Exposure to cold is the most well-known and well-studied mechanism for switching on energy-burning brown fat, which seems to protect mice from developing obesity. It remains to be seen whether the same process can help people.
Humans have three kinds of fat. White adipose tissue, or white fat, comprises the majority of fat in our bodies; its purpose is to store energy for future use. Brown fat is different: Its function is to generate heat to maintain body temperature. Until recently, it was thought that adults did not have brown fat, that it only existed in babies to help them stay warm before they could move around and then essentially vanished. But beginning in 2009, studies have found that many adults have brown fat and that people with more of it tend to be leaner and have lower blood sugar levels.
The third kind of fat, beige fat, appears to convert from white to brown when stressed by exposure to cold, and then back to white. This process is encouraging for scientists trying to figure out how to increase brown fat to improve healthy functioning of the body.
"A balanced diet and regular exercise are the cornerstones of healthy metabolism, but sustaining either is difficult for most people. Understanding how brown fat could benefit our health opens up a new direction in obesity research," says Paul Lee, an endocrinologist at the Garvan Institute of Medical Research in Sydney, where he leads the Brown Fat Physiology Group. "It is not a solution to obesity, but it is an opportunity to explore an alternative strategy for curbing the obesity epidemic."
When the body senses cold, Lee says, the brain releases norepinephrine, a chemical that essentially ignites the fat-burning process within brown fat. When there is not enough brown fat, the body has to turn to less-efficient heat-generating models, such as shivering.
Aaron Cypess, a clinical investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, calls brown fat the principal organ responsible for generating heat in laboratory animals.
"In mice and rats," Cypess says, "chronic activation of brown fat [by exposing them to low temperatures or to drugs that target brown fat] . . . is associated with a reduction in liver fat, a resistance to diet-induced obesity and improvement in insulin release." All of these benefits and others may also apply to people, but it will take much longer to prove because studies in humans have to be conducted differently, he says.
He adds, "While white fat is easy to spot in humans - think abdomen, hips, buttocks and thighs - brown fat tends to be located around the neck and above the collarbone, along the spine and near the kidneys." Additionally, Cypess says, humans are genetically more diverse than lab mice, which produces results with much higher variability.
Lee says that when people are cold and begin to shiver, their muscles release irisin, a hormone that turns white fat into brown fat. The more a person shivers, the more irisin is released into the bloodstream.
A 2014 study by Lee - dubbed "the ICEMAN study" - found that after a month of sleeping at cool temperatures, five men increased their stores of brown fat by 30 to 40 percent and metabolized sugars more efficiently after a meal, which could be helpful for people with diabetes. When the sleeping temperature was raised, the brown stores dropped.
(Interestingly, another recent study found that brown fat also may be stimulated by taking a drug used to treat overactive bladder.)
Cypess says that this research makes it clear that activating or increasing brown fat stores might prevent weight gain, lead to weight loss and provide a new avenue for treating diabetes and obesity.
Can the average person embark on a "shiver diet" to lose weight?
Lee says he believes the current evidence does not support the notion that shivering may be a route to losing weight. (Despite the study's name, ICEMAN - the Impact of Chronic Cold Exposure in Humans - exposed participants to only mild cold, not shiveringly low temperatures.)
Cypess says that shivering to lose weight is an interesting idea, but there are many unknowns.
First, is it safe?
Lee says that "shivering causes stress and could harm the body, which explains why the human body has evolved mechanisms to turn on brown fat or to turn white fat into brown fat."
In most people, Cypess says, shivering causes increases in blood pressure that over the years could damage blood vessels in the brain, heart and kidneys.
Additionally, Cypess says, there is no evidence to prove that a low-temperature regimen could be effective long-term. One of the biggest limitations of weight-loss interventions is that the body learns to compensate to maintain itself, and that might be true with a shiver diet. Lee and Cypess agree that no weight-loss regimen should be recommended without a great deal of evidence that it will work for more than a few weeks or months and that the weight loss can be sustained - evidence that doesn't exist.
Finally, Cypess says, being cold is extremely uncomfortable. "While suggestions exist that long-term activation of brown fat could be beneficial to weight loss and diabetes reduction, this has yet to be proven," he stresses.
Francesco Celi, chair of the division of endocrinology, diabetes and metabolism at the Virginia Commonwealth University School of Medicine, said in an email that he expects "future research will include conducting studies in humans that will test various interventions (drugs or environmental modifications) to expand and activate brown fat to help scientists determine what kind of metabolic improvements can occur. And by studying the various responses to interventions, researchers will be able to determine which patients respond better to brown-tissue expansion and perhaps why they do."
Cypess says he expects scientists to focus on determining to what extent adult brown fat contributes to getting rid of excess calories, how brown fat could be used to bring down blood sugar levels and how brown fat interacts with other organs to keep people healthy.
But even with all that, he adds, "Basically, the issue of losing weight is about controlling the amount of food we put into our mouths."