A small adjustment to your thermostat may do more than just save the planet; it could also help trim your waistline.

THE GIST

Turning down the thermostat or increasing exposure to cold temperatures could help weight loss.

Cooler external temperatures force our bodies to expend more energy to maintain body temperatures.

Even though this effect hasn't yet been proven, it's a strategy with no downside that benefits the environment.

If saving energy isn't enough to convince you to turn down the heat a notch, perhaps this will: Colder temperatures may help you lose weight.

In an article published in Obesity Reviews, Fiona Johnson of University College London and colleagues gather evidence in support of the notion that upwardly creeping indoor temperatures and reduced cold exposure may be a contributor to rising obesity.

Although no studies yet address the question directly, several threads of evidence "seem to suggest that increases in indoor temperatures could be having a significant effect on body weight," Johnson said.

"I think it's quite important to say that we wouldn't expect that this is the major contributor to obesity," she added. Still, many researchers believe that conventional explanations regarding diet and exercise are not enough to fully explain the obesity problem, so this may be part of the picture.

Johnson and colleagues document that household heating has increased in both the United States and the United Kingdom over the last decades.

"What we're seeing is not only people turning up their thermostats a degree or two, but people heating the whole of the home, rather than just certain areas," Johnson said, "People used to turn their heat off at bedtime."

This, combined with adults and children spending less time outdoors in cold temperatures commuting or playing, means people are probably not exposed to as much cold as they used to be, the researchers note.

This reduction could affect how many calories we burn in two ways.

First, it means we use less energy just maintaining our body temperature.

"As the temperature goes down below 27 degrees Celsius (80.6 Fahrenheit), energy expenditure increases," Johnson told Discovery News. "That's simply the expenditure of the body staying warm."

The second effect is that without cold exposure, our inner furnace seems to reduce its ability to stoke our internal fires.

This furnace comes in the form of our stores of brown fat, a type of fat distinct from white fat, which is just stockpiled calories. Brown fat burns energy to create heat, and studies have shown that obese people have less brown fat than thin people, Johnson said.

Babies are born with a lot of brown fat, and those amounts decrease over time. "For a long time it was thought that adults didn't have enough to create a significant effect," she said. Now researchers have found otherwise.

Brown fat becomes activated in response to mild cold and begins to create heat, burning calories. But, "it's use it or lose it," Johnson said. Reduced exposure to cold reduces stores and decreases their effectiveness at burning energy.

Johnson notes that gains from turning down the heat would be tempered by people's natural responses to feeling chilly: seeking more clothes and more food. But evidence suggests layering up and eating more don't completely negate the extra energy expenditures from cold exposure, she said.

All of this points to a connection between shrinking cold exposure and expanding waistlines. However, Johnson notes that direct evidence still is lacking, as is information on how cold one would have to be or for how long to have what effects.

"It is perhaps too early to tell people to turn down the thermostat or make sure they get cold," she said.

It is certainly a case for doing studies to expose people to cold and seeing what effect this has on brown fat levels, energy expenditure, capacity to create heat, and body weight.

Small effects could add up, said Arne Astrup of the University of Copenhagen.

"Even 100 calories a day can mean a lot over a year or in the long term," Astrup said, citing a 2003 Science paper that suggested that reducing net calorie intake by 100 per day could prevent weight gain for most of the population.

"This paper provides some quite good evidence that (cold exposure) is something we should consider," he added. It would be easy to comply with compared to diets or workout regimens, he noted, and comes with other benefits.

"It will save you energy and money, but it will also be more healthy," he said.