FGF21 has been getting a lot of attention lately, and for good reason: its effects are promising for people with obesity.
It is a liver hormone, that is, synthesized by the liver, known for:
- increasing energy expenditure
- reducing food intake
- improving insulin sensitivity
Its role in weight loss has intrigued scientists for several years. But until now, it was unclear where it acted in the brain. That is what Yunfan Lin’s team, under the direction of Prof. Matthew Potthoff, at the University of Oklahoma College of Medicine in the United States sought to clarify.
Let’s decode this study published in April 2026.
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How was the study conducted?
The researchers conducted an experimental study on male mice.
First, the mice were made obese thanks to a very high-fat diet. Then, the scientists injected them with the hormone FGF21, already known to promote weight loss.
To understand the circuits used by the hormone, some neurons were turned off, while others were selectively reactivated. At the same time, injection of a tracer virus made it possible to map the connections between the neurons.
Finally, several parameters were measured: weight, body temperature, energy expenditure, and brown adipose tissue activity, this type of fat capable of producing heat, a phenomenon called thermogenesis.
What do the results say?
The researchers already knew that FGF21 increased energy expenditure, but the neuronal circuits involved remained poorly understood. This study made it possible to identify one, very precise, in the brainstem.
And that is the whole surprise. For a long time, it was thought that the hypothalamus, considered the “conductor” of metabolic regulation, was the main region involved. In reality, FGF21 acts mainly at the level of the brainstem.
More specifically, the hormone first acts on two structures in the brainstem, the nucleus of the solitary tract (NTS) and the area postrema (AP), which then relay the signal to the parabrachial nucleus, involved in the regulation of appetite and satiety. This circuit is essential: when the researchers inhibit it, the weight loss induced by the hormone disappears.
What strikes me: this brainstem is the same region targeted by GLP-1 drugs such as Ozempic and Wegovy. But FGF21 acts there through the opposite mechanism.
Where GLP-1 analogs reduce appetite and therefore food intake, FGF21 increases energy expenditure and helps the body burn more energy. Two different routes to the same goal, and that is what makes this discovery so interesting to me.
A promising discovery, but still insufficient
Even if the results are encouraging, I want to put them into perspective.
First, the study was conducted only on male mice. Second, these mice were given a diet made up of 60% fat right before the hormone injection, which is very far from human reality: obesity is a multifactorial disease that develops gradually, along with a whole host of associated metabolic changes.
It is also important to remember that the effects observed in mice are not directly transposable to humans. Caution is still warranted.
Finally, the researchers themselves note that the long-term effects of FGF21 on energy expenditure remain to be confirmed.
And what about side effects?
This is a point I want to clarify, because it is often misunderstood. Sometimes we read that FGF21 would be “free of side effects.” That is false: the FGF21 analogs already tested have shown adverse effects, including digestive issues and, in some cases, loss of bone mass.
Effects that I also consider similar to those of current GLP-1 treatments (nausea, diarrhea, vomiting).
The researchers’ hope is therefore not that the hormone will be free of side effects, but that by targeting the circuit they have just identified, it may one day be possible to design more targeted and better-tolerated treatments.
To conclude
Obesity is a multifactorial disease, and its dietary management is complex: I see this every week in my consultations. The question of the speed of weight loss, in particular, keeps coming up. Many of my patients get discouraged when results take a long time to appear.
But we know that weight loss that is too rapid is also the hardest to maintain. The more gradual the changes are and the more they are integrated into daily life, the more the new habits last in the long term.
That is precisely where discoveries like this could, in the long term, open up new avenues: in addition to dietary rebalancing, a better understanding of these brain mechanisms could help the people concerned. But we are still only at the stage of basic research, in mice. So this is something to watch with interest and caution!
Scientific reference
Lin Y, Claflin KE, Aklan I, Morgan DA, Sullivan AI, Rudolph MC, Rahmouni K, Potthoff MJ. « Pharmacological administration of FGF21 reverses obesity through a parabrachial-projecting neuron population in the hindbrain ». Cell Reports, 2026;45(4):117093. DOI : 10.1016/j.celrep.2026.117093
