Researchers have known for decades that genetics may be more powerful than environment in predicting who will develop obesity. Identical twins tend to have the same body mass index, even if they are reared apart. Adopted children tend to have a degree of obesity similar to their birth parents rather than their adoptive ones.

Identifying the genetic roots of obesity could aid with prevention starting in childhood. But finding a genetic footprint for obesity has proved challenging. With rare exceptions, there’s not one gene or even a few that are the culprits. Instead, obesity is spurred by thousands of gene variants acting in concert. Each variant exerts a tiny effect.

Now, using genetic data from five million people, an international group of hundreds of researchers reports that it has developed an obesity risk score, known also as a polygenic risk score. It combines thousands of gene variants to estimate individuals’ predicted body mass indexes, which continue to be used by doctors to anticipate weight-related health dangers.

The researchers showed that the scores can predict which young children are at risk of obesity as adults. And, in another test, they found that overweight and obese adults with high risk scores quickly regain any weight that they lose with lifestyle programs.

Their paper was published on Monday in the journal Nature Medicine.

Dr. Joel Hirschhorn, an author of the paper and a professor of pediatrics and genetics at Boston Children’s Hospital, cautioned that genetics cannot account for the effects of environment and is therefore inherently limited in predicting obesity.

“We will almost never be able to say a child will have a BMI of 38 as an adult,” he said. “Genetics is not that predictive.”

However, genetics can offer indications of who is and who is not at risk, he said.

“There is definitely predictive value in genetics,” Dr. Hirschhorn said. And, he added, with the new study, “we are now a lot closer to being able to use genetics in a potentially meaningful predictive way.”

The new study is “intriguing,” said Dr. Pradeep Natarajan, a geneticist at Massachusetts General Hospital who was not involved with the study. And, he added, as obesity risk scores continue to improve, he can see a future when they might be used like risk scores for heart disease, helping doctors determine how aggressively to treat conditions like slightly elevated cholesterol or blood pressure.

But, he said, more testing was needed to see if the obesity score can predict which interventions will help.

Dr. Hirschhorn added that the obesity score that the group derived has the power to account for only about a third of the genetic effect on obesity. To fully account for genetics would require even bigger populations to find variants that, individually, have less power to predict obesity. And because most of the data in the new study were from Europeans, the score was most accurate in predicting obesity in people with European ancestry.

Even so, study researchers said their results looking at obesity risks in young children show what could be achieved.

The weights of children under age 5 do not predict their future obesity risk, noted Ruth Loos of the University of Copenhagen and a principal investigator in the new study. But their polygenic risk scores do, within limits.

The difference in the predicted adult BMI between a child whose risk score is in the bottom 10 percent and one whose score is in the top 10 percent is useful, the study authors said. Dr. Hirschhorn compared it to “the difference between someone in the middle of the normal weight range and someone who is on the borderline of having obesity.”

The group also tested risk scores by looking at the genetics and outcomes of participants in studies using diet and exercise to reduce weight. In those studies, people with the highest risk scores responded best to the lifestyle programs. But they also were the least likely to retain what they’d accomplished — they regained all the weight they lost in the first year after the study ended. Those with lower risk scores did better at maintaining their weight loss.

The researchers said there was more to come as they probe the relationships of genes and obesity.

Dr. Loos and her colleagues have posted a paper online that uses genetic scores to categorize people with obesity into those who will and those who will not have cardiovascular complications (the paper is not yet published in a peer-reviewed journal).

In unrelated research, Dr. Natarajan and his colleagues are using clinical trial data from studies of the new obesity drugs to calculate obesity risk scores of participants and are asking if the score predicts how well people responded to the drugs.

In the future, obesity risk scores, he predicted, will suggest courses of action that are most likely to be successful.

“We want to do something that will improve outcomes,” Dr. Natarajan said.