Hoping to lose a few pounds before summer begins? It’s time to think again.

A new study published this month in American Psychologist suggests that about two-thirds of people trying to lose weight with a reduced-calorie diet will gain back that weight – plus some extra pounds – within four to five years.

The study, led by UCLA associate professor of psychology Traci Mann, examined 31 long-term, doctor-administered diet studies as part of the most comprehensive and rigorous analysis to date of the prolonged effects of calorie-restricted diets.

Mann and her colleagues found that, after a typical loss of 5 percent to 10 percent of the dieter’s starting weight in the first six months, approximately two-thirds regained more weight within four to five years than they had lost.

“”We concluded most of them would have been better off not going on the diet at all,”” Mann said. “”Their weight would be pretty much the same, and their bodies would not suffer the wear and tear from losing weight and gaining it all back.””

Although the weight gain can be partially attributed to the difficulty of adhering to the diet, there are also many adverse physiological effects of such caloric reduction, Mann said.

A severe drop in calories sends the body into “”starvation mode,”” during which lean tissue or muscle is used to provide some of the calories needed to function. This results in a loss of muscle and a lower metabolic rate, causing the body to require fewer calories to maintain weight, slowing weight loss.

“”This is not people failing to follow a diet, but the diet failing them,”” Mann said.

However, Gene Kallenberg, the chief of UCSD School of Medicine’s division of family medicine, said that in his experience, people regain weight when they revert to old dietary habits.

“”The trick is getting folks to really make these permanent behavior changes in their daily lifestyles,”” Kallenberg said in an e-mail. “”This requires counseling skills, an ongoing doctor-patient relationship and perseverance on both the patient’s (and their family’s) and doctor’s part.””

Kallenberg also said that over time, a person’s metabolism naturally slows, making it likely that weight will increase several years later – regardless of dieting.

Although Mann and her colleagues expected a portion of the dieters to gain back their weight, they were surprised by the extent of the gains. The study concluded that only a small minority of participants achieved sustained weight loss, while complete regain was observed in the majority.

“”Diets do not lead to sustained weight loss or health benefits for the majority of people,”” Mann said.

Several studies on dieting indicate that the practice is actually a consistent predictor of future weight gain, co-author and graduate student Janet Tomiyama said, noting that one study that found that both men and women participating in formal weight-loss programs gained significantly more weight over a two-year period than those who had not adhered to a diet.

In addition, Mann said, many previous studies made diets appear more effective than they truly were, suggesting that the true number of people who gained back additional weight may well be significantly higher than her research found.

The majority of the studies that depended on participants self-reporting their weight yielded low follow-up rates­ – eight of the studies had follow-up rates lower than 50 percent. The figure suggests those who did respond may not represent the entire group, as people who gain back large amounts of weight are generally unlikely to attend follow-up tests, according to Mann.

Mann became interested in studying long-term dieting effects after teaching a UCLA graduate seminar on the psychology of eating, in which she and her students found that the majority of diet studies only followed subjects for six months to one year. Mann and her students wondered if the weight gain common among dieters at one year – after reaching their lowest weight around six months – would continue.

The 31 studies Mann analyzed were the only studies out of several hundred that followed patients for more than two years, providing a limited amount of data for the study and a lack of analysis on different types of diets or the effects of exercise.

“”Exercise may well be the key factor leading to sustained weight loss,”” Mann said. “”Studies consistently find that people who reported the most exercise also had the most weight loss.””

Kallenberg also stressed the importance of exercise in efforts to lose weight and keep it off.

He noted that those who maintain an increased daily exercise level, along with improved eating behaviors, are the most successful at losing weight and keeping it off.

In future research, Mann is interested in studying whether a combination of diet and exercise is more effective than exercise alone.

For now, she hopes a greater emphasis will be placed on eating in moderation and exercising to lose weight, but notes the difficulty in changing people’s conceptions about dieting.

Thurgood Marshall College sophomore Jennifer Sims agreed.

“”A lot of college students think food is the only reason for weight gain, and will stop eating, expecting immediate results,”” she said. “”It could be hard to convince them food isn’t the main problem.””