Calories consumed in liquid form don't give stomachs the same satisfied feeling as calories eaten in food. People offset an afternoon snack by eating less at dinner, but they don't do that with beverages.
Making matters worse, the human body is ill-equipped to process the sugar that is concentrated in a glass of juice.
When fructose is eaten in a piece of fruit, it enters the body slowly so the liver has time to convert it into chemical energy. But a single glass of apple juice has the fructose of six apples.
"If you overdose on fructose in a liquid, the liver gets overwhelmed," Lustig said. As a result, he said, the fructose turns to fat. "Eating fruit is fine. Drinking juice is not."
Still, the halo surrounding juice remains strong.
As soda is singled out for its role in the rise of obesity, juice is offered as the sensible alternative. In Los Angeles and elsewhere, it is taking the place of soft drinks in school vending machines alongside water and milk.
Brownell of Yale has waged a high-profile campaign to fight obesity with "sin" taxes on soda and other sugary drinks. It's already an uphill battle, and he said he's loath to provoke the tens of millions of Americans who consider their morning juice sacrosanct.
Dr. Frank Greer, who spent 10 years on the American Academy of Pediatrics' nutrition committee, said he "can't imagine" the group would ever downgrade juice to the status of soda.
"It's such a normal part of the American diet," Greer said. "A glass of fresh-squeezed orange juice for breakfast, my goodness!"