Ambers tackles the question:
We know that a bunch of things contribute to early on-set obesity, but we don't really know in what proportion they contribute, and it probably varies considerably from individual to individual. For a plurality, though, obesity appears to be locked in to a lot of kids by the time they are 3 years old. Think about that. If you believe, as I do, that obesity is a problem that implicates all of us, and therefore requires some sort of a collective solution, that in and of itself is cause for despair. What it means is that smaller changes like expanding recess or building more public parks or adding fruit to a diet won't amount to anything.
Marc explored the subject at length in his May 2010 personal essay, "Beating Obesity."