Brown fat burns calories to keep us warm. Researchers are searching for ways to boost it to help fight obesity and diabetes.
Fat sometimes gets a bad rap. It’s often linked with being overweight and poor health. But fat is an essential part of all living things. In our own bodies, fat lies beneath the skin and hugs our organs. Its job is to store extra calories until needed. Seems like a straightforward role. Or is it?
In fact, body fat is complex. Until recently, scientists thought people had only one type. Called white fat, it stores excess calories in molecules called lipids. Lipids can be broken down for energy when food is hard to find. White fat is what people think of when they think of body fat.
Explainer: All about the calorie
But some 50 years ago, researchers discovered that people also have brown fat. It actually burns calories.
Scientists first discovered brown fat about 500 years ago. They found it in hibernating marmots (also known as groundhogs or woodchucks). Only a small amount of body fat is brown. So for a long time, scientists thought brown fat was a gland that existed only in hibernating animals. Only in the last 100 years have they realized this “gland” is actually a special type of fat.
Scientists are still learning about brown fat. Studies have uncovered its importance for hibernating animals. That research helped show that brown fat plays a key role in how bodies use energy. And that link suggests that boosting brown fat in people might help them lose weight and perhaps treat certain chronic diseases, such as diabetes.
Wake-up call
Most body fat in animals, including people, is white. And it’s a life-saving tissue. Most animals don’t have a constant supply of food. Until fairly recently, most people didn’t either. White fat allows individuals to eat more than they need when food is easy to find. It stores those extra calories until food becomes scarce. Then the body burns it for energy to stay alive until more food shows up.
Explainer: How brief can hibernation be?
Hibernating animals take this to the extreme, explains Mallory Ballinger. She is an evolutionary biologist at the University of California, Berkeley. Hibernation allows many animals to get through harsh winter conditions. Bats, squirrels and bears, for example, all gorge themselves on food in fall. These mammals pack on the pounds — up to half their body weight — in preparation for a long, cold winter.
Animals don’t eat when they’re hibernating. Instead, they burn their white fat to keep their bodies running. But they only have so much fat to burn. To make it last an entire winter, these critters must use it slowly. Much more slowly than they normally would. To do this, they enter a state called torpor.
Scientists Say: Torpor
When in torpor, animals appear to be sleeping. But torpor goes much deeper than that, Ballinger says. In torpor, a body’s activities slow down. The heart may beat only a few times each minute. Breathing may become irregular. An animal may take several breaths and then stop breathing for several minutes — even up to an hour or more. Bodies in torpor also chill down. Instead of wasting energy keeping warm, a hibernating animal’s body can remain just above freezing.
But hibernating animals don’t stay in torpor all winter, Ballinger points out. Every week or two, they waken. And they stay awake for a day or so before going back into torpor. Biologists don’t know what triggers the wake-up or why it happens, she says. The duration and timing varies by species. However, she points out, all true hibernators go through this cycle.
This dormouse will hibernate for a whopping seven months, waking every few weeks for about a day each time. The animal’s stores of brown fat help keep it from freezing. CREDIT: SashaFoxWalters/iStock/Getty Images Plus
In order to wake up, the animals have to warm up. That means going from a temperature just above freezing to their normal body temp in only a couple of hours.
That’s where brown fat becomes important. Brown fat creates heat. If the outside temperature gets too cold, brown fat keeps the animal from freezing. It also pulls the animal out of torpor. As the brown fat gives off heat, it warms the blood, which then carries that warmth to the rest of the body.
This process burns up white fat, including white droplets packed in with the brown fat cells, Ballinger explains. That shows brown fat plays a critical part in how and when the body stores and uses energy. And it turns out that’s true not only in hibernating animals. It’s true in people, too.
