Scientists have discovered why it is often harder to keep weight off than to lose it in the first place.
Many people fail to stay slim
A team at New York's Columbia University has shown the key is falling levels of the hormone leptin, which controls appetite.
They found that giving people who had recently lost weight injections of the hormone helped them to avoid putting the pounds straight back on.
The study features in the Journal of Clinical Investigation.
It is estimated that more than 85% of obese people who have lost weight eventually put at least some of it back on.
Research suggests this is due to a number of changes in the functioning of the body's metabolic, hormonal and nervous systems.
The Columbia team believe these changes are governed by low levels of leptin.
The hormone is made in the fat tissue, and so when a person loses weight their leptin production falls off.
Conversely, putting weight back on should raise leptin levels, and start to reverse the changes that made weight gain more likely.
To test their theory, the researchers gave doses of leptin to lean and obese volunteers who had recently lost weight.
They found that most of the metabolic and hormonal changes which mean people cannot keep the weight from creeping back on were reversed once leptin levels were restored to pre-weight loss levels.
Leptin is known to play a role in controlling appetite, but as yet the exact way that it works is unclear.
Injections of leptin have been used to help morbidly obese people with a deficiency of the hormone to lose weight, but a similar approach has no effect on obese people with normal leptin levels.
New drug hope
The researchers said it might eventually be possible to develop new drugs to keep weight off that work by targeting the way the body monitors leptin levels.
Lead researcher Dr Michael Rosenbaum told the BBC News website that historically it made sense our ancestors to defend their fat reserves, as they were often subjected to periods when food was scarce.
"We would predict that the human genome is heavily enriched with genes that defend body fatness and relatively lacking in genes that would oppose weight gain.
"We essentially have lived through hundreds of thousands of years of an environment that would encourage us to eat more and move less to preserve energy stores.
"We are now in an environment where those traits are maladaptive."
Dr Ian Campbell, chairman of the National Obesity Forum, said it was possible that medication could eventually be developed to target the leptin system.
However, he said: "The common denominator among people who do manage to maintain weight loss is a continuation of physical activity.
"There may be room for medication at some point, but you need to look at lifestyle factors first, and the most important way to manage your weight is to keep a check on your diet, and to take regular physical activity."