VOA常速英语:Shivering Activates Fat-Burning Hormone

With a fourth straight month of snow storms and below normal temperatures, Chicago native Paige Worthy says it is the worst winter she can remember.

“It actually hurts to breathe in because the air is so cold that I actually have to cover my mouth with a scarf to keep my lungs from hurting,” Worthy said.

And it seems that no matter how many layers of clothes they put on, everyone is shivering.

That involuntary reflex is the focus of a new study in Cell Metabolism. It finds that shivering triggers the body to release a natural hormone called irisin, which activates brown fat, the good fat in your body that burns calories and promotes weight loss. Irisin also plays another important role.

“Indeed the purpose of brown fat is maintaining the core temperature, so the temperature of the body whereby all the vital functions can be active and normal,” said lead author Francesco Celi of Virginia Commonwealth University.

Celi worked with a team at the U.S. National Institutes of Health (NIH). They already knew the body produces irisin when we exercise and hypothesized that shivering, which like exercising also makes muscles contract, evolved as a survival mechanism in response to cold.

“This is the last ditch before going into hypothermia and having severe metabolic and life threatening consequences," Celi said. "Now, an hypothesis is only an hypothesis until you prove it.”

Volunteers were recruited to test hormonal changes and energy expenditure with both exercise, and also while lying under cooling blankets where temperatures were gradually reduced to 12 degrees Celsius.

“Most of our volunteers shivered at that time and the shivering was anywhere between five and 10 minutes, not more," Celi said. "And, again, we drew blood before and after the study.”

The volunteers' bodies produced irisin, but with some surprises.

“The amount of increase in irisin was almost identical to what we observed after one hour of exercise," Celi said, "which validated the initial hypothesis, whereby the maximum stimulation of irisin is probably shivering.”

Understanding how irisin activates brown fat, and consequently the burning of calories, could lead to new drugs to fight diabetes or obesity.

"It is a short term effect. We do not know yet the long term consequences of these interventions," Celi said. "So, studies need to be performed to address this very question. We do not know if this is advantageous, which individuals would be more able to gain from intervention.”

Back in Chicago, Paige Worthy says while she is glad to know that shivering may have a physical benefit, as a fitness trainer she is cautious about encouraging it.

“As a newly minted new instructor spinning [exercise bike] instructor, I guess I would say that there are risks that go along with every exercise and every weight loss regimen, but this seems like a perilous way to approach weight loss, if it is going to be this cold,” she said.

Celi agrees, adding that it is too early to jump to conclusions. Just because shivering mimics exercise, he says, that does not mean it should replace it.