New research powerfully strengthens the case against soda and other sugary drinks as culprits in the obesity epidemic.

A huge, decades-long study involving more than 33,000 Americans has yielded the first clear proof that drinking sugary beverages interacts with genes that affect weight, amplifying a person’s risk of obesity beyond what it would be from heredity alone.

This means that such drinks are especially harmful to people with genes that predispose them to weight gain. And most of us have at least some of these genes.

In addition, two other major experiments have found that giving children and teenagers calorie-free alternatives to the sugary drinks they usually consume leads to less weight gain.

Collectively, the results strongly suggest that sugary drinks cause people to pack on the pounds, independent of other unhealthy behavior such as overeating and getting too little exercise, scientists say.

That adds weight to the push for taxes, portion limits like the one just adopted in New York City, and other policies to curb consumption of soda, juice drinks and sports beverages sweetened with sugar.

Soda lovers do get some good news: Sugar-free drinks did not raise the risk of obesity in these studies.

The studies were presented Friday at an obesity conference in San Antonio and were published online by the New England Journal of Medicine.

The gene research in particular fills a major gap in what we know about obesity. It was a huge undertaking, involving three long-running studies that separately and collectively reached the same conclusions. It shows how behavior combines with heredity to affect how fat we become.

Sugary drinks are the single biggest source of calories in the American diet, and they are increasingly blamed for the fact that a third of U.S. children and teenagers and more than two-thirds of adults are obese or overweight.

Consumption of sugary drinks and obesity rates have risen in tandem — both have more than doubled since the 1970s in the United States. But until now, high-quality experiments have not conclusively shown that reducing sugary beverages would lower weight or body fat.

n In one study, researchers randomly assigned 224 overweight or obese high schoolers in the Boston area to receive shipments every two weeks of either the sugary drinks they usually consumed or sugar-free alternatives, including bottled water. No efforts were made to change the youngsters’ exercise habits or give nutrition advice, and the kids knew what type of beverages they were getting.

After one year, the sugar-free group weighed more than 4 pounds less on average than those who kept drinking sugary beverages.

The weight difference between the two groups narrowed to 2 pounds in the second year of the study, when drinks were no longer being provided. That showed at least some lasting beneficial effect on kids’ habits. The study was funded mostly by government grants.

n A second study involved 641 normal-weight children ages 4 to 12 in the Netherlands who regularly drank sugar-sweetened beverages. They were randomly assigned to get either a sugary drink or a sugar-free one during morning break at their schools. After 18 months, the sugary-drink group weighed 2 pounds more on average than the other group.

The studies “provide strong impetus” for policies urged by the Institute of Medicine, the American Heart Association and others to limit sugary drink consumption, Dr. Sonia Caprino of the Yale School of Medicine wrote in an editorial in the journal.

The American Beverage Association disagreed.

“Obesity is not uniquely caused by any single food or beverage,” it said in a statement. “Studies and opinion pieces that focus solely on sugar-sweetened beverages, or any other single source of calories, do nothing meaningful to help address this serious issue.”

The genetic research was part of a much larger set of health studies that have gone on for decades across the United States, led by the Harvard School of Public Health.

Researchers checked for 32 gene variants that have previously been tied to weight. Because we inherit two copies of each gene, everyone has 64 opportunities for these risk genes. The study participants had 29 on average.

Every four years, these people answered detailed surveys about their eating and drinking habits as well as things like smoking and exercise.

A clear pattern emerged: The more sugary drinks someone consumed, the greater the impact of the genes on the person’s weight and risk of becoming obese. For every 10 risk genes someone had, the risk of obesity rose in proportion to how many sweet drinks the person regularly consumed. Overall calorie intake and lifestyle factors such as exercise did not account for the differences.

This means that people with genes that predispose them to be obese are more susceptible to the harmful effects of sugary drinks on their weight, said one of the study leaders, Harvard’s Dr. Frank Hu. The opposite also was true — avoiding these drinks can minimize the effect of obesity genes.

“Two bad things can act together and their combined effects are even greater than either effect alone,” Hu said. “The flip side of this is everyone has some genetic risk of obesity, but the genetic effects can be offset by healthier beverage choices. It’s certainly not our destiny” to be fat.