While a warm, toasty house may feel great in the winter, some researchers suggest warm temperatures also may play a role in the obesity epidemic.
Fiona Johnson of University College London and her colleagues analyzed a number of studies that examined the relationship between exposure to cold temperatures and the ability to burn off energy. Their research is published in the journal Obesity Reviews.
They found evidence that over the past several decades, people in the U.S. and the U.K. have been steadily raising the temperatures in their homes.
They also found indirect evidence that the body's response to cold, which consists of shivering and hormonal actions, plays a major role in energy expenditure. Regulation of body temperature, they say, is associated with weight.
"Both genetic mutations and ablative lesions that result in abnormal energy expenditure contribute to the development of obesity in laboratory animals," the authors wrote.
"As ambient temperatures go down, people tend to move around and have more adaptive behaviors that allow them to burn more energy," said Dr. Peter McCullough, chief academic and scientific officer at St. John Providence Health System in Detroit. McCullough was not involved in the current research.
But other weight loss experts said the connection between cold temperatures and weight isn't yet very strong and they can't say for certain whether there's any association with obesity.
"It's possible that people exposed to cold temperatures induce a process that can lead to weight loss," said Dr. Robert Eckel, professor of medicine at the University of Colorado's Anschutz Medical Campus in Aurora. "I don't know if it has implications for obesity."
Experts do agree that one of the study's findings raises interesting questions about the role of a type of tissue that previously received little attention. The authors found that brown adipose tissue, a kind of fat, plays a very significant role in burning energy when exposed to cold.
"Up until a few years ago, we didn't think it did very much in adults," said Dr. Jana Klauer, a physician in private practice in New York.
Klauer said small animals and babies have brown adipose tissue, but as people get older, they tend to lose it. Studies have shown, however, that some adults do have it, though it hasn't been determined how common it is.
Klauer and Eckel cited a recent study done in Japan that exposed two groups of men to cold. One group had brown adipose tissue and the other didn't.
"They found that in people that had the brown fat, energy expenditure went up 400 more calories per day when they were exposed to cold," said Klauer. "They were using 400 more calories to generate heat."
The results suggest that weight loss in cold temperatures may be easier for people with brown adipose tissue.
"It's not going to make a difference whether you're exposed to cold or a neutral temperature if you don't have any brown adipose tissue," said Klauer.
It's not easy to determine whether a person has this type of tissue. It involves injecting fructose into the body, which then is taken up by the brown adipose tissue.
Experts say the findings are interesting and could lead to a whole new area of obesity research.
"One obvious experiment is to see what happens when temperatures in obesity-prone populations are lowered," said Eckel.
"Establishing the significance and magnitude of the effects of both short-term and long-term thermal exposures on body weight could lead to the development of novel therapies to address obesity on an individual and a population level," the authors wrote.
McCullough said many doctors are telling patients to turn down the thermostat to help lose weight, but Klauer said it's way too soon to send the message that the cold weather can have an impact.
"People shouldn't take away that they'll lose weight if they don't wear a jacket," she said.