If you follow health research these days, you might have the impression that what makes you feel good inevitably either causes cancer or makes you fat.
On the surface, research on the links between indoor temperature and obesity fit right into this puritan streak. According to this argument, spending all our time in climate-controlled buildings that seldom deviate from a small range of comfortable "thermic-neutral" temperatures means we burn fewer calories than we would if we were forced to endure periodic bouts of cold or heat.
But a pair of new studies offer a more cheerful perspective, suggesting that subtle tweaks of the thermostat can ramp up fat-burning – and you don't have to subject yourself to uncomfortable extremes to get the benefits.
The new studies, published last month in the Journal of Clinical Investigation, focus on the role of "brown fat," an unusual type of tissue that was only conclusively identified in humans in 2009. Found primarily in pea-sized deposits in the upper back, brown fat functions like a furnace, burning calories to produce extra heat.
The thermic-neutral point for humans – that is, the ambient temperature at which you don't have to spend any extra energy warming or cooling yourself – is usually somewhere around 23 C. The farther away from that point you get, the more energy you have to spend to keep your core temperature stable, usually by shivering or sweating. But brown fat offers a more subtle way of keeping warm, kicking in long before you feel the need to shiver. Studies have found that brown-fat activity is lower in obese people and declines as you age.
In one of the new studies, researchers at Hokkaido University in Japan put 12 volunteers in a 17 C room for two hours a day for six weeks. During that time, brown-fat activity as measured by a PET-CT scan increased by 58 per cent and body fat decreased by 5.2 per cent; in a control group that didn't undergo cold exposure, both measures were unchanged.
The second study, by researchers at Maastricht University in the Netherlands, exposed lightly clothed volunteers to temperatures of 15 to 16 C for six hours a day for 10 days. Once again, brown fat activity ramped up – and just as importantly, the subjects felt warmer and more comfortable by the end of the study even though the temperature of the room was the same. After just 10 days, they'd acclimated to the cool temperatures.
So the links between temperature and energy consumption are real. But are they significant?
Scraps of circumstantial evidence have been accumulating. A British study in 2011 noted average bedroom temperatures there have risen by about three degrees since the 1970s. And a six-year study of 1,600 Italians, also published in 2011, found those who kept their houses warmer than 20 C were twice as likely to become obese during the study.
Earlier this year, an analysis of obesity rates in the U.S. found that obesity was least prevalent in the counties with the hottest and coldest average temperatures. That suggests being a little warmer than comfortable may have similar effects to being a little cooler, though researchers have yet to test this possibility in controlled experiments.
On the other hand, the effects observed so far are subtle at best. No one, including these researchers, believes cooler offices and bedrooms will eradicate obesity. Instead, ambient temperature joins a long list of factors – work stress, sleep patterns, urban design, convenience food and so on – that combine to create an environment that is "obesogenic." Changing one factor won't fix the problem, but it's a start.
There's also the risk that turning down the thermostat could trigger hunger that causes you to take in more calories than you save, warns Dr. Wouter van Marken Lichtenbelt, the lead author of the Maastricht study. As a result, he suggests starting with small changes: "With the knowledge we have so far, I would recommend to just lower the temperature one to two degrees."
Next year, van Marken Lichtenbelt and his colleagues plan to start another study in which subjects will lower the temperatures in their homes, which will offer a more real-world perspective on whether this tactic will work.
Until then, the most useful message from the study is that you don't need to shiver in an ice-bath to get your brown fat going. You just need to push gently at the edge of your comfort zone and remember that after a week or two, the new temperature should start to feel more comfortable.
And if that doesn't work, you can always use the money you save on heating to hire a personal trainer.
Alex Hutchinson