They have the opposite function of white fat cells, which are used to store excess calories for energy.
The brown fat cells, which scientists initially thought were unique to babies and hibernating animals, also exist in varying levels in adults.
Those with a lower body mass index (BMI) tend to have a higher percentage of brown fat.
Now scientists hope that understanding what stimulates brown fat could lead to new weight regimes and treatments for obesity and diabetes.
Researchers at the University of Nottingham performed stem cell studies to see if coffee - or the caffeine in coffee - could activate brown fat cells.
Once they found an effective dose on stem cells, they ran the tests on humans using a thermal imaging technique that traces the body’s brown fat reserve and assesses its capacity to produce heat and therefore burn calories.
Professor Michael Symonds, who co-authored the study published in the journal Scientific Reports, said: “Brown fat works in a different way to other fat in your body and produces heat by burning sugar and fat, often in response to cold.
“Increasing its activity improves blood sugar control and the extra calories burnt help with weight loss.
“However, until now, no one has found an acceptable way to stimulate its activity in humans.”
He added that the researchers are now running tests to work out whether it is the caffeine or another ingredient in the coffee that activates the brown fat.Daily Mail