April 11, 2007 - Most people who go on diets soon gain back any lost weight, a UCLA study suggests.
Traci Mann, PhD, associate professor of psychology at UCLA, was teaching a seminar on the psychology of eating when she noticed something odd about diet studies. Few of the studies followed up on dieters for more than six months. Even fewer followed dieters for a year or more.
Mann wondered what, in the long term, really happens when people go on diets. So she and her students tracked down 31 studies that, one way or another, had at least one year of follow-up data. They were interested in just one number: the percentage of dieters who, over time, gain back more weight than they lose.
"We found that the average percentage of people who gained back more weight than they lost on diets was 41%," Mann tells WebMD. "In each of the studies, a third to two-thirds of the subjects gained back more weight than they lost."
Does this mean that most of the people in the studies actually lost weight and kept it off? No, Mann says.
"This is actually bleaker than it seems -- even though most people would find that 41% number to be pretty depressing," she says. "We have strong reasons to feel that this number underrepresents the true number of participants who gained back more weight than they lost."
Mann and colleagues report their findings in the April issue of American Psychologist.
Problems With Diet Studies
Diet studies, Mann and colleagues found, more often than not have one or more problems:
- Most of the studies didn't actually weigh the dieters -- they simply asked them about their weight. "If you ask people their weight, they are going to give you a lower number than their real weight. That is obvious to anyone who ever applied for a driver's license," Mann says.
- In many of the studies, a substantial number of subjects dropped out of the study. "This isn't rocket science," Mann says. "A major reason people don't stay in touch with diet researchers is that they are embarrassed because they gained back the weight they had lost."
- Diet isn't the only thing study subjects did to lose weight. Most studies included exercise regimens. So any weight loss could have been due to exercise and not to diet.
- Many people in diet studies lost weight, gained it back, and went back on a diet before the end of the study. Such patients would be counted as having long-term weight loss when they simply lost weight only for a short period of time.
Why don't diets work? Mann says there are two issues. The first is that it's just plain hard for people to change their eating behaviors. And the second reason is that even if you do succeed at a diet, the rule of diminishing returns comes into play.
"When you keep to a reduced-calorie diet, your body makes metabolic adjustments that make it harder and harder for you to lose weight," Mann says. "Your body becomes very efficient, and you have to eat less and less to continue to lose weight. If you had the will to go on a diet, the fact that it steadily becomes less and less effective makes it even harder to stick to it."
But Fernstrom worries that people will get the idea from this that their diets don't matter. And she's worried even more that the Mann report will discourage people from trying to lose weight.
"'Diets don't work' is only half the story," Fernstrom tells WebMD. "Lifestyle change will work if you have realistic expectations, good support, and choose a plan that you can stick with -- a plan that will give you moderate change over a long time."
That doesn't mean weight loss is easy. There's a myth, Fernstrom says, that normal-weight people can eat anything they want and don't need a strict exercise regimen. But that's true for only a very small number of people. Most people who have a healthy weight have to work at it.
"It is really hard to lose weight, and it is even harder to keep it off," Fernstrom says. "You can't cry about this. You must maintain hope. We just have to develop better strategies to keep people on track."
How to Lose Weight for Good
The basic problem is that people think diets are something you do for a little while before going back to your old lifestyle, says obesity expert Rob M. van Dam, PhD, assistant professor of medicine at Harvard Medical School.
"A lot of people go on a low-calorie diet for a few weeks and expect to lose a lot of weight," van Dam tells WebMD. "But if you do a crash diet, you will only regain the weight."
If you take medicine to control high blood pressure or high cholesterol, van Dam says, you would expect your blood pressure or cholesterol level to go back up when you stopped taking your medicine. A weight-control diet works the same way for obesity.
Why is it so hard to lose weight? A big part of the problem, van Dam and Fernstrom say, is that people try to diet in isolation.
"Diet is affected by social issues, by what you do when you are with your family and your friends," van Dam says. "In the current American setting, which really encourages unhealthy eating and dietary patterns, it is difficult to keep these lifestyle changes going."
Fernstrom says it's high time that America treated obesity like the medical problem it is.
"We have to change as a nation and as a culture," she says.
One change she'd like to see is insurance coverage to pay for the cost of professional assistance with lifestyle change.
Meanwhile, Fernstrom says, people who want to achieve and maintain a healthy weight should start working at lifestyle changes they can maintain -- even if it means not losing weight, but just staying at the same weight.
Elements of this lifestyle change, she says, include moderating food intake, increasing physical activity, managing stress without food, and getting treatment for depression and other illnesses that get in the way.
Even though diets don't work all by themselves, Mann agrees that there's much people can do.
"I am not saying 'Don't diet' -- I'm just saying people should try to eat healthy food in moderation and exercise like mad," she says.