Central heating feels good on blustery winter nights, but a new study by researchers at University College London suggests that keeping your house too balmy may be making you fat.
(More on Time.com: 5 Ways to Improve Your Diet on the Cheap)
The idea is that exposure to cold prompts humans to generate their own heat — by shivering, for instance, in extremely cold conditions — which, in turn, helps burn calories. When it’s already toasty indoors, we don’t have to expend any extra energy to get comfortable. The New York Times reports:
And even in mildly cold conditions, like in a chilly room with the thermostat turned down to the lower 60s, people generate extra heat without shivering. The process, called non-shivering thermogenesis, may involve a substance called brown fat that adults carry in certain areas, like the upper back and side of the neck. Unlike regular fat, which stores excess energy and calories, brown fat acts like an internal furnace that consumes lots of calories, but it has to be activated first — and cold temperatures do that.
The authors of the new study, published in the journal Obesity Reviews, note that average indoor temperatures have risen steadily in the U.K. and U.S. over the last several decades, as central heating has become increasingly available — and rates of obesity have risen too. The average temperature in British living rooms went from 64.9 degrees F to 70.3 degrees F, from 1978 to 2008. Living rooms in the U.S. have long been heated to at least 70 degrees F. Indeed, average temperatures have gone up all throughout the house — and in the wintertime, people tend not to leave their homes much anymore, at least not unless it’s in a heated car. (More on Time.com: 5 Fitness Apps to Get You Off the Couch)
“Increased time spent indoors, widespread access to central heating and air conditioning, and increased expectations of thermal comfort all contribute to restricting the range of temperatures we experience in daily life and reduce the time our bodies spend under mild thermal stress — meaning we’re burning less energy,” said lead author Fiona Johnson in a statement. “This could have an impact on energy balance and ultimately have an impact on body weight and obesity.”
Although humans are born with significant deposits of brown fat — the primary purpose of which is to regulate body temperature by burning energy for heat — those stores diminish over time. By adulthood our brown-fat stores have shrunk, having been replaced with the more familiar white fat, the stuff that hangs over belt buckles and swings from the backs of arms. (More on Time.com: 5 Weight Loss Apps that Work)
Studies show, however, that brown-fat activity can be triggered even in adults. So the Times asks: “Could lowering the thermostat make a notable difference in people’s weight?”
Dr. C. Ronald Kahn, a Harvard Medical School professor who does research on brown fat, says it might actually help with weight control over time, provided people stick with it.
“When we put people in a 60-degree room, they increase their energy expenditure by 100 or 200 calories a day if they’re in light clothing,” like hospital scrubs, he said. “They’re not shivering. They activate their brown fat.”
Over a period of several weeks, they will have burned an extra 3,500 calories, which translates into the loss of one pound. Wearing a sweater will dilute the effect.
The problem, Dr. Kahn said, is that “most people won’t stay at that temperature for very long.”