Over the years activists have managed to blame obesity on almost every food imaginable: steak, milk, soda, and even juice. Rather than distancing themselves from this contrarian reputation, food police like Center for Science in the Public Interest director Michael Jacobson are openly "proud about finding something wrong with practically everything."
Instead of hedging their bets, others choose to approach the problem in a more analytical fashion. For instance, researchers in the Avon Longitudinal Study (a project of the University of Bristol) carefully examined multiple factors and then placed all of their chips on one square: physical activity.
Today, after eleven years of research, the scientists announced their conclusion in a British newspaper: "The findings point to a lack of exercise, rather than gluttony, as the key to obesity in young people."
In a groundbreaking report published in PLoS Medicine (the Public Library of Science journal), the researchers acknowledge the changing diet as a popular but overplayed card in the obesity debate:

There is a continuing debate as to which is most important — eating too much food or the lack of sufficient activity. However, in several countries it looks like, on average, people are eating less than they were a few years ago … Analysis of the results showed a consistent trend-the greater the fat mass the lower the level of physical activity.