April 10, 2009

Update: Brown Fat Metabolism in Humans and Mice

Three studies in the New England Journal of Medicine made headlines this week for providing evidence that humans have brown fat – the same stuff that bears burn up so as not to freeze to death while hibernating. Well, at least we have a few grams of it, in strange places far removed from our big bellies. These studies hark back to a story we reported last week on mutant mice that eat fat with near impunity.

Robert Farese Jr.

Why the excitement over brown fat? Unlike more familiar fat stuff, called white adipose tissue, or simply white fat, which serves as a storage depot for energy, brown fat appears made for burning up energy. It’s brown, researchers say, because of a superabundance of fuel-processing factories within these fat cells, called mitochondria, which produce heat as a byproduct and tint the tissue. Researchers wonder whether we can manipulate our brown fat to burn more unwanted calories through this same process of thermogenesis, instead of having so many calories converted to white fat. Robert Farese Jr., MD, of the UCSF-affiliated Gladstone Institutes wonders whether the brown fat in his lab mice might play a role in the elevated energy burning and the higher body temperatures he measured in mutant animals lacking a key fat-processing enzyme. When the animals ate the same high-fat diet as normal mice that still had the enzyme, the mutant animals barely gained weight in comparison with the normal mice. But Farese is still tracking down the exact connection between enzyme loss and increased thermogenesis that he observed. The enzyme, called MGAT, may become the target of efforts to come up with a new weight loss drug. “It looks like adult humans do indeed have brown fat cells,” Farese says. “These may be interspersed with white adipose tissue, and studies show collections around the collarbones and neck. No one knows why it’s there, but it may be there to warm blood that goes to the head. This clearly looks to be inducible by cold exposure.” Maybe if we turn down our thermostats, we can lose weight and fight global warming at the same time. Or maybe folks up north could learn to hibernate.

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