most knowledgeable people disagree with him. for an intro look at his wikipage: http://en.wikipedia.org/wiki/Robert_Lustig or i.e. http://www.alanaragonblog.com/2010/01/29/the-bitter-truth-ab...
Furthermore, something being "unproven" (which is what his Wiki says about his propositions) is very different from it being "disproven." Even the most brilliant scientific breakthroughs of our time were, at the time of their inception, unproven.
Whether or not you agree, I still highly recommend watching the whole video. It's rife with good material for discussion.
It's not "some people" but instead the clear majority of nutrition researchers / specialists. Your argument about "unproven" vs "disproven" is a straw man. Having extensive meta studies that show no support for Lustigs thesis is as close to disprove something as it is possible to do so in nutrition discussions (even though most nutrition studies have major problems in their quality and samplesize quantity).
Further, in your original comment you wrote "modern science has shown us that it's simply not true." and your only support for this was the linking of Lustigs youtube-video (which, to say it nicely, is at least heavily contested). I agree with you that there are many open nutrition questions (and that it is important to keep an open mind for new concepts / ideas), but to postulate that science has shown that "calories in, calories out" is wrong, is untrue. I think this is definitely the wrong place to have an indepth discussion about the topic, but i just couldn't let your original statement stand by itself without a reply.