The drink seems to stimulate a type of fat in the body known as 'brown fat' or brown adipose tissue. The fat helps generate body heat by burning sugar and fat, often when the temperature drops, whereas white fat is the consequence of storing excess calories.
But other than a cold day, nobody has worked out how to stimulate brown fat—although, until recently, scientists believed only babies and hibernating animals possessed it. Brown fat in adults is mainly found around the neck.
Researchers from the UK's University of Nottingham used stem cells from brown fat and thermal imaging to demonstrate it responds to coffee. "In earlier studies, we were able to image someone straight after they drank coffee to see if the brown fat got hotter," said researcher Michael Symonds.
The researchers aren't sure if it's the caffeine in coffee or one of its other compounds that stimulates the fat. They are next planning to test caffeine supplements to see if they also stimulate the fat.