A decade ago, scientists discovered that energy-consuming brown fat may have an important role for adults. Since then, researchers have sought to understand how it arises and how it can help to counteract obesity. Research confirms that activating brown fat among adult humans increases the metabolic rate. The focus is now on understanding the interaction between the brain and brown fat to ensure that the brain does not respond to any increased energy consumption by increasing appetite.
The thought is captivating. Imagine if you could get your body to burn more energy – without having to lift a finger. A special type of fat cells called brown fat cells contain more of the body’s power plants, mitochondria, than white fat cells. Brown fat mitochondria specialize in converting chemical energy into heat, which expends energy. This enables brown fat tissue to burn fat instead of just storing it as in regular white fat. For years, it was thought that only infants have brown fat, which regulates their body temperature. But over the past decade, researchers have learned that many adults either have this brown fat or can develop it.
“We know that certain factors can promote and activate brown fat, but we also know that activating brown fat does not immediately result in weight loss. The human body is more complex than this, and during evolution we have developed systems to maintain our body weight. In our latest review article, we discuss whether the brain responds to temporary higher energy expenditure with a feedback mechanism that makes us feel hungry in an attempt to maintain body weight,” explains Camilla Schéele, Associate Professor, Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen.
This could explain the lack of weight loss in studies performed over several weeks without limiting food intake. However, several studies have shown that the amount of active brown fat is negatively correlated with obesity. In the new review, the researchers explore the idea that the endocrine capacity of brown fat could explain this discrepancy.
"In other words, in the long term, when the brown fat is better “trained”, we expect it to play a more active role in regulating appetite by sending signals to the brain to increase appetite upon activation of the brown fat when we are cold and, conversely, decrease appetite when we warm up again. Adults with detectable levels of brown fat are generally healthier, but we need to understand the signals sent back and forth between the brain and the fat tissue to understand how the brown fat can promote weight loss."
Brown fat produces heat and consumes energy
Brown fat is scarcely a new concept. Scientists have known for decades that infants rely on it to regulate temperature. They cannot produce heat by moving muscles because the links between the nervous system and muscles are still developing during the first year of life.
"Brown fat has also been widely studied in rodents and other small mammals, helping to maintain body temperature in cold weather. The feeling of cold causes the nervous system to release catecholamines, such as norepinephrine, into the brown fat, and this stimulates energy consumption and heat production in brown fat. A decade ago, scientists confirmed that adults also have brown fat,” explains co-author Jo B. Henningsen, a postdoctoral fellow from the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen.
Researchers discovered that this was not a mere artefact among some adults but that most adults can recreate the brown fat and that age inactivates it among many people.
Research has shown that the brown fat can be reactivated and that both exposure to cold and sustained pharmaceutical activation improve the thermogenic capacity of brown fat, although activation declines with age.
Brown fat communicates with the brain
Artificially activating brown fat was quickly considered as an opportunity to stimulate weight loss among obese people. Unfortunately, this turned out to be more complex because the body has natural feedback mechanisms so that it does not run out of energy.
Our review article discuss the idea that the extra energy consumed by brown fat results in signals being transmitted to the brain about this extra consumption. The brain responds by activating its hunger control centre in the hypothalamus, making the person hungry.
In the short term, the increased energy consumption can therefore result in compensatory increased food intake.
"But we hypothesize that the increased active brown fat will play a greater role in the body’s metabolism and result in better appetite regulation and thus counteract obesity in the long term,” says Jo B. Henningsen.
Have you had a cold shock today?
Brown fat has many other positive effects, including on insulin sensitivity. Researchers do not yet completely understand how and why, but future research should try to elucidate the communication and signalling pathways between the brain, fat tissue and other metabolic organs.
The research group is currently working diligently to understand the interactions between brown fat and the brain. Camilla Schéele has just received a European Research Council Consolidator Grant, one purpose of which is to investigate this interaction in more detail.
“We hope that improved understanding of the interaction between brown fat and the brain can give us a key to reset some of the imbalances among people with obesity and type 2 diabetes. We hope that the research can reveal new mechanisms that will help us to better understand metabolic disorders and to identify new drug targets to combat obesity and type 2 diabetes,” concludes Camilla Schéele.