Scientists discover a new brain pathway that rapidly depletes diet-resistant body fat

Scientists have identified a potent pathway originating in the brain that can cause the rapid depletion of all body fat, including stubborn fat deposits that typically resist diet and exercise. This process operates independently of the nervous system signals usually responsible for fat loss, and it instead relies on a specific state of low blood sugar and low insulin. The findings were published in the journal Nature Metabolism.

Survival depends on the ability of the body to store and mobilize calories during times of severe need. Most fat cells release energy in response to typical signals like fasting or physical activity. However, humans and other mammals possess specialized populations of fat cells, known as stable adipocytes, that remain largely unchanged during normal calorie restriction or exercise.

These stable fat cells make up about 70 percent of the fat located deep within human bone marrow. Scientists wanted to understand why these specific fat cells resist typical weight-loss cues and how the body eventually breaks them down during extreme conditions. In severe states of starvation or wasting diseases, the body ultimately consumes these stable fat reserves. Until now, the exact biological mechanisms controlling this end-stage fat depletion were unknown because scientists lacked a reliable animal model.

“Certain fat cells within the body are stable and resistant to diet and exercise. We wanted to figure out why and how to deplete them,” said study author Erica L. Scheller, an associate professor at the Washington University School of Medicine and executive director of the Washington University Center of Regenerative Medicine.

To explore this phenomenon, the researchers developed a new procedure to rapidly trigger the loss of all body fat in adult male mice between 12 and 17 weeks of age. They continuously delivered a hormone called leptin, which normally regulates energy balance, directly into the brains of the mice.

The delivery was achieved using microscopic pumps implanted under the skin and connected to the brain. The researchers administered doses of either 10 or 100 nanograms of leptin per hour over a nine-day period. They strictly controlled the food intake of these mice, matching it exactly to a control group of mice that received a harmless saline solution.

Over the nine days, the mice receiving the highest dose of brain-directed leptin lost an average of 19.3 percent of their body mass. This occurred even though they consumed the exact same amount of food as the control mice. The scientists observed a specific, cascading pattern of fat loss.

Regular fat deposits under the skin and around the organs disappeared within the first few days of the experiment. The stable fat hidden deep within the bone marrow was much more resistant to the treatment. This deep skeletal fat only depleted completely by day nine in the mice receiving the highest leptin dose.

The researchers initially suspected that local nerves or stress hormones called catecholamines, such as adrenaline, were driving this extreme fat loss. To test this, they surgically severed the sciatic nerve in one leg of several mice to disable the local nervous system. In a separate experiment, they used a specialized chemical to destroy sympathetic nerves throughout the entire body of another group of mice.

The scientists also tested genetically modified mice, aged nine to 12 months, that were completely unable to produce certain stress hormones. To their surprise, removing the nerves and stress hormones did not prevent the extreme fat loss. This provides evidence that the brain communicates with these stable fat cells through an entirely different method via the bloodstream.

Further testing revealed that the continuous leptin delivery caused the mice to experience concurrent low blood sugar and low insulin levels. This specific physical state is clinically known as hypoinsulinemic hypoglycemia.

To see if this physical state was responsible for the fat loss, the scientists implanted insulin pellets under the skin of a new group of leptin-treated mice. This artificial implant restored the circulating insulin in the mice back to normal levels. Restoring insulin selectively protected the stable bone marrow fat from being broken down, though regular body fat still disappeared. This suggests that the exact combination of low blood sugar and low insulin is necessary to unlock these stubborn fat reserves.

“We were very surprised that the activation of stable adipocyte loss by the brain occurred through signals in the blood and did not involve the peripheral nervous system,” Scheller told PsyPost. “This is different than standard neural systems that regulate traditional fat depots.”

The scientists then examined the genetic makeup of these stable fat cells using advanced genetic sequencing techniques. They found that, under normal conditions, stable fat cells produce high levels of specific proteins that act as internal brakes. One major brake is a protein called G0S2, which blocks the internal cellular machinery that breaks down stored fat.

When the mice experienced low blood sugar and low insulin, the production of this G0S2 protein dropped significantly. The removal of this internal brake allowed an enzyme called adipose triglyceride lipase to finally break down the stored fat. This biological process is known as lipolysis, which is how the body turns stored fat into usable energy.

The researchers also observed this exact same process in a separate group of mice experiencing severe, tumor-induced weight loss. They injected colon cancer cells into 12-week-old adult mice to induce a severe wasting disease known as cachexia. In the final days of the cancer progression, these mice experienced the exact same drop in blood sugar, insulin, and the G0S2 protein. This suggests that this newly discovered biological pathway represents a universal response to extreme physical stress.

“In mice, activation of stable fat catabolic pathways drives loss of all body fat within 9 days without reducing food intake,” Scheller said. “Future titration of the effect could be used to inform therapies for fat loss and to support healthy fat storage in patients with cachexia and wasting disorders.”

While these findings provide insights into how the body burns fat, the researchers caution against viewing this as a potential weight-loss strategy. These stable fat deposits tend to provide necessary mechanical and physical support to important body structures.

Depleting these specialized fat cells is considered dangerous. In human patients, the loss of bone marrow fat is associated with severe consequences like bone fractures. One potential misinterpretation of the study is that standard diets could trigger this extreme fat loss, but the researchers note that typical dieting does not produce the severe biological conditions required to activate this pathway.

The study was primarily conducted in mice, meaning the exact timelines and biochemical thresholds might differ slightly in humans. Additionally, the exact blood-based signals that interact with the fat cells once the internal brakes are removed remain unidentified. The scientists believe there may be multiple circulating factors working together to complete the final fat breakdown process.

The scientists hope that understanding this newly discovered biological pathway will eventually inform targeted medical interventions. By blocking this specific fat-loss pathway, doctors might be able to support healthy fat storage in severely ill individuals. This could ultimately improve survival rates and the overall quality of life for these vulnerable patients.

“Wasting is a debilitating consequence of diseases including cancer, chronic infection, and end-stage organ failure,” Scheller said. “Atrophy of muscle and fat can decrease a patient’s ability to withstand necessary chemotherapy and treatment, contributing substantially to loss of life. We hope to use this work to discover new intervention points to prevent pathological fat loss and improve survival.”

The study, “A catecholamine-independent pathway controlling adaptive adipocyte lipolysis,” was authored by Xiao Zhang, Sreejith S. Panicker, Jordan M. Bollinger, Anurag Majumdar, Rami Kheireddine, Lila F. Dabill, Clara Kim, Brian Kleiboeker, Fengrui Zhang, Yongbin Chen, Kristann L. Magee, Brian S. Learman, Adam Kepecs, Gretchen A. Meyer, Jun Liu, Steven A. Thomas, Irfan J. Lodhi, Ormond A. MacDougald, and Erica L. Scheller.