Could the cold weather boost weight loss?

University of California-San Francisco (UCSF) researchers have found that the cold temperature calorie-burning effect can be achieved biochemically - without the chill - raising hopes for a weight-loss strategy.

Led by Ajay Chawla, MD, PhD, an associate professor of medicine at the UCSF Cardiovascular Research Institute, researchers determined that two signalling molecules secreted by immune system cells trigger the conversion of fat-storing white fat cells to fat-burning beige fat cells.

Cold activates the immune system and a molecule in fat

Working with mice, it was discovered that the signalling molecules, called interleukin 4 and interleukin 13, activate cells known as macrophages, which in turn drive the fat conversion.

The finding builds on previous work by Chawla's team, which reported that cold activates part of the immune system, and specifically activates interleukin 4 in fat.

In the new study, they determined that both interleukin 4 and interleukin 13 recruit macrophages to fat and that the production of molecules called catecholamines by the macrophages causes the browning of white fat.

When the researchers inhibited interleukin 4 signalling in white fat, they found that the mice made less beige fat, burned less energy, and could no longer maintain normal body temperature in the cold.

How does shivering help you stay slim?

We shiver in the cold to keep warm, but cold also triggers the growth of fat cells that burn fuel, instead of the fat cells that store it.

If you keep humans indoors at 16 degrees to 17 degrees Celsius, without allowing them to bundle up, they adapt by generating more fat-burning cells to help them keep warm. In other words, they start losing weight!

Although many researchers now believe that the potential to exploit brown fat for weight loss is significant, the amount of individual variation when it comes to brown fat reserves and the potential to generate more brown fat is unclear.

"We don't know what the dynamic range is," Chawla said. "It appears that women have more, that we have less as we age, and that obesity is associated with having less brown fat."

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