One thing that I noticed in particular was a part where they found anorexia cases in a society where being fat is considered beautiful, and that the number of cases was proportional (given the population size) to the number of cases in the UK and the US. This ties in with what I believe, which is that societal pressures to be thin aren't that big of a contributer compared to the biological factors. My main belief for this comes from finding statistics about eating disorders in Finland (I don't have the link). I've spent a fair amount of time in Finland the population in general has a very healthy attitude to food and exercise. Yet the statistics for anorexia and anorexic behaviours were actually slightly higher for a national Finnish survey than for the reported cases in America.
The documentary talks a lot about stress and the biological causes/effects of stress behaviours. I'm certain it was stress that was the biggest contributor in my disorder because I have brief relapses whenever I have to study for exams.