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Maggie Caudill, 15, has entered a weight loss program at Boston Children's Hospital.

Her goal is to lose about 40 pounds so she can get back to cheerleading and softball.

"I want to be able to run as fast as I can through those bases. I don't want to get winded and have to use my asthma inhaler," she says.

Her first step toward weight loss is to cut out sodas.

She'd been drinking about a liter a day.

"Sugary beverages can affect body weight quite quickly -- perhaps more so than any other single food product," notes Dr. David Ludwig.

Dr. Ludwig oversees Maggie's weight loss program.

He led a new study that found teens gained four pounds fewer than their peers when they replaced sugary beverages with zero calorie drinks for one year.

"The effect was especially pronounced among Hispanics, who gained 14 pounds less during that first year," he says.

In a statement response to the new study the American Beverage Association points out:

"We know, and science supports, that obesity is not uniquely caused by any single food or beverage."

It's true obesity can run in families, like Maggie's.

"My brother is overweight, both of my parents are overweight," she admits.

And new evidence from the Harvard School of Public Health finds that even people who are genetically predisposed to becoming obese are much more likely to do so if they drink sugar-sweetened beverages on a daily basis.

The risk for obesity was about 30-percent for those with a family history, but who did not indulge in soft drinks.