The Good Fat

Fat is bad, right? I'm not talking about the fats we eat here. I mean the fat on your body. The less fat, the healthier you are. Everyone knows that.

But maybe everyone is wrong.

New research in the laboratory of C. Ronald Kahn at the Joslin Diabetes Center suggests that some fat, namely subcutaneous fat, may actually be healthy and may protect people from getting metabolic diseases like diabetes.

There are two basic classes of fat in the body: brown fat and white fat. Brown fat seems to serve as a heat source, and this is an important function in many animals. For example, newborn lambs have brown fat around their kidneys, and when they're born in subzero weather, this fat is burned to produce heat and keep them from freezing to death.

I used to breed my sheep, and if I found a dead recently born lamb, I could look at the kidneys to see if they contained brown fat. If they did, the lamb had probably been stillborn. If the brown fat was gone, they'd probably lived for a short period of time and then had died.

Many animals, including rodents and hibernating animals, use brown fat throughout their lives to provide heat when needed. In humans, however, although babies use some brown fat to provide warmth, most adults don't have very much at all. This is one of many reasons we have to keep studies in mice in perspective. In some ways their metabolism is different from ours.

The white fat used to be thought of simply as an organ to store energy to be used in lean times. Now we know that white fat is also an endocrine organ, meaning it secretes hormones like leptin that are important for controlling metabolism.

For a long time, researchers have known that visceral fat, or the fat that surrounds your internal organs, is especially dangerous, and a high level of visceral fat often precedes diseases like type 2 diabetes. People with a high ratio of visceral fat to subcutaneous fat -- the fat that is just below your skin and in your buttocks -- have more insulin resistance. But it was assumed that subcutaneous fat was essentially the same type of fat, just in a different location.

The Joslin researchers tried transplanting some of the visceral fat into subcutaneous locations and some of the subcutaneous fat into visceral locations in mice, wondering if the function of the fat would change with the location.

It didn't. The subcutaneous fat turned out to protect the mice from disease even when it was transplanted into visceral locations. The mice with extra subcutaneous fat gained less weight than the control mice and had less total fat despite having identical diets and exercise patterns. They also had less insulin resistance, lower blood glucose levels, and less glucose production by the liver.

Kahn speculates that subcutaneous fat secretes a yet-undiscovered hormone that has all these beneficial effects.

The glitazone drugs, like Avandia and Actos, stimulate the body to produce more fat cells. Some bloggers have said this is one reason to avoid these drugs. What person with type 2 diabetes would want more fat cells? they ask.