Using a PET camera that identifies a radioactively labelled glucose analogue, the researchers found brown fat in the lower part of the neck, just above the clavicle. This technique is normally used to locate metastases (daughter tumours), and false-positive results have previously been observed in such examinations in the area around the neck, cells that have been suspected of being brown fat.

"We have been able to show that this is the case for the first time, by taking biopsies from the area and examining morphology, biochemistry and gene expression. The analyses were in precise agreement with those for brown fat from rodents," says Sven Enerbäck, who has directed the study.

Brown fat cells contain large quantities of mitochondria, which convert energy to heat and also give the adipose tissue its characteristic colour. Brown fact is present in significant quantities in animals that hibernate, to enable them to retain heat properly.

"Cold activates the sympathetic nervous system and the formation of catecholamines in the adrenal medulla, which in turn activates UCP1, uncoupling protein 1, which in the mitochondria of brown fat cells generates heat," says Dr Enerbäck.

In the study, healthy volunteers were examined using a PET camera, after staying at an outdoor temperature of 22 degrees and after staying lightly dressed at a room temperature of around 17 degrees. A clear difference was seen in the quantity of active brown fat, with a clear increase after staying in the somewhat cooler room.

"The uptake of glucose, which is a pretty pure energy source, then increased around 15 times. According to our calculations heat equivalent to the energy content of four kilos of white adipose tissue is burnt annually with the quantity of activated brown fat we have measured.

An appealing idea is to develop new methods of treatment for obesity that focus on increasing the body´s burning of fat instead of reducing energy intake, Dr Enerbäck feels.

"Obesity is a complex condition, and several different measures will probably be required, but increasing the amount of active brown fat might be one way. We are now trying to find out which genes control the formation of brown fat.

By:Felicia Lindberg (www.mentoronline.se)