The winter's frigid temperatures could be having one desirable side effect. They may be revving up your metabolism.  Shivering in the cold sparks biochemical reactions within the body that alters fat cells and bolsters metabolism, much as exercise does, according to a new study. 

Scientists affiliated with several branches of the US National Institutes of Health recruited 10 adult men and women and invited them to the lab on three separate occasions. 

During one lab visit, the volunteers completed a short but intense session of stationary bicycling, riding as hard as they could. Then, on another day, they rode the bike at a gentle pace for an hour. Throughout, the lab temperature was at 65 degrees.

On their final visit, the researchers had each volunteer lie in bed, lightly clad, for 30 minutes as the temperature dropped to 53 degrees. By the end, they were shivering. After each session, the scientists gathered blood and other samples. In particular, they wanted to see what was happening with the volunteers' white and brown fat.

Until a few years ago, it was believed that adults do not have brown fat, a tissue that is metabolically quite active. Unlike white fat, it burns calories and generates heat. Rodents and infants harbour brown fat, which helps 

keep them warm, since they are not good at shivering. But scientists thought that after childhood, humans lost their brown fat and substituted shivering to stay warm.

Newer studies, however, have found brown fat stores in humans of all ages. Scientists were unclear why, as a species, we continue to shiver if we have brown fat. 

A 2012 study in mice seemed at first to provide some clues. In that study, animals that produced more of a hormone called irisin had more brown fat than other rodents. Irisin was created during muscle contractions, then travelled through the blood to white fat cells, where it transformed those cells into calorie-burning brown fat.

Intriguingly, the study also found, exercise, which involves muscle contractions, inspired an enormous surge in irisin production.

But scientists were puzzled. Exercise creates a great deal of heat as working muscle cells burn fuel and generate energy, they pointed out. In fact, in some situations, the body can't dump the excess heat and heat illness ensues. So why would the body produce a hormone during exercise that would increase body heat still more?

The NIH scientists, however, suspected that the question was not why exercise would instigate the production of irisin. Instead, they wondered whether irisin production might have been sparked by a deeper, older biological process.

And, as their results suggest, shivering might have been that spark. For while the volunteers' blood levels of an irisin marker were higher after exercise, the markers were just as high after the volunteers had lain quietly in the cold for 30 minutes, not moving except to shiver. What seemed to matter, the researchers concluded, was not the exertion of the exercise, but the contraction of various muscles, which occurred during shivering as well as cycling. The implications of this finding are important, said Dr Francesco S Celi, a professor at Virginia Commonwealth University who oversaw the experiments.

At one level, they bolster the idea that exercise increases irisin production, leading to larger stores of brown fat in those who exercise. But just as important, the new study also helps to explain why exercise would cause physiological changes that could so significantly increase body heat.

“We think that shivering came 

first,” Celi said. In this theory, irisin originally was created by the muscular contractions occurring during shivering and exercise increases irisin production not because it's exercise, but because it is basically an exaggerated form of shivering.

Unfortunately, there are no indications that exercising in the cold amplifies the production of irisin and brown fat, Celi said. But the new study does suggest that if you can't get to the gym, at least consider lingering outside and shivering.

—© The New York Times