector, which is charged with nutrition education writ large for the American public, has never had the kind of funding that it ought to have, to be doing the job. We haven't been able to compete in a consistent message with the private sector, which of course spends millions and billions of dollars on food advertising.
The food industry's behavior, is that not predictable?
I think the food industry is very resourceful. I was not intending to damn the food industry. In fact, one of the great ironies is that in the surgeon general's goals for the year 2000, the only goal in the nutrition arena that was met was the percentage of low-fat products that should be available to the American public by the year 2000. Industry was charged with producing those products. It was a government document that said industry should do this, and they did it.
Industry turns on a dime. They can do things quickly and resourcefully, and they gave it their best shot. Clearly, also, they were thinking in a competitive environment. That piece of it, in no way should anyone point a finger.
We failed through succeeding?
We've failed through succeeding in doing what was asked. Or industry failed through succeeding. If I ruled the world, I'd go back to basic foods entirely. But I don't rule the world.
You've also got the obesity community banging the low-fat drum.
Well, that was pretty consistently so until the people who were banging the low-carbohydrate drum came along. But actually, the people banging the low-carbohydrate drum began with -- I think his name was Dr. Banting, in 1862, who wrote a pamphlet called "A Letter on Corpulence," addressed to the general public, in which he proposed a low-carbohydrate, high-fat diet. … So the low-carbohydrate craze has been around a long time.
You can only practically jigger carbohydrates and fats. And what I honestly believe is that different diets work for different people -- work in the sense of if you need to lose weight, whatever is the most comfortable model for you to follow for a reasonable period of time, is probably what you ought to go with. The trick is what are you then going to stay with? It's not the weight loss. It's the weight maintenance which is the difficult thing. And I really believe that the low-carbohydrate, high-protein, high-fat mix is not the way to go long term.
Two anomalies: (1) in the low-fat era, people got fat; (2) people eating high-caloric food lost weight in the short term. The nutrition community wished this away?
Well, I don't think the nutrition community functions in lock step to have wished it away. It's a nutritionally imbalanced diet. I don't think anybody can really argue that one seriously. It is, as literally practiced, it's an atherogenic diet, it's followed long-term.
But I think the other thing is that nobody really looked at caloric intake over a long term. And I think we have not violated the law of thermodynamics. The fact is that while people are really excited about eating a steak-and-eggs diet at the outset, it gets old reasonably quickly. The people who've been successful on that diet really do cut back on their calories, so maybe they feel more satiated on a high-protein, high-fat diet, but they are not consuming appreciably fewer calories long term.
Satiety. You've got a diet with double the calories, but somehow you eat less.
That's because protein is digested more slowly. Fat has a very high satiety factor, and it does, for many people, allow them to feel more comfortable when dieting. Anyone who has ever deprived themselves of calories consciously has -- I haven't yet met the person who's said, "Oh boy, this is fun. I'm really enjoying it." The idea is: When can I get back to doing what I like to do? I think for most people, getting back to what they could do, what they like to do, if they'd only do a little bit less of it, it would be fine. The problem is, we go back to business as usual. It's very difficult in our environment to control weight. …
Is this a disaster?
My prediction is that the low-carbohydrate foods will, long run, be just about as successful as all the low-fat foods were. People are demanding them. Industry is responding to the demand. At the end of the day, it's not how people really want to eat. And really, food is fun, it's pleasure, it's what people like to do. These aberrant diets don't fit with what people really have become accustomed to, what they enjoy. It's not going to be the answer to the weight problem.
Are you saying all diets are artificial so they don't [work]?
No. What I'm saying is that the diet which sustains is probably a diet which is relatively higher in carbohydrate, complex carbohydrate, on the higher end in protein, and relatively low in fat.
But diet is only part of the story. You've got to put the exercise piece into the equation. Until recently, that's really been left out. It isn't that you can't burn off whatever you eat unless you're a marathoner and train all the time. But you do have to be physically active to manage to control your weight, unless you are willing to eat a very restricted diet.
Measuring what people eat and how much they exercise is a pretty hard thing to do.
It's extremely difficult to measure food intake and energy expenditure, and there's lots of error in the measurement. There's error in what people report. They can be as dedicated to reporting as they possibly think they can, but they can't possibly remember what they ate yesterday. They can't possibly accurately communicate what they usually eat. Those are difficult questions, and there are measures for validating what people tell you, but it's a very expensive technique and you can't do that sort of thing as field studies. So yes, there is error in measurement.
[Some] have criticized the USDA food pyramid as simplified. Would you defend the USDA pyramid?
I would defend the USDA food pyramid. In the interest of full disclosure, I have to say I was the principal investigator on the study which led to the release of the food guide pyramid after it was withdrawn.
I think one of the problems in the confusion about the food guide pyramid is that probably one person in 10,000 doesn't understand its origins. Its origins were as a graphic to explain the content of a booklet which laid out some of the specifics of the concept of variety, which was part of a government document called "The Dietary Guidelines for Americans." It was just a graphic that was supposed to illustrate some of these points. Somehow it got from that point into a debate which made it look like a policy document, as "This is one way to eat," and then there are a myriad of other pyramids which say, "This is the way to eat."
I wish the food guide pyramid were as powerful as its has been positioned as being, because if it were, and if Americans were following the food guide pyramid with the caloric bases on which it's underpinned, a lot of the obesity problem and a lot of the eating problems that we have would just go up in smoke. I think it isn't about whether it's the food guide pyramid or a pyramid that has a different configuration. We're just eating too much, and too much of the wrong things. But the food guide pyramid didn't cause obesity. It just is not that powerful.
One of the real problems in using it though is, if you look at the food guide pyramid, or any pyramid, it contains a bunch of basic foods -- breads, fruits and vegetables, dairy products, meat, so on. Everybody's seen it. The world is acquainted with it. What happens when somebody eats a pizza, which is a simple but very popular food? Where does that fit in the food guide pyramid? And what person is going to do the mental arithmetic to look at the food guide pyramid and say, "Okay, I had a pizza. Now, the crust belongs in the base. The tomatoes belong over here. The oil belongs at the top." You see where I'm going? I mean, all it is, is a graphic, which should be used as a tool that's part of a comprehensive learning program for kids and adults to learn how to choose a diet.
Some epidemiologists argue that all fat has been unfairly lumped in; that there should be a lot of fats at the bottom. Good[fat]-bad fat, good[carb]-bad carb.
… It's not a stand-alone graphic. There's a lot of information that goes into choosing a healthful diet. I don't care how good anyone is at designing graphics, they'll never put it into one triangle, pagoda, or any other model -- and we tried large numbers of models that might look more effective. I don't think that should be the use. I think we ought to come back to defining what is the use of the food guide pyramid, and begin from there. Wipe the slate clean and say: What are we going to do with this document? Or should we get rid of it?
Some people feel fats have got too bad a deal and carbs have got too good a deal -- [for example] the New York Times article. What is a nutritionist's response?
When we swing to the low-carbohydrate, low-fat, I think, "Oh no, not again." I really wish we could get a consistent message of balance intake and expenditure. Eat lots of whole grains, fruits and vegetables, limited amounts of fat.
I think for the public, really understanding all of this chemistry about trans fats and saturated fats, it runs counter to the way people choose a diet. If you go up to somebody in the supermarket and say, "Excuse me. Can you tell me whether you looked at the trans fat information? Or what information did you look at, on the food label?" The odds overwhelmingly are that they didn't look at the nutrition label. Many of the labels, they've looked at and learned about and moved on. But if they looked at anything, they looked at calories, and hopefully they looked at calories and serving sizes.
I think we really need to get back to helping people to understand what portions are. If we could get back to the portion issue, and understanding that the plate of spaghetti on the bottom of the food guide pyramid was never meant to be one serving of spaghetti. Many people looked at that and said, "Oh my God, if I ate six of those, I'd be like a house in a week." That was never the intent. The intent was to make a recognizable graphic that looked like pasta.
I think we're trying to throw too much at people, and we [should] really try to get back to a whole message about what should your diet look like over a week, and yes, it's okay to have some of your favorite foods some of the time, just not every day, all of the time, and as much as you like.
The idea that you can take a starchy food like a potato and it's turned into [sugar] very quickly, is genuinely novel, shocking to people.
… The problem with glycemic index is that it's extremely complicated. Setting aside whether controlling the spikes in blood sugar are a good idea or not, the form in which the food is cooked affects the glycemic index of the food. The mix of foods in which it's eaten affects the glycemic index. Measuring the glycemic index of carrots doesn't do any good unless you know what other foods the carrots are going to be eaten with.
So in my view, using glycemic index as something that we teach people how to live by is like selling them a pet rock. To me, it's unfathomable. It doesn't translate. Even if there is, down the road, proven to be some effect on controlling blood sugar more evenly throughout the day, I think we've got to be a little bit more practical about how we do it. And the notion of telling people to limit potato consumption -- I mean, yes, limit french fry consumption; yes, [don't] eat giant portions; but edging [potatoes] into a bad food category, to me, just doesn't make a whole lot of sense as a practical thing.
If part of the reason people overeat in a low-fat era is because of unrefined carbohydrates, through a physiological mechanism making them hungry, there is this question of what makes you hungry?
I'm not sure that we all agree that what makes you keep going back and eating is always hunger. I would say that a large part of it is that we have done a wonderful job of undermining the satiety mechanism. I don't think most of us realize that we've had enough and put down the fork. The notion of the clean plate club, which is a remnant of the postwar era, I think, and maybe even earlier than that, but we've overwhelmed the satiety mechanism. We eat too much because we're accustomed to it, we're served it. That doesn't have to do, I don't think, with glycemic index necessarily. I think that we need a retraining there, in terms of recognizing what's enough food, and what should be served as enough food.
Whether the concept of glycemic index can be translated into true meal planning, I have yet to be convinced of that. It's like all of the other very complicated things. Getting an entire population or a large part of the population to say, "Oh, I should plan my meal this way because my glycemic response will be lower," is not likely to go anywhere in large numbers very quickly. Even if it works for some people, it's going to be a very hard sell. It's going to be very difficult to do. Our food supply is so complex and so highly processed; this requires a cognitive load that most people aren't willing to take on. Their lives are complicated enough without it.
I also think that love of things like pasta are not going to be easily displaced. Now, having said that, the diet is too low in whole grains, and we really ought to work on stressing more whole grains and less refined carbohydrates. I think there's no question about that. The fiber content of the diet is way low. …
Over this period, the public health situation regarding obesity has become more and more alarming.
The rise in the prevalence of obesity, to me, is really scary. Maybe because I'm a nutritionist, it just scares me in terms of public health. I worry so much about all the gains that we've made in decreased incidence of cardiovascular disease and certain cancers -- we've done some good work. And with the rise in obesity, the rise in the early onset of type 2 diabetes -- it has been said many times, but we're just seeing the tip of the iceberg of a huge problem.
Probably the scariest part of that problem is the problem of obesity in kids. Because once kids become overweight and overweight to extreme, [it is] something they're not easily going to outgrow at adolescence; it's very difficult to reverse it. And you look at adults who are suffering in so many ways, and if you listen to their tales of how they can't fit in an airplane seat and all the rest of it, it's not the way they would prefer to live, but once having arrived there, very, very difficult to undo. It's going to be a huge public health burden unless we do something to reverse the trend.
Characterize type 2 diabetes.
Type 2 diabetes is a disease which we usually see in people in their mid-40s and older. Usually they're overweight, and if they lose a few pounds, very often you can get the symptoms to go away with the weight. Problem is, people get scared when they have a diagnosis, and they're real good for a short time, and then there is a high recidivism rate. But it has been typically a disease of middle-aged and older people, and it just has to do with the wearing out of the cells in the body that produce insulin so that there aren't enough, and they don't produce enough insulin. But now we're seeing this in kids. Now clearly, the kids in whom it's occurring, there must be a genetic predisposition to the expression of the disease at very much younger age.
But the problem is that it isn't the diabetes itself; it's the problems that are associated with uncontrolled diabetes long term -- devastating effects on the eyes, on the renal system, on the cardiovascular system. So it's not just about having diabetes and being on a diet, which unfortunately many of these people don't do. ...
As a society, are we facing a huge cost? Are we facing a generation that may not live as long?
We are facing huge medical costs. I mean, just the fact that we're beginning to have to treat kids for type 2 diabetes is just the tiniest, tiniest tip of the iceberg. The complications of obesity that are associated with type 2 diabetes, and the type 2 diabetes itself, will lead to increased incidence of blindness, kidney malfunction, and kidney decline and malfunction, more cardiovascular disease, and kids not living as long as their parents. But also, it's about quality of life, and I think we can't just talk about the end stage. We really need to talk about the fact that these kids, even though it's becoming more commonplace, the discrimination against kids who are overweight is right out there, just like it was when I was a kid.
Solutions? Does it seem hopeless?
Well, I'm an optimist. And I know that we're going to get on top of this problem. We have got to. It's too urgent a public health problem. It's too pervasive in our society. We're not going to build wider airplanes and all the rest of it, and wider seats on buses, to accommodate. That's just really a band-aid approach.
I think we need vast environmental changes. We need individuals to recognize that they've got to take ownership of the problem, because at the end of the day, it's the calories you put [in] your mouth and the energy that you burn, walking to get a newspaper, or don't burn by riding to get a newspaper, that really make the difference.
On the other hand, we also have to make it possible for kids to get lots of physical activity. That is not available to kids now, for a bunch of reasons. I think right now there are lots of good programs, many of them funded as research projects -- we have one ourselves in Somerville, [Mass.] called "Shape Up, Somerville" -- which are trying to change the environment to create a healthy environment, so that everything about the day provides opportunities for healthier choices.
I don't think there's one solution. But I think that encouraging kids to walk to school, and if it's not safe to walk alone, creating mechanisms by which they can walk in groups, is one teeny little thing, but it's a piece of something that we threw away without looking back. I think parents of very young kids have got to get them out of those little strollers that they use until ages way past when -- certainly when I pushed my kids around, and most other people did. It's convenient, it's fast, it's easy, and the kids are riding around in buggies until they're 7 and 8 years old, and some even older, because it's convenient.
There are a myriad of small things that need to be done: Parks cleaned up so kids can play in them, in all communities. The Rails to Trails program, converting bike routes, bike paths, so we have safe places to bicycle. We need more opportunities to make outdoor activity a way of life. We need school recess. We need activity programs in after-school programs. We need to make activity part of daily life.
That's the expenditure side.
That's the expenditure side. We also need to have food environments with healthy choices, all the time. Airports are a great place. Actually there is some small trend in the direction of being able to get something reasonable in an airport, but most airports most of the time are travesty on nutritional balance. School lunches need to be real healthful meals with foods that kids like to eat. They should not be fried nuggets and hot dogs and little else. Fried nuggets and hot dogs are okay some of the time, but not all the time. The snack line certainly needs to have lower fat choices, more healthful choices for kids. There are loads and loads of small things we can do. There isn't one solution.
What about family meals? Is that just hopeless?
I think family meals are extraordinarily important, and I think the notion that nobody ever has time to sit down is silly. It's as silly a notion as the fact that nobody understands what they ought to eat. People can make time, and they can make it a value. The family meal table needs to be a place where people get together and exchange ideas. It's part of a set of values. I don't think it's reasonable to basically let the eating process take itself where it may. "There's a microwave, there's food in the freezer, eat what you want," is not okay. I think that's probably the extreme. I certainly have witnessed it. But I like the other extreme, which is the family sits down to dinner every night. [If] they can't sit down to dinner every night, and everybody eats the same thing, and meals are meals and snacks are snacks, and people know the difference, they can certainly do it some of the time.
Can you draw a distinction between diet [and weight maintenance]?
Well, a weight-loss diet has got to be a diet that has a caloric deficit, that is, one where you consume fewer calories than you burn. And that requires a conscious attempt at seeing where you are. I think what happens is that people say, "Okay, I'm going to go on a diet," and they react by putting themselves on some semi-starvation thing. "I'm going to accomplish all this in an unreasonably short period of time. If not by tomorrow, at least in 3 weeks." It doesn't work, because they're so hungry and so miserable, and don't like themselves, that the rebound effect is really terrible. They often gain back more weight than they lost in the first place.
The weight reduction diet really should begin with a goal: It is reasonable for me to lose a pound a week if you're older, 2 pounds a week if you're younger. But it's got to be a combination of assessing what you eat and cutting back on that, and assessing what you do to move, and adding to that. Once you get to maintenance, the idea is to do some monitoring. You can add things back, but to do some sort of monitoring so that you realize that when you're a pound or two over, whoa, this is the time to go back. Not when you're 5 pounds over or 10 pounds over. The longer you wait to pull in the reins, the harder it gets, but I think most people who are in a phase of maintenance will tell you that they never stop keeping track of what they're doing.
You can't let up.
I think saying you can't let up is a little bit harsh. That makes it like a life sentence. I think keeping it somewhere in your consciousness that if you're going out to dinner at night, you eat a light lunch. It's not brain surgery. It's just common sense.