Scientists have used gene therapy to transform fat-storing cells into fat-burning cells.
Obesity is a widespread problem
They hope the finding, published in Proceedings of the National Academy of Sciences, may one day lead to new treatments for obesity.
The researchers, from Switzerland and the US, triggered the change by adding a protein called leptin to cells.
Rats given the treatment shed weight dramatically without any apparent side-effects.
Some doctors have used leptin injections to treat people with extreme obesity. However, the work is still at an experimental stage.
The new study further raises hopes that leptin treatment will eventually become more widespread.
The researchers focused on rats bred to be genetically predisposed to develop diabetes.
They injected the rats with a virus containing the leptin gene. The rats decreased in weight from an average of 280 grams to 207 grams in 14 days.
They also ate 30% less food, but remained healthy and active.
Microscopic evaluation of individual fat cells found that the cells shrank in size, and developed more energy centres called mitochondria.
In addition, levels of enzymes known to promote fat metabolism increased while those that impede fat metabolism decreased.
The rats showed no signs of the side effects associated with fat loss induced by starvation or insulin deficiency. These can include loss of lean body mass, hunger and the build up toxic substances called ketones in the blood.
The researchers also found that when they force-fed rats, those given the leptin gene put weight on at a slower rate.
The researchers stress that more work is needed to reveal the precise mechanism behind the changes.
However, they suggest that the results may have important implications for the treatment of obesity in humans.
Writing in the journal, they say: "The fat loss induced here was far more rapid and profound than can be induced by caloric restriction."
Lead researcher Dr Roger Unger, director of the Touchstone Center for Diabetes Research at the University of Texas Southwestern Medical Center in Dallas, said: "The structure of the cells changed from the normal appearance of a fat cell to a very novel cell that's really never been seen before.
"There's no precedent for a cell that appears like this."
Leptin is normally produced by fat cells, or adipocytes, but is somehow prevented from interfering with the accumulation of surplus fat.
Scientists believe this is to ensure fat cells maintain their vital function of storing fuel during times of food shortage.
Dr Unger said: "We hope to identify the barrier to leptin action erected by the obese fat cells.
"Then, if we could overcome or bypass the block, we could transforn fat fat cells as we did normal fat cells and prevent and/or reverse the obesity."
However, Dr Unger stressed much more work was required before this could be achieved.
Dr Andrew Hill, chairman of the Association for the Study of Obesity, told BBC News Online a quick and effective treatment for obesity would be extremely welcome.
But he said: "There is an enormous distance between what these researchers have done and a GP using it to treat 400-500 patients on their list for obesity."
Dr Hill said many scientific advances had promised the prospect of a quick fix for obesity, but for most people the only solution was to work hard at maintaining a low weight throughout their life.