Previous research has shown that moderate exercise can help turn energy-storing white fat into brown fat - the "good" fat that burns energy and helps keep us warm. But new research published in Cell Metabolism suggests that shivering for 10-15 minutes can do the same job.
According to the research team, led by Dr. Paul Lee of the Garvan Institute of Medical Research in Sydney, Australia, the findings indicate that brown fat could be a therapeutic target against obesity and diabetes.
Humans and other mammals have two types of fat in the body - brown fat, also known as brown adipose tissue, and white fat.
The main function of brown fat is to generate heat, which is why higher levels of brown fat are found in newborn babies and hibernating animals.
However, the researchers say that it has only been in recent years that scientists have discovered that brown fat is present in adults, and that adults with high levels of brown fat tend to be slimmer than those with low levels.
"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," says 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."
According to the investigators, approximately 50 g of white fat stores more than 300 kilocalories of energy, while 50 g of brown fat may burn up to 300 kilocalories a day.
The researchers already knew that cold temperatures stimulate brown fat, causing it to burn more energy. But the underlying mechanisms were unknown.
"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," explains Dr. Lee.
Hormones released by muscle and brown fat
The research team recruited a set of volunteers for their study. They exposed the volunteers to increasing cold, from 18 degrees Celsius down to 12 degrees, until it caused them to shiver. The volunteers started to shiver at around 16 degrees.
Because the body can sense and communicate environmental changes to different organs though nerves and hormones, the researchers set out to investigate hormones that are triggered by cold environments.
"We drew blood samples to measure hormone levels and detected shivering by special devices placed on the skin that sense muscle electrical activity," says Dr. Lee.
From this, the researchers discovered that shivering muscle releases hormone called irisin, while brown fat exposed to cold environments releases a hormone called FGF21.
Explaining the hormones' activity, Dr. Lee says:
"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."
Exercises mimics shivering
The investigators point out that a research team from Harvard University first discovered irisin in 2012, identifying it as a muscle hormone that was triggered by exercise that turned white fat into brown fat.
But the researchers of this most recent study note that exercise produces heat in itself. Therefore, they questioned why exercising muscle would prompt a mechanism that produces more heat.
In order to find out, the same volunteers carried out exercise tests. The process in which muscles release irisin during exercise was compared with the process in which they release the hormone through shivering.
The researchers found that when exercising on a bicycle for 1 hour, the muscles of the participants released the same amount of irisin as they did when they shivered for 10-15 minutes.
The investigators hypothesize that exercise may be mimicking shivering. Dr. Lee notes that the muscles contract during both processes, and that exercise-stimulated irisin may have evolved from shivering in cold environments.
Overall, he notes that these findings could open the doors for new treatments for obesity:
"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."
Medical News Today recently reported on a study suggesting that as we age, our brown fat becomes less efficient at burning calories.
Written by Honor Whiteman