Anorexia and obesity are both symptoms and responses of our culture's obesogenic climate. Western countries especially suffer from food preferences high in sugar, increasingly sedentary lifestyles, and an obsession with overindulging in food. We make it part of our tradition (huge meals at christmas and easter for example, and the fact that almost every special occasion or celebration is marked by the over consumption of sugar,) and the response to this has been people are getting bigger. 60% of people in the UK are overweight, and by 2050 half the UK population will be classed as obese.
The flip side to this is anorexia and bulimia. These conditions have been around for ages (anorexia nervosa was first characterised in 1873, but has probably existed in human history for millennia.) I have nothing to back this next part up with, as it is my opinion: the uptick in anorexia prevalence might be reactionary. As we have seen obesity rates rise, we have seen an increase in body scrutiny and critique, high profile diets, and in general conversation about how fat people are 'unhealthy' and 'undesirable' from a social standpoint (just look at the tropes from 80's and 90's media, the unpopular, bullied, put-upon fat kid was always the butt of jokes.) As people have internalised this "fat bad" mantra over the decades, people have taken extreme measures to ensure they do not become fat. Diet and weight loss culture notwithstanding, the extremest form of control results in the mental disorders we call anoreixa, bulimia and body dysmorphic disorder. The more people who resort to extreme measures and succumb to these conditions, the more prevalent it becomes, the more profile it gains, and the more vulnerable people become ensnared in the cycle.
Both are 'unhealthy' In vastly different ways. It is possible to function relatively normally while not at your optimal weight, but the further you diverge from optimal, the less healthy you will become, whether this be towards the under or overweight ends of the spectrum. Point is, we have never lived in a culture so obsessed with weight as a marker of success, or in a food environment so detrimental to our ability to maintain a healthy weight.
'Fat acceptance,' 'body-positivism,' 'pro-ana' are really all sides of the same dice. We, are a society, have lost control of our bodies, and have not been in control for some time. Our culture is to blame, and yet we readily attack those who do not fit with our prescribed notion of who looks too fat or who looks too thin. Acceptance movements do not address the root of the problem, they only provide a temporary respite to people who feel -rightly- that they have lost control and hope over their own situations. We, as a society, must take responsibility for the state of our food pyramid, food culture, our bodies and our health. If we want to be healthy, we must identify the antecedents of our disease and remove them, rather than bargain with ourselves and pretend that we, and everything else, is ok.