It may not sound like much, but obesity experts meeting in Prague were impressed. Nearly all dieters eventually return to their old weight, and maintaining a 5 percent loss is considered a health benefit.
The study is the most rigorous investigation yet of a commercial weight loss program. Experts say that although dieters would probably hope for better results, a 5 percent reduction is enough to reduce the risks of developing such diseases as diabetes, heart disease and cancer.
Experts said the study indicated the value of programs that are well-rounded, promote gradual weight loss and include a strong support system.
Studies conducted on obese people enrolled in hospital-based experiments have indicated that within a year, dieters regain one-third to one-half of the weight they lost. Within five years, most of them gain almost all of it back. Studies on commercial programs are rare.
The latest study, paid for by Weight Watchers, involved a random sample of 870 lifetime members of Weight Watchers in the United States — those who reached their goal weight and maintained it for six weeks.
The dieters, most of whom were married, middle-aged, affluent women, were followed for five years. A total of 649 were interviewed by telephone about their starting weight when they entered Weight Watchers, their goal weight and their current weight. Another 226 were brought in to be weighed, as a way of seeing how accurate the weights reported over the telephone were.
The average weights reported over the telephone were 3 percent lower than the weights confirmed by scales, so all the self-reported weights were adjusted upward by 3 percent.
The average starting weight was 75 kilograms (165 pounds) — overweight, but not obese. The dieters lost about 10 kilograms (22 pounds), or 13.3 percent, on the program.
Weight Watchers is a method that restricts calories using a point system so that dieters lose a maximum of 1 kilogram (2 pounds) a week. Daily points are spent by eating and earned through exercise. Dieters are weighed during regular support group meetings, where tips and experiences are shared.
Five years later, most of the dieters had regained 5 kilograms (11 pounds), or half their original weight loss.
Half of them still weighed 5 percent less than they had when they had entered Weight Watchers. Only a small fraction of dieters in studies based at universities or hospitals achieve that.
"This is impressive," said Dr. Pierre Lefebvre, emeritus professor of medicine at Liege University in Belgium, who was not connected to the study nor to Weight Watchers.
"Treating overweight and obesity is a very difficult task and every means to achieve the goal is good," he said. "Drugs do not work regularly; drugs have side effects. Exercise is not easy."
"Motivation is very important. Weight Watchers is part of behavioral management of a serious condition and they do it very very well," said Lefebvre, president of the International Diabetes Federation. "It has a place."
Andrew Prentice, an obesity expert from the London School of Hygiene and Tropical Medicine in London, agreed that the results shown in the study are enough to seriously improve health.
"Most of these patients would have gained weight in those five years," said Prentice, who was not connected with the research. "Even if we just stop them gaining weight, that would be success."
Though the success rate seemed better in the Weight Watchers study than in studies run by hospitals or universities, the results cannot be directly compared.
Only successful dieters were included in the Weight Watchers study. Hospital-based studies also usually involve people who are much fatter and who have more severe eating and emotional problems, said lead investigator Dr. Michael Lowe, a clinical psychologist at Drexel University in Philadelphia.
Lowe suggested his findings indicate it may be harder for obese people to maintain weight loss than it is for people who are merely overweight.