Brown fat, also known as brown adipose tissue (BAT) is one of two types of fats found in humans, the other two being white or yellow fat. Hibernating mammals and newborns have especially high levels of brown fat. Its main function is to generate body heat in animals and newborns. White fat cells (adipocytes) contain a single lipid droplet, as opposed to brown adipocytes, which contain several smaller lipid droplets and a considerably higher number of iron-containing mitochondria. The high iron is said to give brown fat its brown color. There are more capillaries in brown fat than white fat, because its oxygen requirement is greater.
White fat accumulates around the waist and thighs, while brown fat appears to be concentrated in the front and back of the neck.
Experts say much remains to be known regarding brown fat, but the main differences between these two types appear to be:
- Brown fat burns through calories in order to generate heat
- White fat is a storage area for excess calories
Scientists do not yet know how humans might be able to increase the amount of brown fat in their bodies. In this study, endocrinologist, Dr. André C. Carpentier, from Universite de Sherbrooke, and team set out to determine how humans might be able to switch on the brown fat so that it uses up fat. They found that exposure to cold temperatures seems to be the best trigger.
They found that when healthy adult volunteers were exposed to even slightly cold environments, their brown fat "turned on".
The team enrolled six healthy young men, whose weights ranged from normal, overweight to obese. An obese person has a BMI (body mass index) of at least 30.
All the participants had their brown fat levels measured before the experiment began. They were then placed into cooling suits which lowered the temperature of their skin by 3.8 Celcius. However, their core body temperatures stayed pretty much the same.
In an interview with CTV News, Dr. Carpentier said:
"During this exposure, these patients were slightly shivering. They were at
the threshold of shivering."
The researchers found a link between levels of brown fat and when people started to shiver from cold. The more brown fat a person had, the longer it took before he would start to shiver. They also reported that as soon as the men felt cold, their brown fat went into gear (became active and started burning calories).
Over a three-hour period, the authors explained that 250 extra calories were burned while the brown fat cells were active. This represents a 1.8 times higher calorie burn compared to a those men when resting at normal temperatures - equivalent to a walking rate of burning calories.
The scientists stressed that theirs was a very small study, consisting of only six volunteers. However, since their findings were consistent between subjects, they are sure they could be applied to other populations. They added that further, larger studies are required to confirm their findings.
How this may be relevant to weight loss is not clear, Carpentier added. He said that nobody yet knows whether activating brown cells to help weight loss or to treat diabetes type 2 is a good idea. Further studies are required to decide whether activating brown fat might have any therapeutic benefits.
Dr. Carpentier said to CTV news:
"It is still too early to cool yourself in a suit and in the hope that you will lose weight because we don't know how the body adapts over the long run to this type of stimulation, whether these stimulations can increase appetite or change the metabolism of the body over time."
The authors concluded in an Abstract in the journal:
"In sum, our study provides evidence that BAT acts as a nonshivering thermogenesis effector in humans."