Until recently, most scientists thought that adults had no brown fat. But in the past few years, scanty deposits teaspoonfuls, really of the tissue have been detected in the necks and upper backs in many adults. This is important because brown fat, unlike the more common white stuff, is metabolically active. Experiments with mice have shown that it takes sugar out of the bloodstream to burn calories and maintain core temperature.
A similar process seems to take place in humans. For the new study, published in June in Diabetes, researchers affiliated with the National Institutes of Health persuaded five healthy young male volunteers to sleep in climate-controlled chambers at the NIH for four months. The men went about their normal lives during the days, then returned at 8 pm. All meals, including lunch, were provided, to keep their caloric intakes constant. They slept in hospital scrubs under light sheets.
For the first month, the researchers kept the bedrooms at 75 degrees Fahrenheit, considered a neutral temperature that would not prompt moderating responses from the body. The next month, the bedrooms were cooled to 66 degrees, a temperature that the researchers expected might stimulate brown-fat activity (but not shivering, which usually begins at more frigid temperatures). The following month, the bedrooms were reset to 75 degrees, to undo any effects from the chillier room, and for the last month, the sleeping temperature was a balmy 81 degrees. Throughout, the subjects blood-sugar and insulin levels and daily caloric expenditures were tracked; after each month, the amount of brown fat was measured.
The cold temperatures, it turned out, changed the mens bodies noticeably. Most striking, after four weeks of sleeping at 66 degrees, the men had almost doubled their volumes of brown fat. Their insulin sensitivity, which is affected by shifts in blood sugar, improved. The changes were slight but meaningful, says Francesco S Celi, the studys senior author and a professor at Virginia Commonwealth University.
Just by sleeping in a colder room, they gained metabolic advantages that could, over time, he says, lessen their risk for diabetes and other metabolic problems. The men also burned a few more calories throughout the day when their bedroom was chillier. The metabolic enhancements were undone after four weeks of sleeping at 81 degrees; in fact, the men had less brown fat than after the first scan.
- GRETCHEN REYNOLDS