Willyou lose weight and keep it off if you diet? No, probably not, UCLA researchersreport in the April issue of American Psychologist, the journal of the AmericanPsychological Association.
"You can initially lose 5 to 10 percent of your weight onany number of diets, but then the weight comes back," said Traci Mann, UCLAassociate professor of psychology and lead author of the study. "We found thatthe majority of people regained all the weight, plus more. Sustained weightloss was found only in a small minority of participants, while complete weightregain was found in the majority. Diets do not lead to sustained weight loss orhealth benefits for the majority of people."
Mann and her co-authors conducted the most comprehensive andrigorous analysis of diet studies, analyzing 31 long-term studies.
"What happens to people on diets in the long run?" Mannasked. "Would they have been better off to not go on a diet at all? We decidedto dig up and analyze every study that followed people on diets for two to fiveyears. We concluded most of them would have been better off not going on thediet at all. Their weight would be pretty much the same, and their bodies wouldnot suffer the wear and tear from losing weight and gaining it all back."
People on diets typically lose 5 to 10 percent of theirstarting weight in the first six months, the researchers found. However, atleast one-third to two-thirds of people on diets regain more weight than theylost within four or five years, and the true number may well be significantlyhigher, they said.
"Although the findings reported give a bleak picture of theeffectiveness of diets, there are reasons why the actual effectiveness of dietsis even worse," Mann said.
Mann said that certain factors biased the diet studies tomake them appear more effective than they really were. For one, many participantsself-reported their weight by phone or mail rather than having their weightmeasured on a scale by an impartial source. Also, the studies have very lowfollow-up rates — eight of the studies had follow-up rates lower than 50percent, and those who responded may not have been representative of the entiregroup, since people who gain back large amounts of weight are generally unlikelyto show up for follow-up tests, Mann said.
"Several studies indicate that dieting is actually aconsistent predictor of future weight gain," said Janet Tomiyama, a UCLAgraduate student of psychology and co-author of the study. One study found thatboth men and women who participated in formal weight-loss programs gainedsignificantly more weight over a two-year period than those who had notparticipated in a weight-loss program, she said.
Another study, which examined a variety of lifestyle factorsand their relationship to changes in weight in more than 19,000 healthy older menover a four-year period, found that "one of the best predictors of weight gainover the four years was having lost weight on a diet at some point during theyears before the study started," Tomiyama said. In several studies, people incontrol groups who did not diet were not that much worse off — and in manycases were better off — than those who did diet, she said.
If dieting doesn't work, what does?
"Eating in moderation is a good idea for everybody, and sois regular exercise," Mann said. "That is not what we looked at in this study.Exercise may well be the key factor leading to sustained weight loss. Studiesconsistently find that people who reported the most exercise also had the mostweight loss."
Diet studies of less than two years are too short to showwhether dieters have regained the weight they lost, Mann said.
"Even when you follow dieters four years, they're still regainingweight," she said.
One study of dieting obese patients followed them forvarying lengths of time. Among those who were followed for fewer than twoyears, 23 percent gained back more weight than they had lost, while of thosewho were followed for at least two years, 83 percent gained back more weightthan they had lost, Mann said. One study found that 50 percent of dieters weighedmore than 11 pounds over their starting weight five years after the diet, shesaid.
Evidence suggests that repeatedly losing and gaining weightis linked to cardiovascular disease, stroke, diabetes and altered immunefunction. Mann and Tomiyama recommend that more research be conducted on thehealth effects of losing and gaining weight, noting that scientists do notfully understand how such weight cycling leads to adverse health effects.
Mann notes that her mother has tried different diets, andhas not succeeded in keeping the weight off. "My mother has been on diets andsays what we are saying is obvious," she said.
While the researchers analyzed 31 dieting studies, they havenot evaluated specific diets.
Medicare raised the issue of whether obesity is an illness,deleting the words "Obesity is not considered an illness" from its coverageregulations in 2004. The move may open the door for Medicare to considerfunding treatments for obesity, Mann noted.
"Diets are not effective in treating obesity," said Mann."We are recommending that Medicare should not fund weight-loss programs as atreatment for obesity. The benefits of dieting are too small and the potentialharm is too large for dieting to be recommended as a safe, effective treatmentfor obesity."
From 1980 to 2000, the percentage of Americans who wereobese more than doubled, from 15 percent to 31 percent of the population, Mannnoted.
A social psychologist, Mann, taught a UCLA graduate seminaron the psychology of eating four years ago. She and her students continued theresearch when the course ended. Mann's co-authors are Erika Westling, Ann-MarieLew, Barbra Samuels and Jason Chatman.
"We asked what evidence is there that dieting works in the longterm, and found that the evidence shows the opposite" Tomiyama said.
The research was partially supported by the NationalInstitute of Mental Health.
In future research, Mann is interested in studying whether acombination of diet and exercise is more effective than exercise alone.