Scientists have identified "good" fat in adult humans

Contrary to previous assumptions, brown fat is a type of fat that uses energy and the new research shows that it remains functional in adults

PET/CT scans of a person exposed to cold (left) and at room temperature (right) show the dark signature of brown fat.

Credit: WD van Marken Lichtenbelt et al., The New England Journal of Medicine 360, 1500 (2009)

"Good" fat - is there such a thing?

Those who maintain their health, and spend hours in gyms and walking at a regular pace, do so mainly to burn fat. Is it possible to mobilize the body itself for the purpose of burning the "bad" fats, regardless of one activity or another. It turns out that there is such a thing, as shown by three studies published simultaneously in the New England Journal of Medicine.

Fat keeping the body lean sounds like a logical contradiction, but that's what brown fat does. Three new studies reveal this truth, contrary to previous assumptions, which have existed for a long time. Brown fat is a type of fat that uses energy and the new research shows that it remains functional in adults. The findings increased the researchers' hopes of channeling brown fat to control body weight. White fat, or white adipose tissue, is the vibrating substance that stores excess energy from food in our body. In contrast, brown adipose adipose tissue consumes energy to produce body heat.

The tissue connects to the mitochondria that maintain food-based metabolism, hence the brown color. Brown fat is adipose tissue that differs from white fat tissue in its anatomical distribution and composition. The yellowish-brown color (from which its name is derived) is obtained from the content of the mitochondria, cytochromes and fat molecules in the tissue. Brown fat is found mainly in small mammals and also in hibernating mammals. Non-ATP heat production is the main function of brown fat. It means generating heat that is not by way of muscle activation. The mechanism is through a protein molecule found in the mitochondrial membrane called UCP1 (uncoupling protein).

"It's like the burner of the heater" says the medical geneticist, Sven Enerback, from the University of Gothenburg in Sweden, who led one of the studies on brown fat. Other mammals and human infants rely on brown fat to maintain their body heat. For many years, physiologists assumed that brown fat tissue largely disappears by the time we reach adulthood.

But now, scientists have shown that brown fat is muscle and exists and works in adults as well. Physiologist Water Van Merken Lichtenblatt from the University of Maastricht in the Netherlands, along with his colleagues, conducted an experiment on 24 young men and fed them radioactive glucose, the content of which can be used to monitor metabolically active tissues. The experiment was conducted under conditions of 16 degrees Celsius to encourage metabolic activity to heat the body among the subjects. A combination of PET and CT scans, which identify tissue that consumed the radioactive glucose, revealed the presence of brown fat tissue in the neck, chest and abdomen in all but one of the men tested. This fat began to act during exposure to the cold. "It was exciting to see the brown fat in so many people and in so many tissues," says Van Marken Lichtenblatt. When the researchers reexamined three subjects at room temperature, the brown fat was no longer visible (it didn't disappear… it just stopped working so hard and consuming the glucose).

In search of the brown fat, the endocrinologist Ronald Kahn from Harvard, Boston, and his colleagues, delved into the PET/CT reviews of about 2000 patients who came to the hospital for various purposes. Brown fat was detected in most of them, while in 5% of the subjects, the brown fat could be seen even without cooling and lowering the temperature. It was also discovered that women generally have more brown fat than men and that the amount of brown fat decreases with age.

In the third study, Enerback and his colleagues used PET/CT scans to identify and precisely locate layers of brown fat in volunteers in cold conditions and then confirm the identity of the tissue through genetic activity. Samples of these tissues displayed the molecular signature of brown fat, including a key protein for heat production that is not present in white fat tissue.

Kahn and van Merken Lichtenblatt's groups also found that overweight people had less brown fat. "Not only do adult humans have brown fat, but it is metabolically active and is correlated with thinness," says Kahn. Why overweight people have less brown fat is a question that still has no answer. Thin people may be thin because they have an excess of brown fat, which helps them burn calories. Or maybe obese people need less brown fat to keep their body warm because they have a thicker insulating layer of white fat.

"Together, these studies represent a turning point in the way we treat brown fat tissue" says endocrinologist Mitchell Lazar from the University of Pennsylvania. Now, he says, researchers should focus on the question of whether brown fat can be harnessed, perhaps by speeding up its activity with drugs, to help overweight patients burn unhealthy and unhelpful white fat.

Sources

Investigators' press release

Beshaar Association, Prof. Yoram Epstein

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