Obese people are being warned they face a virtually impossible battle to lose weight because their bodies are programmed to regain any kilos they shed.
Professor of medicine at the University of Melbourne, Joseph Proietto, said once someone lost weight, changes in both energy expenditure and hunger-controlling hormones encouraged weight regain.
"It is likely that it is these physiological adaptations that make it so difficult to maintain weight loss," he wrote in an article published by the Medical Journal of Australia.
"Importantly, if this regulatory mechanism is operating in those who are already obese, public health messages encouraging people to eat healthy food and to exercise are unlikely to have long-term impact on their weight.
"Several studies have shown that although obese people who make the effort can achieve and maintain significant weight loss for one to two years, the weight is usually regained in the longer term."
Professor Proietto said obesity was often attributed to people eating too many high-calorie foods and not exercising enough.
But he noted not everyone who indulges in that type of behaviour becomes obese.
He said this was largely because the body acts to prevent obesity by increasing the levels of a protein hormone, leptim, as fat accumulates.
But when that process failed, people were likely to become obese.
Professor Proietto said such cases were rare and more people were likely to become obese because of genetic changes caused by environmental factors, either while a baby was still in the womb or in their early years.
He said the battle against obesity should focus on stopping children from becoming obese in the first place and making bariatric surgery more widely available as it was nearly impossible for obese people to lose weight any other way.