Genetics is only one piece of a much larger puzzle
A large genetic study has found that variations in appetite-related genes may partly explain why the same weight-loss medication produces dramatically different results in different people — a discovery that places what often feels like personal failure into the quieter language of biology. Analyzing data from fifteen thousand users of GLP-1 medications like Wegovy and Mounjaro, researchers identified two genetic variants influencing both weight loss magnitude and side effect severity. The findings, published in Nature, open a door toward more personalized treatment, though scientists caution that genetics is only one thread in a far more complex human tapestry — and that clinical practice will not change until the evidence grows stronger.
- Millions of people use GLP-1 injections worldwide, yet outcomes range wildly — some losing 30% of their body weight, others barely losing anything — creating confusion, frustration, and unequal expectations.
- A study of 15,000 people has now identified two genetic variants that appear to drive part of this variation: one linked to greater weight loss, another to severe nausea and vomiting in Mounjaro users.
- The genetic effects are real but modest, and experts warn they sit alongside a crowded field of other influences — sex, age, ethnicity, dose, diet, exercise, and quality of clinical support all shape outcomes just as meaningfully.
- The dream of precision medicine — matching patients to medications using genetic profiles — remains on the horizon, as scientists stress the findings must be replicated and tested in rigorous clinical trials before reshaping prescribing practice.
- Meanwhile, access to these medications is expanding globally, with Rio de Janeiro becoming the first Brazilian city to offer semaglutide through its public health system following the expiration of its patent in March 2026.
A study of fifteen thousand people has found that genetic variations in appetite-related genes may help explain one of the most puzzling features of weight-loss injections: why they work so differently from person to person. Researchers analyzing data from users of medications like Wegovy and Mounjaro identified two variants that appear to influence both how much weight someone loses and how severely they experience side effects like nausea and vomiting. Published in Nature, the findings suggest that what can feel like personal failure may be partly encoded in biology.
These injectable medications mimic a natural intestinal hormone that signals fullness to the brain, and their popularity has surged — in the United Kingdom alone, an estimated 1.6 million people used weight-loss drugs in the past year. Clinical trials show average losses of 14% with semaglutide and 20% with tirzepatide, but those averages conceal enormous variation. In this study, participants lost an average of 11.7% of body weight over eight months, with some losing as much as 30% and others losing almost nothing.
Working with genetic data from 23andMe, researchers found one variant — common in people of European descent but present in only about 7% of African Americans — linked to an extra 0.76 kilograms of weight loss on average, doubling in those carrying two copies. A second variant appeared connected to severe vomiting episodes in some Mounjaro users. Reviewer Ruth Loos noted that the variant associated with better weight loss also correlated with nausea, suggesting the two effects may share a common biological mechanism.
Yet experts were careful to keep the findings in proportion. Women are more than twice as likely as men to lose 15% or more of their body weight on Mounjaro. Age, ethnicity, medication type, dose, exercise, diet, and clinical support all shape results significantly. Researcher Marie Spreckley emphasized that behavioral and clinical factors remain the primary drivers of success, with genetics as just one piece of a much larger puzzle.
Specialists including metabolic health professor Naveed Sattar stressed that these findings remain far from changing clinical practice, and that replication in other studies — which has not yet occurred — will be essential. The broader vision of precision medicine, using genetic data to personalize treatment selection, remains a future possibility rather than a present reality. At the same time, access to these medications is expanding: Rio de Janeiro recently became the first Brazilian city to offer semaglutide through its public health system, following the expiration of its patent in March 2026 — a development that may accelerate both access and the urgency of understanding who benefits most.
A study of fifteen thousand people has found something that might explain why weight-loss injections work so differently from one person to the next: your genes. Researchers analyzing data from users of medications like Wegovy and Mounjaro discovered that variations in two genes linked to appetite and digestion appear to influence both how much weight someone will lose and how intensely they'll experience side effects like nausea and vomiting. The findings, published in Nature, suggest that what feels like a personal failure—losing less weight than a friend, or suffering through worse nausea—may partly be written into your biology.
These injectable medications, now used by millions across the world, work by mimicking a natural intestinal hormone that signals fullness to the brain. They have become remarkably popular, particularly in countries where they're available through private pharmacies. In the United Kingdom alone, an estimated 1.6 million people used weight-loss drugs in the past year, a number expected to climb. The medications produce real results: clinical trials show average weight loss of 14 percent with semaglutida and 20 percent with tirzepatida. But those averages mask enormous variation. In this study, participants lost an average of 11.7 percent of their body weight over roughly eight months—but some shed as much as 30 percent while others lost almost nothing.
The research team, working with genetic data from the company 23andMe, identified a pattern in millions of genetic variations across the fifteen thousand participants. One variant, particularly common in people of European descent, appeared linked to greater weight loss. Those carrying a single copy of this variant lost an extra 0.76 kilograms on average; those with two copies lost roughly twice that amount. The variant is present in 64 percent of Europeans with one copy and 16 percent with two, but only about 7 percent of African Americans carry even one copy. Ruth Loos, a professor at the University of Copenhagen who reviewed the study, noted that the same genetic variant associated with better weight loss also correlated with nausea—suggesting that the mechanism driving one effect drives the other.
A second variant emerged as potentially linked to severe side effects in users of tirzepatida, the active ingredient in Mounjaro. The research suggests that up to 1 percent of people taking this medication might experience vomiting episodes occurring nearly fifteen times more frequently than typical. These are not trivial differences. Yet Loos emphasized that while the genetic effect is real, it remains modest—comparable in magnitude to other known factors but hardly determinative on its own.
The larger picture is far more complicated. Women are more than twice as likely as men to lose 15 percent or more of their body weight on Mounjaro. Younger age, white ethnicity, and Asian ethnicity all correlate with greater weight loss, though researchers don't yet understand why. The medication itself matters: different drugs, different doses, and different treatment durations all produce different outcomes. So do behavioral and clinical factors—daily exercise, eating well while on the medication, the quality of medical support received, and existing health conditions all shape results. Marie Spreckley from Cambridge University stressed that genetics is only one piece of a much larger puzzle. The main drivers of success, she argued, are behavioral, clinical, and treatment-related factors.
Experts caution that this research, while scientifically interesting, has not yet changed how doctors prescribe these medications. Naveed Sattar, a metabolic health specialist at the University of Glasgow, noted that the findings remain far from clinical practice. What's needed now, he said, is more robust data from clinical trials to better define the balance between benefits and risks. The vision of precision medicine—using genetic information combined with other patient data to select the ideal treatment for each person—remains distant. The findings need to be replicated in other studies, something that has not yet happened.
In Brazil, access to these medications is beginning to shift. Rio de Janeiro became the first city in the country to offer Ozempic through the public health system on March 18, following the expiration of semaglutida's patent on March 20. With Novo Nordisk's exclusive rights ended, other companies can now develop their own versions of the drug. The combination of genetic research and expanding access suggests that how these medications are prescribed and personalized may change significantly in the years ahead—but that transformation will require far more evidence than currently exists.
Citações Notáveis
The study identified a genetic variant associated with greater weight loss, which also correlates with nausea—people with this variant tend to lose more weight.— Ruth Loos, University of Copenhagen
Genetics is only one part of a much more complex picture; the main determinants of results are behavioral, clinical, and treatment-related factors.— Marie Spreckley, University of Cambridge
A Conversa do Hearth Outra perspectiva sobre a história
Why does this genetic finding matter if the effect is so modest?
Because it's not just about the size of the effect—it's about prediction. If you know someone carries this variant, you can tell them they're likely to lose more weight and experience more nausea. That's useful information for setting expectations and managing side effects.
But the study says other factors matter more. Doesn't that undermine the genetic story?
Not really. It means genetics is one tool among many. The point isn't that genes are everything; it's that they're something. When you're trying to help someone lose weight, you want every piece of information you can get.
Why is the variant so much rarer in African Americans than Europeans?
That's a question the research doesn't answer. It's a reminder that most genetic studies have been done on European populations. We don't know if this variant is actually less common in African Americans or if it's just been less studied.
So could this lead to personalized medicine—picking the right drug for the right person?
Theoretically, yes. But the experts are clear: we're not there yet. The findings need to be replicated, and we need much more data from clinical trials. Right now it's a promising direction, not a practical tool.
What about the side effects? The study mentions some people get severe vomiting.
That's where the second genetic variant comes in. If you carry it, you're at much higher risk of intense nausea and vomiting on Mounjaro. That's clinically important—it could help doctors decide whether to use a different medication or prepare patients for what to expect.
Does this explain why women lose more weight than men on these drugs?
No. That's a separate finding that researchers don't yet understand. It suggests there are multiple biological and behavioral factors at play that have nothing to do with these appetite genes.