Have you lost all of your baby fat? Although conventional wisdom says that you have, three new studies present evidence that adults contain significant amounts of "brown fat," a kind of fat that keeps you warm by torching calories.
"Brown fat is a heat-producing engine which is believed to be essential to keep babies warm and which is very important in helping animals wake from hibernation," said Richard Wahl, director of the Nuclear Medicine and PET Facility at Hopkins, who was not involved in the current research.
Brown fat, or brown adipose tissue (BAT), is present in newborns because of their inability to generate heat by other means, such as by shivering or cranking up the thermostat. Because of their smaller size, infants are also much more susceptible to heat loss because they have a larger surface area-to-volume ratio.
Adults, on the other hand, have access to countless ways of keeping warm. There are many physiological differences that make heat retention easier for adults, including more body hair and the ability of the nervous system to constrict blood vessels that run just below the skin.
Brown adipose tissue, compared to its white counterparts, has many more mitochondria, thus giving it the darker tint from which its name comes. These additional mitochondria allow BAT to very quickly transfer the energy obtained from food into large amounts of heat.
BAT comprises up to 5 percent of an infant's total body weight and is distributed as a large sheet on the upper back, and therefore, is able to protect the infant from extreme cold and provide additional means of body temperature regulation. Scientists had believed that BAT deposits shrink with age and are completely lost by adulthood because this added mechanism of heat retention was thought to be unneeded.
However, according to three studies published in the New England Journal of Medicine last week, almost all adults possess active deposits of BAT, which becomes activated when individuals are exposed to colder temperatures, around 60 degrees Fahrenheit. The studies also correlate brown fat with sex and body mass: Women tended to have more BAT, and overweight individuals had less BAT.
In these studies, subjects were placed in a cold environment for a couple of hours, and then their bodies were scanned using integrated positron emission tomography and computed tomography (PET-CT). The PET scan was able to identify metabolically active areas in the patient's body, and this image was superimposed over the CT scan, which can identify adipose tissue. Some of the subjects had the areas that lit up on the PET scan biopsied to confirm that it was indeed BAT that was consuming calories.
But how could more fat make you thinner? "A tissue that burns calories and dissipates the calories as heat can easily be considered as a possible target for managing hyperglycemia and, if activated for a long period of time, obesity," Wahl said. "Very minor but persistent changes in net caloric balance result in weight loss or gain."
Scientists hope to make use of the calorie-burning abilities of brown fat in order to tackle weight-loss and obesity issues. Obesity, which results from an imbalance in energy intake and energy expenditure, could be combated by awakening these dormant BAT reserves.
Mice, which maintain large amounts of BAT well into adulthood, have been shown to lose weight when simply placed in a cold environment, even though they were fed a diet higher in calories and fat than the control group. Furthermore, in mice whose genomes were modified so that their brown fat was unable to burn calories, they became overweight.
But before you make the move to Siberia and donate all of your winter clothes to Goodwill, there is some skepticism regarding whether adult human BAT is capable of producing the same effects that were observed in mice.
"Can adults utilize or generate enough BAT to facilitate weight-loss, or even a significant degree or warming?" Bernard T. Engel, a retired professor of Behavioral Sciences at Hopkins, said. And even if the answers to these questions are yes, a key question still remains: "Is BAT the most efficient way to enable and maintain weight loss?"
There are still efforts to see if BAT has therapeutic ?potential. "If such treatments can be developed, human BAT activation therapy may ultimately be viewed as a very potent tool in managing both diabetes and obesity," Wahl said.
