Fat tissue and the brain talk both ways over nerves. Sympathetic nerves carry the brain’s signal to burn fat, and sensory neurons inside fat tissue, confirmed by Scripps researchers in the journal Nature, carry messages back to the brain. They act as a brake on fat burning.
Adipose tissue (body fat) stores extra energy. When you eat fewer calories than you burn, the body burns fat for fuel. Fat tissue also makes many signaling molecules that shape metabolism and well-being. But in recent years researchers have found that fat is also rich in nerves and talks to the brain all the time. Fat is wired to the sympathetic nervous system, and more sympathetic activity means more fat burning. Fat also has sensory neurons. When these neurons fire more, they act as brakes and stop fat burning.
What does fat say to the brain? And how does it talk to the brain? This matters for many reasons. If science grasps the details of this conversation, it can better understand obesity and metabolic disorders. Most people picture fat as dead weight. That view is changing fast. Science now knows fat speaks many languages. Fat cells can talk to many other parts of the body.
In recent years, researchers have learned more about fat as a hormone gland. For example, they now know fat releases many hormone-like compounds. These shape appetite, metabolic rate, the immune system, and much more. But fat seems to talk to the body and brain in more than one way. So it is wrong to think of fat only as tissue that sends messages through hormones and other compounds. Instead, fat and the brain seem to hold a two-way conversation that never stops.
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A new study confirms that adipose tissues produce hormones responsible for sending various signals, and fat cells are also rich in nerve cells. In the new study published in the journal Nature, Scripps Research scientists focused on sensory neurons in fat, because they carry key information from fat to the brain1. The proof that fat has sensory neurons taking part in this live fat-brain conversation is mind-blowing. It is the first study to confirm that fat and the brain talk all the time over nerve pathways. Scientists at Scripps Research say the impact of these findings is huge.
This is not the first study to show that fat has nerves. Researchers have long known that nerve cells link fat to the brain. But they did not know these nerve cells also sense things. In recent years, science has seen more and more how much sensory neurons matter for health. In mammals, fat mainly serves as an energy store. But science has also learned a lot about the hormones and signaling molecules fat cells release. These signals, and their breakdown, play a key role in metabolic disorders like obesity, diabetes, atherosclerosis, fatty liver disease, and more.
Until now, researchers believed most nerve cells in fat belonged to the sympathetic nervous system. They thought the brain and fat held a one-way conversation. When the brain senses the body needs more energy, it signals fat to turn white-fat cells into brown fat cells and switch on fat burning. In the new study, researchers used a special method called HYBRID to see where and how signals travel between fat and the brain. They found that many nerve fibers leaving fat did not connect to the sympathetic nervous system. Instead, they connected to the dorsal root ganglia. These are clusters of sensory nerve cells that sit beside the spinal cord.
Researchers then used another technique to confirm what they found. They used a special virus to destroy the sensory neurons in fat. This showed what changes in the body when these sensory neurons are missing or damaged. It also helps show what happens when they are overactive. When the brain did not get these sensory signals, more white-fat cells turned into brown-fat cells. Fat breakdown rose, and so did thermogenesis (heat production). Sympathetic nervous system activity also went up.
So researchers concluded that sensory and sympathetic neurons work against each other. Sympathetic neurons are needed to burn fat. Sensory signals act as brakes and slow the sympathetic neurons down. This two-way traffic between fat and the brain, over sensory and sympathetic nerves, keeps fat cells in balance. Think of it this way: sympathetic neurons are the gas pedal, and sensory neurons are the brake.
Signaling of this kind is part of why weight is approached as a medical problem in our medical weight loss program.
Frequently asked questions
Are there nerves in fat?
Yes. Fat tissue is full of nerves and stays in constant contact with the brain. Most of those nerves belong to the sympathetic nervous system, which switches fat burning on. Scripps researchers, publishing in Nature, confirmed that fat also holds sensory neurons. They found that many nerve fibers leaving fat tissue do not connect to the sympathetic system at all.
How does fat talk to the brain?
In two ways. Fat cells release hormone-like signaling molecules that shape appetite, metabolic rate and the immune system. Fat is also wired to the brain through nerves, and that conversation runs both ways. The brain sends the signal to burn fat, and sensory neurons in the fat send information back. Fat is not dead storage. It talks to the body in more than one language.
What do the sensory nerves in fat do?
They act as a brake. Researchers used a special virus to destroy the sensory neurons in fat, so the brain stopped getting their signals. The result was more white fat turning into brown fat, more fat breakdown, more heat production and higher sympathetic activity. Sympathetic nerves work like the gas pedal for fat burning. Sensory nerves work like the brake.
Why does fat-brain signaling matter for weight and health?
The signaling molecules fat releases, and their breakdown, play a key role in metabolic disorders. These include obesity, diabetes, atherosclerosis and fatty liver disease. Knowing how fat and the brain talk helps science understand obesity itself. It is also why our medical weight loss program treats weight as a medical problem.
Dr. Gurpreet Singh Padda, MD, MBA, MHP
