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.

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

Collectively, the results suggest that sugary drinks cause people to pack on the pounds, independent of other unhealthy behaviour 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.

“You may be able to fool the taste” and satisfy a sweet tooth without paying a price in weight, said an obesity researcher with no role in the studies, Rudy Leibel of Columbia University.

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 four pounds (1.8kg) less on average than those who kept drinking sugary beverages.

A second study involved 641 normal weight children ages four 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, and were not told what kind they were given.

After 18 months, the sugary-drink group weighed two pounds (approximately 1kg) more on average than the other group.

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

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