San Francisco: 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.
Investigating how brown fat works in mice, a team of researchers at the University of California, San Francisco (UCSF) has uncovered what may be a holdover from our evolutionary past: in response to cold, tiny immune cells known as macrophages can switch on the brown fat, inducing it to burn energy to make heat.
Prior to this research, scientists had assumed that brown fat metabolism was completely controlled by the brain. But the UCSF research 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.
“This is a very important secondary system that the body uses to provide a backup for the thermal stress response,” said Ajay Chawla, MD, PhD, an associate professor at 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,” he added.
The modern human immune system relies on these macrophages to gobble up bacteria, helping protect us against infection. Macrophages were never known to play a role in metabolism, but the evidence Chawla and his colleagues gathered suggests otherwise.
Using brown fat to burn calories and produce heat is one of the ways that mammals maintain thermoregulation-an essential adaptation that defines warm blooded creature and enables them to thrive in the face of challenging environmental extremes. Not all animals share this ability.
The work of the UCSF team showed that macrophages residing in brown and white fat 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.
The study has been published in the journal Nature.