Study details

For the study, which was published in Nature Medicine, researchers from 500 institutions — working under the Genetic Investigation of Anthropometric Traits (GIANT) Consortium — analyzed the genomes of more than 5 million people. The study was led by scientists at the University of Copenhagen's Novo Nordisk Foundation Center for Basic Metabolic Research (CBMR) in collaboration with 23andMe.

Researchers developed a polygenic risk score (PGS), a tool that aggregates the effects of thousands of genetic variants associated with body mass index (BMI) and fat storage.

"What makes the score so powerful is its ability to predict, before the age of five, whether a child is likely to develop obesity in adulthood, well before other risk factors start to shape their weight," said Roelof Smit, the study's lead author and assistant professor at CBMR.  

The new PGS significantly improves on earlier genetic screening tools. According to NBC News, the new score explains up to 17.6% of a person's risk of developing obesity in adulthood — more than double the predictive power of the next-best method, which accounted for just 8.5%. 

"This new polygenic score is a dramatic improvement in predictive power and a leap forward in the genetic prediction of obesity risk," said Ruth Loos, a professor at CBMR. "[It] brings us much closer to clinically useful genetic testing."

However, performance varied widely across racial and ethnic groups. The test explained only 2.2% of obesity risk in rural Ugandans and about 16% in East Asian Americans. According to NBC News, roughly 70% of the genetic data used in the study came from people of European ancestry. About 14% were Hispanic, 8% were East Asian, and just under 5% were of predominantly African ancestry.

"The next step is to collect more — and more diverse — data on people with African ancestry in particular," Loos said. Broader representation will be essential to make the score more accurate for all populations. 

Still, genetics alone do not determine outcomes. "Obesity is not only about genetics, so genetics alone can never accurately predict obesity," Loos said. "For the general obesity that we see all over the world, we need other factors such as lifestyle that need to be part of the predictions.” 

Juliana Simonetti, codirector of the Comprehensive Weight Management Program at the University of Utah, has been using genetic tests with her adult patients for several years. "Obesity is not homogeneous. We have different kinds and different presentations," she said.

Genetic testing can be helpful for identifying people who struggle with satiety — those who eat but don't feel full.

People who have these mutations "tend to have higher weight," Simonetti explained. But that's just one piece of a much larger picture. "The genes that a person inherits from either parent, even if they are not mutations, also determine how a person's body stores weight or uses energy," she said.

Genetics may also influence treatment effectiveness. "We are talking about three out of 80 mutations that we can treat," Simonetti noted. "We are getting better, and the more data we have, I'm hopeful that we are going to do a better job in being more precise in understanding treatment responses."