Scientists think they understand how our brains monitor our body fat levels: by monitoring fat-related hormones in our blood.
But in a new study, researchers discovered a whole additional messaging system.
It turns out that we have an entire sensory system dedicated to carrying messages from our adipose tissue (adipose tissue) to our brains.
“The discovery of these neurons suggests for the first time that your brain is actively investigating your fat, rather than just passively receiving messages about it,” says neuroscientist Li Ye of the Scripps Research Institute.
“The implications of this discovery are profound.”
Understanding this system may one day help the growing number of us who struggle with weight and related health problems like heart disease and diabetes.
It adds another layer to the already complex interplay between our genes, environment, diet and microbiome, all of which contribute to our levels of these important insulating energy stores.
Although researchers have long known that mammalian fat is full of neurons, these nerves have been linked in animal models to the mammalian sympathetic nervous system—the system that controls our body’s automatic, unconscious responses like an increased heart rate or widening of the eyes.
They promote the breakdown of fat for use during physical activity, hunger and other stresses.
But while these messages going from the brain to our fat have been established, questions remain about what signaling happens in the opposite direction in our nerves.
“When we first started this project, there were no existing tools to answer these questions,” explains neuroscientist Yu Wang of the Scripps Research Institute.
So Wang and his colleagues developed the tools, including a new imaging technique called HYBRiD and a method of targeted cell manipulation called ROOT, to overcome the technical difficulties of reaching neurons deep in our body’s fat without confusing them.
The researchers designed HYBRiD (Hydrogel Reinforced Purified Mammalian Tissue) to enable close examination of large intact tissue samples. It uses solvents to remove molecules that give fabrics their opacity, resulting in transparent fabrics still in their original configurations.
The addition of fluorescent proteins that target specific tissue types allows researchers to clearly image their structures of interest.
The resulting visualizations allowed Wang and team to clearly see that almost half of the fat neurons do not connect to the sympathetic nervous system, but to the sensory nervous system.
They then used ROOT (retrograde vector optimized for organ tracing) to selectively target and kill different subsets of neurons in mice.
Loss of sensory neuron signaling leads to more fat in mice, with especially high levels of brown fat. The mice also had higher body temperatures, which makes sense because brown fat helps our bodies convert other fats and sugar into heat.
The researchers concluded that their newly discovered sensory neuron system must act to regulate signals from the sympathetic nervous system, instructing the body to burn our fat—by turning it down or off.
“This tells us that there is not simply a universal instruction that [the] the brain sends fat,” Li says.
“It’s more nuanced than that; these two types of neurons act as gas pedal and brake for fat burning.’
The team suspects that these nerves may also play a crucial role in interoception – the perception of sensation coming from our body, as is the case with similar neurons found in other organs. But they haven’t studied this yet and want to study this system further.
This research was published in Nature.