How semaglutide lowers body weight: inside a 2020 rodent study

Semaglutide is best known today as the active ingredient in Ozempic and Wegovy. In people, it produces large, sustained weight loss: in the STEP 1 trial, adults with obesity lost about 15% of their body weight on average over 68 weeks, compared with roughly 2% on placebo. What took longer to pin down was how a drug injected under the skin changes appetite in the brain. A 2020 rodent study from Novo Nordisk researchers, published in JCI Insight, is one of the most detailed attempts to answer that.

What the researchers did

The team gave semaglutide to diet-induced obese mice and rats and tracked body weight, food intake, and energy expenditure for periods ranging from a few days to about eleven weeks. They then used several techniques to follow the drug and its effects inside the brain:

Fluorescently and radioactively labeled semaglutide, to map where the molecule actually ends up.
c-Fos staining, which marks neurons that have recently been activated.
Electrophysiology on hypothalamic neurons, to see how individual cells respond.
RNA sequencing of six small brain regions, to see which genes switch on or off.
Mice lacking the GLP-1 receptor, to confirm the effects depend on that receptor.

What they found

Weight loss came from eating less, not burning more. Semaglutide lowered body weight in a dose-dependent way, by roughly 10 to 22% from baseline in the obese mice. Food intake dropped sharply in the first days of treatment. Energy expenditure did not rise to explain the difference, so reduced intake was the main driver.

It changed food choice, not just quantity. When animals could choose, semaglutide cut intake of high-fat food while leaving standard chow intake relatively unchanged. That matches what many people on GLP-1 drugs describe: less interest in rich food, not just a smaller appetite overall.

It doesn't cross the blood-brain barrier, but it still reaches the brain. Labeled semaglutide was found in around ten brain areas, including the arcuate nucleus of the hypothalamus and the area postrema and nucleus of the solitary tract in the brainstem. Rather than crossing the blood-brain barrier, it got in through circumventricular organs, small regions where that barrier is naturally leaky. From those entry points, the effects spread through connected neural circuits that weren't directly exposed to the drug.

The GLP-1 receptor is required. In mice without the receptor, the weight and appetite effects disappeared.

Limitations

This is an animal study. Rodent brains and metabolism differ from ours, and the authors found differences even between mice and rats in some of the gene-expression changes.
It explains mechanism, not safety. Side effects in people, such as nausea, muscle loss alongside fat loss, and weight regain after stopping, aren't addressed here.
The study was run by the drug’s manufacturer. That doesn’t make it wrong, and the findings fit independent work, but it’s worth knowing.

Why it matters

The study helps explain why GLP-1 drugs feel different from older appetite suppressants: they act on several appetite and reward circuits at once, reached through the brain’s natural entry points, and seem to shift preferences away from energy-dense food. It also set the template for studying the newer multi-receptor drugs, such as tirzepatide and retatrutide, that build on the same GLP-1 foundation.

Sources

Gabery S, Salinas CG, Paulsen SJ, et al. Semaglutide lowers body weight in rodents via distributed neural pathways. JCI Insight. 2020;5(6):e133429. doi:10.1172/jci.insight.133429
Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021;384:989–1002. doi:10.1056/NEJMoa2032183