Shivering Turns White Fat Into Brown Fat
February 5, 2014

Shivering Could Have Same Benefits As Exercise

April Flowers for - Your Universe Online

Most of us put off exercising when it is cold outside. However, new research is showing that shivering, which is the body's way to stay warm, is as equally capable of stimulating the conversion of energy-storing "white fat" into energy-burning "brown fat" as bouts of moderate exercise. The findings were published in a recent issue of Cell Metabolism.

For example, 1.8 ounces of white fat stores 300 kilocalories of energy and the same amount of brown fat could burn up to 300 kilocalories a day.

Endocrinologist Dr Paul Lee, from Sydney's Garvan Institute of Medical Research, recently undertook the study at the National Institutes of Health (NIH) with Dr. Francesco S. Celi of the National Institute of Diabetes and Digestive and Kidney Diseases.

The study uncovered specific hormones that act as a means of communication between muscle and fat – turning white fat cells into brown fat cells in order to protect us from the cold.

The levels of the hormone irisin, which is produced by the muscles, and FGF21, which is produced by brown fat, rose during cold exposure and exercise. For example, they found 10-15 minutes of shivering resulted in equivalent rises in irisin as an hour of moderate exercise. Laboratory results revealed that irisin and FGF21 turn human white fat cells into brown fat cells over a period of six days.

Humans are all born with supplies of brown fat around our necks to keep us warm as infants. Until recently, scientists thought the brown fat vanished in early infancy. Now, we know that brown fat is present in most, if not all, adults. Those with more brown fat are slimmer than those without.

"Excitement in the brown fat field has risen significantly over last few years because its energy-burning nature makes it a potential therapeutic target against obesity and diabetes," said Dr Lee.

"White fat transformation into brown fat could protect animals against diabetes, obesity and fatty liver. Glucose levels are lower in humans with more brown fat."

The researchers wanted to understand the underlying mechanism for activating the brown fat. Prior studies had already shown that cold temperatures stimulate brown fat, but it was unclear how the body signals that message to its cells. Nerves and hormones are used by the body to sense and relay environmental changes to different organs. Lee and his team investigated the hormones stimulated by cold environments.

"When we are cold, we first activate our brown fat because it burns energy and releases heat to protect us. When that energy is insufficient, muscle contracts mechanically, or shivers, thereby generating heat. However we did not know how muscle and fat communicate in this process."

"So we exposed volunteers to increasing cold, from 18 to 12 degrees, until they shivered. We drew blood samples to measure hormone levels and detected shivering by special devices placed on the skin that sense muscle electrical activity."

"Volunteers started to shiver by around 16 or 14 degrees, varying between individuals."

"We identified two hormones that are stimulated by cold – irisin and FGF21 – released from shivering muscle and brown fat respectively. These hormones fired up the energy-burning rate of human white fat cells in the laboratory, and the treated fat cells began to emit heat – a hallmark of brown fat function."

In 2012, a team of Harvard University researchers discovered irisin, identifying it as a muscle hormone stimulated by exercise that turned white fat in animals to brown fat. The find was still puzzling. Exercise produces heat, so the scientists couldn't find a reason why exercising a muscle would initiate a process that would generate even more heat.

To compare the two processes, Lee invited cold-exposure study participants to take part in exercise tests. "We found that exercising for an hour on a bicycle at a moderate level produced the same amount of irisin as shivering for 10-15 minutes," he said.

"We speculate exercise could be mimicking shivering – because there is muscle contraction during both processes, and that exercise-stimulated irisin could have evolved from shivering in the cold."

"From a clinical point of view, irisin and FGF21 represent a cold-stimulated hormone system, which was previously unknown, and may be harnessed in future obesity therapeutics through brown fat activation."

The findings suggest that exploiting this muscle-fat crosstalk could also represent a new strategy for treating or preventing obesity.

"Perhaps lowering the thermostat during the winter months could help both the budget and metabolism," said Dr. Celi.

These findings coincide with earlier research from Maastricht University Medical Center. Last month, redOrbit reported that people wanting to lose weight could benefit from regular exposure to cold.