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

A 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 teens calorie-free alternatives to the sugary drinks 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.

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

The studies were being 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.

Having many of these genes does not guarantee people will become obese, but if they drink a lot of sugary beverages, "they fulfill that fate," said an expert with no role in the research, Jules Hirsch of Rockefeller University in New York.

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 teens and more than two-thirds of adults are obese or overweight.

Consumption of sugary drinks and obesity rates have more than doubled since the 1970s in the United States. But that doesn't prove that these drinks cause obesity. Genes, inactivity, and fatty foods or just too much food also play a role.

The studies "provide strong impetus" for policies urged by the Institute of Medicine, the American Heart Association, and others to limit sugary drink consumption, 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.

The genetic research was part of health studies that have gone on for decades across the nation, led by the Harvard School of Public Health.