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December 21, 2011
How Now, Brown Fat?
Throughout the interior spaces of humans and other warm-blooded creatures is a special type of tissue known as brown fat, which may hold the secret to diets and weight-loss programs of the future.
Unlike ordinary "white" fat, in which the body stores excess calories, brown fat can burn calories to heat up the body. It's one of the things that helps keep wild critters warm on cold nights.
Leapin' lizards! They -- and more sedentary types -- were part of a research project on body fats.
Investigating how brown fat works in mice, a team of researchers has uncovered what may be a holdover from our evolutionary past: In response to cold, tiny immune cells can switch on the brown fat, inducing it to burn energy to make heat.
Prior to this research, published last month in the journal Nature, scientists had assumed that brown fat metabolism was completely controlled by the brain. But the University of California at San Francisco research team suggests that the immune system plays a backup role in this process -- a legacy, perhaps, of some ancient ancestral creature whose metabolic and immune systems were much more intertwined.
Reported Dr. Ajay Chawla of UCSF's Cardiovascular Research Institute, who led the research: "It raises the possibility that we can perhaps modulate this program and enhance it in humans to rev up metabolism."
The modern human immune system relies on these immune cells to gobble up bacteria, helping protect us against infection. These cells were never known to play a role in metabolism, but the evidence Chawla and his colleagues gathered suggests otherwise.
Many animals, like lizards, are "cold blooded," or exothermic. They maintain their body temperature through completely external means, sunbathing at certain times of the day and huddling in warm, protective places at night. This naturally limits their range and explains why lizards, so abundant in tropical climates, are far rarer in cold climates.
Mammals, on the other hand, are "warm-blooded" (endothermic). They produce heat internally by a variety of means: shivering, sweating, regulating the size of their blood vessels and burning off excess calories in brown fat.
Scientists have known for years that brown fat burns calories in response to signals from the brain. These signals break down molecules known as triglycerides in white fat, which are then released into the bloodstream as fatty acids.
These circulating fatty acids are taken up by the brown fat and are then burned to generate heat. Brown fat is full of blood vessels, and the heat warms the blood, which in turn circulates and warms the body.
The brain controls this process by monitoring the body's temperature and, in face of extreme cold, releasing a hormone called norepinephrine, which kick-starts the brown fat.
The work of the UCSF team showed that the immune cells within the brown fat can also do this directly.
They produce an enzyme that makes norepinephrine when mice are exposed to the cold. This leads to the production, the breakdown and mobilization of stored fat, which is then burned in brown fat to produce heat.