Brown fat burns energy when we are cold. Now scientists are trying to harness its powers to fight obesity and type 2 diabetes.

When the nights draw in, the leaves fall off the trees and the outside world becomes blanketed in frost, many of us will be tempted to reach for the thermostat and crank up the heating. When venturing outdoors, we'll likely wrap up in a scarf and hat, piling on the layers in a desperate attempt to keep warm. Yet being cold and uncomfortable could be the key to improving health and losing weight.

The reason is all down to a special type of fat called brown fat. Unlike white fat, which clings stubbornly to thighs, waists and hips, brown fat burns calories, converting them into heat when we are cold. Some believe that if harnessed properly it could make us thinner. Intriguingly, all we theoretically need to do to activate it is to hop into a cold shower or eat lots of spicy foods and caffeine.

But before you jump into an icy lake or down that fourth cup of coffee, it is worth exploring whether the hype behind brown fat is justified.

When we are born, each of us has plentiful stores of brown fat which act like an internal heater. Babies don't have enough muscle mass to shiver, so they rely on brown fat to convert sugars and fats into heat.

Brown fat cells have a disproportionately high number of mitochondria – the energy producing factories inside cells. But unlike normal mitochondria which produce adenosine triphosphate (ATP) – a kind of universal energy currency the body can use – the mitochondria inside brown fat cells contain a protein called thermogenin, or UCP1, which allows them to convert calories from food directly into heat instead.

"When stimulated, brown fat has the capacity to produce 300 times more heat per unit mass than any other tissue or organ in the body," says Michael Symonds, professor of developmental physiology at the University of Nottingham in the UK.

Most of what we know about this brown fat comes from studies on small mammals such as mice and rats. These rodents have large reserves of it, which helps them keep toasty and warm during the winter months when they go into hibernation.

"For our ancestors, or for small animals like mice, cold temperatures would have been a major threat to survival, so it was advantageous to have a tissue that could convert energy sources into heat," says Paul Cohen, an associate professor at The Rockefeller University, New York, who studies molecular metabolism.

Decades of research shows that, in mice at least, brown fat sucks up sugars and fats from the bloodstream. Brown fat also appears to protect mice from obesity and metabolic diseases linked with weight, such as diabetes and heart disease.

Adult humans are, however, adept at finding other ways to keep warm – turning the heating on, snuggling under a thick duvet or pulling on an extra-warm coat. It was widely assumed that brown fat all but disappeared by puberty for this reason, with the traditional white thigh-hugging fat we are all familiar with taking its place.

And this also meant that most scientists believed it was impossible for brown fat to play a meaningful role in health beyond infanthood. This changed in 2009, however, when researchers from Finland and Sweden showed that human adults not only still have brown fat, but in temperatures below 16C (60.8F), the fat activated and started absorbing glucose and fats from the bloodstream.

Not only that, there was a clear correlation between a person's body weight and their brown fat reserves. Thinner, leaner individuals had more, and obese people less. Those with more brown fat also had higher metabolic rates in the cold. Together, this suggested that brown fat could be a target for the treatment of obesity.

The field received a boost in 2021, thanks to a study by Cohen and colleagues at The Rockefeller University in New York. Cohen and his team examined positron emission tomography (PET) scans of over 52,000........

© BBC