A newly discovered type of "beige" fat could speed up the development of obesity treatments.
The calorie-burning tissue, found in adults, is the third type of fat to be identified.
It is produced from ordinary "white fat" cells that store energy and contribute to excess bodyweight.
In addition, there is heat-generating "brown fat" carried by infants to keep them warm.
Previous research suggested adults also had small amounts of brown fat, which helped to prevent obesity.
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The new work shows that what was previously thought to be adult brown fat is actually genetically distinct "beige fat".
Like brown fat, it has the ability to burn calories instead of storing them in waistline-expanding deposits.
Beige fat is now seen as a key target for new strategies to fight the obesity epidemic.
A natural hormone made by exercising muscles, irisin, stimulates beige fat to burn calories nearly as effectively as brown fat. US scientists have now set up a biotech company to develop irisin as an obesity and diabetes treatment.
Lead researcher Dr Bruce Spiegelman, from Harvard Medical School, said: "We've identified a third type of fat cell. There's white, brown and now there is this third type that is present in most or all human beings."
Dr Spiegelman's team found beige fat cells in scattered pea-sized deposits beneath the skin near the collarbone and along the spine in human adults.
The findings are reported today in the latest online issue of the journal Cell.
"Going forward, it means that what you want to study for potential therapies are the beige fat cells in these 'hotspots' we're all walking around with," said Dr Spiegelman.
The research showed that beige fat cells spring from stem cell precursors of white fat cells. In contrast, brown fat cells emerge from muscle stem cells.
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Beige fat cells behaved like a hybrid of the two cell types, being able both to store and burn calories.
Stimulated the right way, their burning capacity was ramped up, the researchers found. They could be stimulated either by cold or certain hormones, such as irisin.
The Harvard scientists reported the discovery of irisin earlier this year.
Ember Therapeutics, a biotic company founded by Dr Spiegelman, now plans to develop the hormone's therapy potential.