They said subcutaneous fat, or fat that collects under the skin, helped to improve sensitivity to the hormone insulin, which regulates blood sugar.

Mice that got transplants of this type of fat deep into their abdomens lost weight and their fat cells shrank, even though they made no changes in their diet or activity levels.

"It was a surprising result," said Dr. Ronald Kahn of Harvard Medical School in Boston, whose study appears in the journal Cell Metabolism.

"We actually found it had a beneficial effect, and it was especially true when you put it inside the abdomen," Kahn said in a telephone interview.

Kahn said he started the study to find out why fat located in different parts of the body seems to have different risks of metabolic disease such as diabetes.

Researchers have known for some time that fat that collects in the abdomen known as visceral fat can raise a person's risk of diabetes and heart disease, while people with pear shaped bodies, with fat deposits in the buttocks and hips, are less prone to these disorders.

Now it turns out that subcutaneous fat fat found just under the skin may be actively protecting people from metabolic disease.

Kahn and colleagues conducted a series of experiments on mice where they transplanted subcutaneous fat from donor mice into the bellies and under the skin of mice.

Mice that got subcutaneous fat transplanted into their bellies started to slim down after several weeks, and they also showed improved blood sugar and insulin levels compared to mice that underwent a sham procedure.

"What we found was that when we put it in either place, there was some improvement in metabolism," Kahn said.

"I think it's an important result because not only does it say that not all fat is bad, but I think it points to a special aspect of fat where we need to do more research," he said.

Kahn's team is working to find the substances produced in subcutaneous fat that provide the benefit with the hope of developing a drug that might copy this effect. Although fat is known to produce several hormones, Kahn said none of the known hormones appeared to be involved in this process.

"If we can capture those (substances), we might have an opportunity to convert them into drugs or use them as guides to help develop drugs," he said.

On the other hand, the build-up of visceral fat has been linked to an increased risk of heart disease and type 2 diabetes.

The researchers found a specific protein is produced in much greater amounts by visceral fat than fat that lies just beneath the skin.

The study, by a US and German team, appears in the journal Cell Metabolism.

The researchers, from Harvard University and the University of Leipzig, analyzed 196 people.

They showed the genes which produce the protein - RBP4 - were up to 60 times more active in obese people with high levels of visceral fat than in lean people.

In contrast, the same genes were only 12 times more active in obese people with a preponderance of fat lying just under the skin.