Dieting Does Not Work
Will you lose weight and keep it off if you diet? No, probably
not, UCLA researchers report in the April issue of American Psychologist,
the journal of the American Psychological Association.
"You can initially lose 5 to 10 percent of your weight on
any number of diets, but then the weight comes back," said
Traci Mann, UCLA associate professor of psychology and lead author
of the study. "We found that the majority of people regained
all the weight, plus more. Sustained weight loss was found only
in a small minority of participants, while complete weight regain
was found in the majority. Diets do not lead to sustained weight
loss or health benefits for the majority of people."
Mann and her co-authors conducted the most comprehensive and
rigorous analysis of diet studies, analyzing 31 long-term studies.
"What happens to people on diets in the long run?"
Mann asked. "Would they have been better off to not go on
a diet at all? We decided to dig up and analyze every study that
followed people on diets for two to five years. We concluded most
of them would have been better off not going on the diet at all.
Their weight would be pretty much the same, and their bodies would
not suffer the wear and tear from losing weight and gaining it
People on diets typically lose 5 to 10 percent of their starting
weight in the first six months, the researchers found. However,
at least one-third to two-thirds of people on diets regain more
weight than they lost within four or five years, and the true
number may well be significantly higher, they said.
"Although the findings reported give a bleak picture of
the effectiveness of diets, there are reasons why the actual effectiveness
of diets is even worse," Mann said.
Mann said that certain factors biased the diet studies to make
them appear more effective than they really were. For one, many
participants self-reported their weight by phone or mail rather
than having their weight measured on a scale by an impartial source.
Also, the studies have very low follow-up rates -- eight of the
studies had follow-up rates lower than 50 percent, and those who
responded may not have been representative of the entire group,
since people who gain back large amounts of weight are generally
unlikely to show up for follow-up tests, Mann said.
"Several studies indicate that dieting is actually a consistent
predictor of future weight gain," said Janet Tomiyama, a
UCLA graduate student of psychology and co-author of the study.
One study found that both men and women who participated in formal
weight-loss programs gained significantly more weight over a two-year
period than those who had not participated in a weight-loss program,
Another study, which examined a variety of lifestyle factors
and their relationship to changes in weight in more than 19,000
healthy older men over a four-year period, found that "one
of the best predictors of weight gain over the four years was
having lost weight on a diet at some point during the years before
the study started," Tomiyama said. In several studies, people
in control groups who did not diet were not that much worse off
-- and in many cases were better off -- than those who did diet,
If dieting doesn't work, what does?
"Eating in moderation is a good idea for everybody, and
so is 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. Studies consistently find that
people who reported the most exercise also had the most weight
Diet studies of less than two years are too short to show whether
dieters have regained the weight they lost, Mann said.
"Even when you follow dieters four years, they're still
regaining weight," she said.
One study of dieting obese patients followed them for varying
lengths of time. Among those who were followed for fewer than
two years, 23 percent gained back more weight than they had lost,
while of those who were followed for at least two years, 83 percent
gained back more weight than they had lost, Mann said. One study
found that 50 percent of dieters weighed more than 11 pounds over
their starting weight five years after the diet, she said.
Evidence suggests that repeatedly losing and gaining weight is
linked to cardiovascular disease, stroke, diabetes and altered
immune function. Mann and Tomiyama recommend that more research
be conducted on the health effects of losing and gaining weight,
noting that scientists do not fully understand how such weight
cycling leads to adverse health effects.
Mann notes that her mother has tried different diets, and has
not succeeded in keeping the weight off. "My mother has been
on diets and says what we are saying is obvious," she said.
While the researchers analyzed 31 dieting studies, they have
not evaluated specific diets.
Medicare raised the issue of whether obesity is an illness, deleting
the words "Obesity is not considered an illness" from
its coverage regulations in 2004. The move may open the door for
Medicare to consider funding 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 a treatment for obesity. The benefits
of dieting are too small and the potential harm is too large for
dieting to be recommended as a safe, effective treatment for obesity."
From 1980 to 2000, the percentage of Americans who were obese
more than doubled, from 15 percent to 31 percent of the population,
A social psychologist, Mann, taught a UCLA graduate seminar on
the psychology of eating four years ago. She and her students
continued the research when the course ended. Mann's co-authors
are Erika Westling, Ann-Marie Lew, Barbra Samuels and Jason Chatman.
"We asked what evidence is there that dieting works in the
long term, and found that the evidence shows the opposite"
The research was partially supported by the National Institute
of Mental Health.
In future research, Mann is interested in studying whether a
combination of diet and exercise is more effective than exercise