A single gene can keep in check the tendency to pile on fat, scientists have shown.
Obesity may be at least partly in the genes
The University of Texas team manipulated the gene, called adipose, to alter the amount of fat tissue laid down by fruit flies, worms and mice.
If the same effect could be achieved in humans, which also carry the gene, it is hoped it could lead to new ways to fight obesity and diabetes.
The study is published in the journal Cell Metabolism.
Lead researcher Dr Jonathan Graff said: "From worms to mammals, this gene controls fat formation.
"It could explain why so many people struggle to lose weight, and suggests an entirely new direction for developing medical treatments that address the current epidemic of diabetes and obesity.
"Maybe if you could affect this gene, even just a little bit, you might have a beneficial effect on fat."
The adipose gene was discovered in fat fruit flies more than 50 years ago, but scientists had not pinned down its exact role.
The Texas team used several methods to turn the gene on and off at different stages of the animals' lives and in various parts of their bodies.
Their work suggested that the gene acts as a high-level master switch that tells the body whether to accumulate or burn fat.
Mice with experimentally increased adipose activity ate as much or more than normal mice.
However, they were leaner, had diabetes-resistant fat cells, and were better able to control insulin and blood-sugar metabolism.
In contrast, animals with reduced adipose activity were fatter and less healthy, and had diabetes.
The researchers also showed that gene activity could be turned up or down, not just on or off.
Dr Graff said this increased the potential to manipulate its effect to treat obesity.
The next step will be to probe further the exact mechanisms by which the gene exerts its control.
Dr David Haslam, clinical director of the National Obesity Forum, warned that it could take many years to develop genetic treatments for obesity.
In the meantime, he said, the only way to tackle the problem effectively was to encourage people to eat healthily and take exercise.
"I don't want patients coming to me saying: 'It's not what I eat, it's all in my genes'," he said.
"Don't give my patients another excuse to be victims."