Fat may be a villain of our time, but it's not fair to paint it all with the same brush. There are actually several types of the stuff – white fat, which stores excess energy, and brown fat, which the body readily burns to keep warm. Now, researchers at the Salk Institute have identified the protein that allows brown fat to expend energy, opening up a new potential avenue for treating obesity and its related diseases.

Scientists once thought only babies possessed brown fat, and used it as a way to keep warm until they became able to shiver. But it was discovered in adults in 2009 and has been the subject of intense study ever since. Brown fat cells have far more mitochondria – the energy-producing components of cells – than their white counterparts, and this is what allows them to burn energy more efficiently. Scientists have been hunting for a so-called "fat switch", which might be able to convert white fat to brown, and found promising leads in fish oil and nanoparticle injections.

Following suit, the Salk scientists set out to study how brown fat is maintained, even when the body isn't exposed to cold. The team looked into the role of a gene known as estrogen-related receptor gamma (ERRγ), which is highly expressed in brown fat.

The team found that brown fat cells constantly express ERRγ, while white fat cells don't express it at all. The researchers also studied which other genes ERRγ triggers, and were able to link several which are known to be related to brown fat.

"We uncovered the factors that are both involved in protection against the cold and underpin brown fat identity," says Michael Downes, co–senior author of the paper.

To test the difference between white and brown fat cells, the researchers engineered mice that lacked the ERRγ gene completely. They found that in these mice, the brown fat cells didn't look much different to regular old white ones, and the mice weren't able to regulate their body temperature under chilly conditions. None of the engineered mice were able to tolerate exposure to cold, as compared to 80 percent of the normal mice that shrugged it off.

"This not only advances our understanding of how the body responds to cold, but could lead to new ways to control the amount of brown fat in the body, which has links to obesity, diabetes and fatty liver disease," says Ronald Evans, senior author of the study.

The research was published in the journal Cell Reports.

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