Dear Doctors: I was very interested to read your column that talked about the fat around your organs, and how when you have too much, it is bad for your health. I have also heard of something called brown fat, and that it’s actually a good thing. Can you please explain?
Dear Reader: When we think of fat, it is often subcutaneous fat that comes to mind. That’s the layer of fat that lies just beneath the skin that you can see (it jiggles) and pinch. Visceral fat, by contrast, is found deep within the abdomen. It helps cushion and protect the internal organs, including the stomach, intestines, liver, pancreas and kidneys.
A certain amount of visceral fat is important to health and well-being. But this type of fat is hormonally active. Having too much can be a danger to health. As we explained in the column you read, excessive visceral fat is linked to an increased risk of diabetes, coronary artery disease, heart disease, high blood pressure and systemic inflammation.
Another important kind of fat produced by the body is brown fat. Also known as brown adipose tissue, it plays an important role in regulating body temperature. As the name suggests, brown fat is darker in color than the rest of the fat in the body. This is due to a high concentration of cellular organs, known as mitochondria, which produce energy. It means unlike white fat, which only stores calories, brown fat can burn them, too.
That’s where temperature regulation comes into play. When the body gets cold, brown fat is activated. It begins to generate the heat the body needs by burning calories. Recent research also suggests that brown fat plays a role in blood sugar metabolism.
Brown fat has an interesting history. Although it was visually identified as early as the 16th century, it wasn’t until 1961 that scientists confirmed it generates heat. It was also initially believed that brown fat is only present in babies. Newborns have a significant amount of brown fat -- up to 5% of their body weight -- which helps to keep them warm. It is found primarily in the upper back, neck and shoulders. Those fat cells dissipate as children grow into adolescence. They were believed to vanish completely in adults. In 2003, thanks to advancements in imaging, it was discovered that brown fat persists in adults, but in much smaller amounts.
In adults, brown fat constitutes less than 0.5% of body weight. People who are lean have more of it than those with a higher body-fat percentage. And unlike the centralized deposits seen in newborns, scans of adults find small amounts of brown fat in the tissues of the neck, kidneys, adrenal glands, chest cavity and along the spine.
Due to its thermogenic nature -- that means it burns energy -- brown fat has become a focus of research into obesity and weight loss. You can protect the brown fat in your own body by eating a healthful diet, getting regular exercise and spending periods of time in cooler temperatures.
(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)
