14 DAY TRIAL // Just some numbers: from 1980 to 2000, the percentage of obese Americans rose from 15 % to 31 % of the population.
This came also accompanied by the diseases associated with overweight.
Dieting is the most recommended solution against this condition.
But is it effective?
"You can initially lose 5 to 10 % of your weight (in the first six months) on any number of diets, but then the weight comes back," said Traci Mann, UCLA associate professor of psychology who led a new research on this issue.
"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."
Mann's team made a complex meta-analysis of 31 long-term diet studies (two to five years), without evaluating specific diets.
"We concluded most of the people 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 all back." said Mann.
At least 30 % up to 70 % of people on diets got more weight than they lost within four or five years, and the real number could be even higher.
"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 %, 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 co-researcher Janet Tomiyama, a UCLA graduate student of psychology.
"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," she said.
One research examining a variety of lifestyle factors linked to changes in weight in over 19,000 healthy older men over a four-year period, revealed 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," she said.
In this case, what's the solution for overweight?
"Eating in moderation is a good idea for everybody, and so is regular exercise," Mann said.
"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 loss." said Mann.
"Diet studies of less than two years are too short to show whether dieters have regained the weight they lost. Even when you follow dieters four years, they're still regaining weight," she added.
"One study of dieting obese patients followed them for varying lengths of time. Among those who were followed for fewer than two years, 23 % gained back more weight than they had lost, while of those who were followed for at least two years, 83 % gained back more weight than they had lost," Mann said.
What's worse is that repeatedly losing and gaining weight is connected to cardiovascular disease, stroke, diabetes and altered immune function.
"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."