Being cold can burn calories but no one wants to freeze just to sculpt their muffin-top. Soon we may not have to. Researchers have identified immune molecules triggered by cold temperatures that make obese mice lose weight – without the need for the mercury to drop.
Humans and other mammals respond to cold in two ways. On the surface, we shiver to burn energy and produce a quick burst of heat. On a deeper level, as Ajay Chawla at the University of California, San Francisco, and his colleagues recently discovered, cold temperatures send signals to immune molecules called macrophages. They, in turn, release other molecules that convert energy-storing white fat into another type that burns energy.
Babies and some hibernating animals have lots of these energy-burning cells – known as brown fat – but it almost all disappears as people age. We now know that cold temperatures can trigger a "browning" of white fat in adults – converting some of their white fat into an intermediate form called beige fat.
It may seem counterintuitive for our bodies to use up fat stores when we get cold, but think of the white fat as the wooden walls of a log cabin – having them there is a good way to keep warm generally, but when the cold sets in, you're going to want firewood – brown or beige fat, to burn.
Now Chawla's team have identified interleukin-4 and interleukin-13 as the signalling molecules that kick-start the transition of white fat to its darker counterpart.
What's more, by injecting mice with interleukin-4 four times over a period of eight days, the team was able to bypass the physical cold stimulus and activate the pathway biochemically.
Two weeks later the mice, bred to have so much white fat they were classed as obese, had lost 12 per cent of their body weight. Four grams of their body weight was beige fat, whereas they'd had none before, and they were using 10 per cent more energy.
According to Chawla, that's broadly equivalent to what might happen if a generally healthy person added an extra 30 minutes of moderate exercise to their daily routine.
However, in reality, the effect might be less dramatic, says Chawla. While the mechanism is the same, people are less efficient at producing brown or beige fat than other animals, and there is high variability among individuals – for example, women have less than men and obese people have less than people of normal weight.
"But increasing energy expenditure even by a few per cent can still have a huge effect because it's cumulative," he says.
Before this study, energy metabolism was thought to be governed by the brain and the endocrine system, but now it's clear that the immune system is also part of the mix. "This places the immune system in the realm of homeostasis. There's no pathogen – no infection, but the immune system is at work keeping the body's temperature stable," says Chawla.
The immune system is more easily manipulated than the nervous system so it could be a promising path in the pharmacological quest to treat obesity and melt away excess weight, he adds.