Sunholm. A hormone that tricks the body into believing it has been exposed to cold temperatures causes mice to lose weight.

It sounds like every scam diet you've ever had advertised at you, but physiologists at the University of California, San Francisco are working on a way to cause fat to burn more calories. So far their success is restricted to rodents, but it's a start.

Humans have two types of fat, the white adipose tissue that dominates in adults, and brown adipose tissue which provides babies protection against the cold by burning food. Adults exposed to cold will produce a mixture known as beige fat. Ajay Chawla says cold is “the only stimulus we know that can increase beige fat mass or brown fat mass.” Strangely, among all the marketing one thing you don't see is people offering you an icebath for a low, low price.

However, Chawla wondered if there was a way to trick the body into thinking it was being exposed to cold. In 2011 he revealed the role of immune cells called macrophages in the process, with low temperatures activating the macrophages to turn fat brown. Mice lacking the interleukin-4 and 13 proteins develop fewer beige fat cells when exposed to cold than other mice.

While this may be a great way to produce obese mice, the far more pressing question is whether the process can be reversed. That is, would extra IL-4 or IL-13 result in more brown fat, and therefore less fat in total.

Now Chawla has announced his results in Cell. “The events that lead from the sensing of cold to the development of beige fat remain poorly understood,” the paper notes. Nevertheless Chawla and his colleagues report that, “administration of IL-4 to thermoneutral mice increases beige fat mass and thermogenic capacity to ameliorate pre-established obesity. Together, our findings have uncovered the efferent circuit controlling biogenesis of beige fat and provide support for its targeting to treat obesity.”

Mice given extra IL-4 produced 15 times as many brown fat cells as other mice. This increase their energy expenditure for a particular body mass and level of activty by 15-20% even at 30°C.

Chawla's graphical representation of the way macrophages and cold temperatures produce energy burning brown fat cells.

The same edition of Cell carries another paper by a separate team at Harvard who have gone upstream, finding that the hormone Metrnl increases IL-4 concentrations, as well as some of the brown fat generating macrophages.

While the researchers are keen to see if the results can be repeated in humans, ads for a product claiming to be Metrnl will probably be appearing all over the web well before this is confirmed, let alone safety established.

The Harvard team's representation of their work showing the role of Metrnl