(Inside Science) -- If you're the type of person who gets anxious about going to the gym, you may be already partway to your weight-loss goal. New research shows that mild psychological stress can activate calorie-burning brown fat, and may even help you lose weight.
Brown fat is different from the white fat that makes up most of the fat in your body. Rather than storing excess energy for later, brown fat's job is to burn energy quickly to produce heat. Until recently it was thought that only newborn babies and hibernating animals had much brown fat, but we now know that adults also have brown fat, mostly around their collarbone and neck.
The fat can be activated by cold temperatures, but Michael Symonds, a physiologist at the University of Nottingham in the U.K., noticed during testing that people's temperatures would begin to rise even before they were exposed to the cold -- the anticipation seemed to be enough to "turn on" the brown fat. So he devised a way to test whether stress is one of the triggers that can induce brown fat to begin burning energy.
Symonds took five lean, healthy young women and subjected them to a short mental arithmetic test. To add to the stress, he sometimes told the subjects that they had gotten a question wrong, even when they were right. Using an infrared camera, he measured the change in skin temperature around the brown fat deposits in their chest and neck before, during and after the test, and took saliva samples to measure levels of the stress hormone cortisol. A few days later they came back and repeated the experiment, but instead of doing a math test viewed a relaxing video.
They found a strong correlation between the levels of cortisol and the skin temperature near brown fat deposits -- the higher the levels of the stress hormone, the higher the temperature of the fat. Surprisingly, the temperature and cortisol didn't go up further during the test. Higher cortisol levels before the test resulted in a high initial temperature going into the test. The results were published today in Experimental Physiology.
"The mild psychological stress of just coming in to the study knowing that you're going to have to do these math questions was sufficient to raise their cortisol, and we think that was the trigger for switching on the brown fat," he said." Having the brown fat already activated, it couldn't be further stimulated by the math test."
Symonds also suggests that, being highly educated graduate students, the subjects may not have found the test sufficiently challenging to really stress them out. The anxiety of waiting for the test may have been worse than the arithmetic itself.
Strangely, the subjects also had elevated brown fat temperatures in the relaxation portion of the experiment. Symonds thinks this could be a result of the subjects having to sit still for a long period of time in order to allow the thermal imaging cameras to get a good read on their skin temperature. That could cause their core temperature to drop, and the brown fat may have been recruited to help counteract that. But he believes that the strong statistical link between cortisol and temperature in the stressful situation shows that stress is playing a real role in stimulating brown fat.
Jan Nedergaard, a physiologist at Stockholm University in Sweden who was one of the first scientists to indicate that adult humans have brown fat, says that although the sample size was quite small, these preliminary results point to the possibility that even in humans, stress can activate brown fat. "If this turns out to be the case, the good news is that it has been seen in mice that also positive stress -- and not only negative stress as used here -- can stimulate brown adipose tissue. So we could keep slimmer just through the anticipation of nice things happening to us," he said.
But Nedergaard cautions that the amount of fat burnt, and weight lost, through this process is likely to be very small.
Symonds says determining whether stress-activated brown fat can contribute to weight loss is "the million-dollar question." It is important to note, however, that the study was only looking at very mild, short-term stress. Severe chronic stress can be dangerous and raises the risk of heart disease, depression and metabolic problems, including weight gain.
"Stress can be good," says Symonds, but it "depends on the type and frequency, and one's ability to cope."