Researchers have discovered that adult humans have a type of "good" fat that may actually keep us lean by helping to burn calories.
Unlike white or yellow fat, which is what most fat around the waist is made of, this good fat, called brown fat, seems to be concentrated around the neck. While white fat acts as storage for excess calories, the brown kind is active in burning energy to generate heat.
Scientists had long thought that brown fat existed in humans only during childhood, giving babies and toddlers that adorable pudge that helps to keep them warm. But new research published Wednesday has found brown fat in adults.
The findings are reported in three studies from Boston, Finland and the Netherlands in the New England Journal of Medicine. Experts says the discoveries could pave the way for new treatments both for obesity and type 2 diabetes.
Dr. C. Ronald Kahn, senior author of one study and a researcher of the Joslin Diabetes Center in Boston said the research might mean a way to stimulate brown fat growth to control weight.
"There has been a long debate as to whether brown fat exists in adult humans and whether it was important physiologically," Kahn said in a statement.
"This study demonstrates that it is both present and appears to be physiologically important in terms of body weight and glucose metabolism. We hope this opens up a new therapeutic area for obesity and type 2 diabetes by modifying the activity of brown fat."
Reviewing PET and CT scans from nearly 2,000 patients, the researchers found tiny deposits of brown fat in many of the patients, mostly in the neck and shoulder area. They didn't find much -- about 50 grams, on average -- but the researchers believe such a small amount is enough to burn calories.
Using the example of one patient who had 63 grams of brown fat in their neck, researchers led by Dr. Kirsi Virtanen of the University of Turku in Finland noted: "If the brown adipose tissue in this example were fully activated, it would burn an amount of energy equivalent to approximately 4.1 kg (9 pounds) of adipose tissue over the course of a year."
More brown fat among the young
Not only did the researchers find brown fat in adults, they found differences in the amount of brown fat in each patient, based on such things as their age and level of obesity.
Not surprisingly, younger patients were more likely to have larger amounts of brown fat. But Kahn's research team also noted that brown fat was more common in adults who were thin and had normal blood glucose levels.
"What is of particular interest is that individuals who were overweight or obese as measured by higher Body Mass Index (BMI) were less likely to have substantial amounts of brown fat," said Kahn.
"Likewise, patients taking beta-blockers and patients who were older were also less likely to have active brown fat. For example, individuals both over age 64 and with high BMI scores were six times less likely to have substantial amounts of brown fat."
It's not clear why slim people might have more brown fat. They may be slim because they have extra brown fat, or they have more brown fat because they are thin. And it may be that overweight people have less brown fat because they have more white fat to keep them warm.
Two other studies found that the fat is particularly metabolically active in cold environments.
In the Netherlands study, researchers gave 24 young men radioactive glucose, which tags metabolically active tissue, and turned down the thermostat to 16 degrees Celsius. The PET and CT scans, which spot tissues that are feeding on radioactive glucose, found that the brown fat in the men "lit up" during exposure to cold.
"The cold quickly turned on the activity of this tissue and it began to burn a lot of energy, a lot of calories," explained Dr. Mary-Ellen Harper from the Mitochondrial Bioenergetics Laboratory at the University of Ottawa.
When the researchers rescanned three of the subjects at room temperature, the brown fat was no longer visible, presumably because it had stopped working to create heat.
Researchers now hope to unlock more mysteries of this unique fat. In so doing, they might devise treatments to activate the fat, increase its production and thus speed up metabolism, which should lead to weight loss.
With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip
