The Brain 'Switch' That Explains Why Opposite Obesity Drugs Both Work

Cambridge researchers have solved a long-standing puzzle in obesity medicine: why drugs that activate the GIP receptor and drugs that block it can both drive weight loss. In mice, activating the receptor in the brainstem curbed appetite, while blocking it in the hypothalamus released a 'brake' on fullness signals. The findings, published in Nature Metabolism, could help design stronger drug combinations with existing GLP-1 medicines.

Key Takeaways

Cambridge researchers have solved a long-standing puzzle in obesity medicine: why drugs that activate the GIP receptor and drugs that block it can both drive weight loss.
In mice, activating the receptor in the brainstem curbed appetite, while blocking it in the hypothalamus released a 'brake' on fullness signals.
The findings, published in Nature Metabolism, could help design stronger drug combinations with existing GLP-1 medicines.

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A new generation of weight-loss drugs has transformed obesity treatment, but one question has puzzled scientists for years: medicines like Mounjaro and Zepbound activate a receptor called GIPR, while others such as MariTide block it — yet both approaches can promote weight loss. How can opposite actions on the same target achieve the same result?

Researchers at the Institute of Metabolic Science, University of Cambridge, say the answer lies in geography. Their study, published in Nature Metabolism, shows that the outcome depends on which part of the brain is affected.

Using genetically engineered mice, the team removed GIPR selectively from different brain regions. They found that GIPR agonists — drugs that activate the receptor — work primarily through the brainstem, the region at the base of the brain involved in appetite and nausea. Activating the receptor there reduced appetite and led to lower body weight.

GIPR antagonists, which block the receptor, take a different route. They work through the hypothalamus, where GIPR acts as a kind of 'brake' limiting how strongly the brainstem responds to signals that the body is full. Blocking the receptor releases that brake, letting fullness signals hit harder.

'Understanding which brain circuits respond to these medications — and how they do so — could help us design better drugs that produce more weight loss with fewer side effects,' said Dr. Jo Lewis, the study's first author.

The team also found evidence that blocking GIPR could strengthen emerging medicines that target the amylin receptor, hinting that GIPR antagonists might eventually boost several classes of obesity treatments. The findings also help explain why MariTide — a phase 3 candidate combining GIPR antagonism with GLP-1 agonism — appears effective.

'Our work also strengthens the idea that the brain is central to obesity treatment. Obesity drugs are not acting simply on the gut or pancreas,' Lewis said. The research was funded by the Medical Research Council and Wellcome.