In a report in the International Journal of Obesity, they note the focus of such programmes may need to change if they're really going to have a lasting effect.
To test the potential impact of different diets, the researchers ran two computer simulations: one included a low-fat diet, the other a diet rich in whole grains and vegetables plus 180 minutes of exercise per week.
According to the models, people lost an average of eight to 12 pounds on the diets and kept the weight off for an average of six months. But the pounds slowly crept back on, and after less than six years, the dieters were back where they started - negating any improvement in health from the weight loss.
In addition, the researchers estimate that only about 3% of Australia's population would participate in weight-loss programmes.
"Interventions that try to change the behaviour of individuals but do nothing about the environment in which these people live, are likely to have modest and temporary effects at best," said study author Lennert Veerman, who studies population health at the University of Queensland in Brisbane.
Not a solution
"They are not the solution for the obesity epidemic - more, and different, interventions have to be taken," he added.
Along with continued counselling to help people keep weight off long-term, Veerman suggests a 'junk food tax' and better nutrition labelling. These did seem to have an overall positive effect on health in a parallel study, published in the same journal.
In the US, 73% of adults are overweight or obese, according to the Centres for Disease Control and Prevention.
The extra pounds increase the risks for a multitude of ailments, including diabetes, heart attack, stroke, high blood pressure, and some types of cancer. And according to the surgeon general, they rack up a yearly bill of $117 billion in the US, counting health care spending and lost productivity.
"We understand that weight loss programmes that we have now are really helpful for individuals," said Hollie Raynor, associate professor of nutrition at the University of Tennessee in Knoxville. "But the challenge seems to be, how do we help individuals maintain this weight loss over time?"
Financially speaking, the Australian study shows a dieter who also exercises would save an average of $1,088, and someone in the low-fat diet program would save about $1,040 in health care costs.
Nonetheless, Veerman said, "the fact that not everybody (who's) overweight wants to, or can, participate reduces its overall effect on the overweight-related burden of disease in the population."(Reuters Health/ January 2011)