Diets Don't Work Long-Term

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.