Scientists have discovered an elusive, alternative type of fat in adults which could hold the key to effortlessly shedding excess kilos.

Researchers in the Netherlands have found evidence of "brown fat" working in a group of young men, a discovery which runs counter to the conventional view the unique substance disappears during childhood.

"The function of brown fat in infants is to burn energy to produce heat, to maintain body temperature," Sydney-based Dr Tim Gill said of brown fat's amazing extra ability over ordinary white fat.

"In your body you use energy to grow and move and breathe (and) if you want to burn up more energy you have to exercise, that's the only thing you can do."

"But with brown fat it uncouples that, it can burn off energy just to produce heat."

In short, white fat is an energy store which is used up during human movement and body functioning.

Brown fat can take these stores and convert it straight into heat, causing a rise in body temperature but the process is otherwise effortless.

Dr Gill said if scientists could find a way to harness the mechanism used by brown fat, people could quit their gym and instead flick an internal switch to "waste" those excess calories as heat.

"Up till now we've not found effective way to do that that is safe," he said.

"The body has mechanisms for wasting energy when necessary but normally it is extremely good at conserving energy (i.e. as conventional white fat)".

The Maastrich University study, published in the New England Journal of Medicine, took 24 men and split them into "lean" and "overweight or obese" groups.

The men were exposed to a pleasant 22 degrees celsius, and they were put through a range of complex tests designed to shown any heating effect from brown fat.

The tests were run again at a cool 16 degrees, and all but one of the men showed evidence of brown fat in operation.

The overweight men also showed a significantly weaker response, leading researchers to conclude "the percentage of young men with brown adipose tissue is high but its activity is reduced in men who are overweight or obese".

Dr Gill said this could be because men who had lower levels of brown fat found it easier to put on weight.

"This is one study which is suggesting we need to revise our thinking which dismissed the role of brown fat in obesity and weight gain," he said.

There's also bad news for dieters - there's no way to replace fat in our food with this good brown fat.

"It's like being tall, you can't change your height - brown fat is something you inherit," he said.

Dr Gill is Associate Professor at the University of Sydney's Institute of Obesity, Nutrition and Exercise.

- AAP