If you're searching for a villain in America's obesity epidemic, most nutritionists tell you to put one picture on the wanted poster: a cold, bubbly glass of soda pop.

"Liquid candy" to detractors, sweetened soft drinks are so ubiquitous that they contribute about 10 percent of the calories in the American diet, according to government data.

In fact, said Dr. David Ludwig, a Harvard endocrinologist whose 2001 paper in the Lancet is widely cited by obesity researchers, sweetened drinks are the only specific food that clinical research has directly linked to weight gain.