The parents in my focus group were visibly upset by the skeletal selfies posted on these sites and the messages of encouragement accompanying them. As a former sufferer who used to look that way, I too was shocked. I had never come across such honest discussions on how to starve yourself.
I recently interviewed a 17-year-old girl whose anorexia had been sparked by one of these websites. She showed me a thin red bracelet sent to her "by some guy in America when I joined the pro-ana movement. When the hunger gets too much, when I think I'm going to give in, I touch the bracelet and remind myself it's stronger to starve."
Who runs these sites, and what their motivation might be, is anyone's guess. The fact that they exist, unregulated, and accessible to any teenage girl (or boy) is a scandal.
Go online and have a look. Every parent needs to know that "thinspiration" sites are dangerous. They glamorise anorexia, they promote photogenic waif-like models and starvation, and ignore the misery of eating disorders.
Anorexia is not glamorous, not from the inside. I was anorexic for more than 10 years. When I started at Oxford University, aged 19, I weighed 57kg and was happy, healthy and well-adjusted. When I left, aged 21, I weighed about 32kg. None of the myths around anorexia applied: I did not dislike my body, and never had done. I did not think I looked fat, or wear skimpy clothes, or even look in the mirror much. I had declared war on myself.
We live in an age in which we are confronted by thousands of images each day. The focus on women's bodies is intense, in magazines, websites or TV adverts, on posters and celebrity shots, and in the conversations of those around us. We can't avoid the overwhelming message that weight loss is good, weight gain is bad, that thinner is better.
The effect can be profound, and yet still eating disorders are dismissed as a teenage, female condition (though male eating disorders are rising) or misrepresented as faddy dieting, body hang-ups, a phase they'll "grow out of".
In fact, the opposite is true: eating disorders are addictive, and self-starvation becomes involuntary. Anorexia has the highest mortality rate of all mental illnesses. One in five anorexics will die, either from physical complications or suicide.
The long-term health consequences of anorexia include infertility, amenorrhoea (cessation of periods), loneliness and depression; bulimia can lead to kidney and heart problems as well as electrolyte imbalance. Osteoporosis (low bone density) is an invisible but growing problem among young women who avoid calcium-rich dairy products such as milk and cheese - and up to 90 per cent of anorexics show some degree of bone loss.
Meanwhile, a form of disordered eating known as orthorexia is becoming more mainstream, fuelled by the mania for healthy eating and our growing anxiety around obesity. Orthorexia is somewhere between being health conscious and a health obsessive. What begins as an attempt to improve one's lifestyle can morph into an unhealthy fixation. It can lead to self-loathing, social isolation and even malnourishment. Orthorexics combine restrictive diets with punishing exercise regimes, are fanatical about calorie-counting, green juicing and honing their gym bodies. Orthorexia thrives in a society in which we're urged to count calories and "eat clean", to avoid artificial additives and preservatives, to beware plastic packaging and hidden toxins and, above all, never to get fat.
Disordered eating goes hand in hand with excessive levels of physical activity. When I was at my sickest with anorexia, I would run 8km a day, no matter how cold or exhausted I felt. Hardcore workouts have surged in popularity among middle-class women, who go for the burn at their "skinny-bitch collective" classes and then Instagram the results. An 18-year-old student I know says she "feels bad" when she sees the post-workout selfies of her role models Ellie Goulding, Kylie Jenner and Miley Cyrus.
Parents, especially mothers, are in a difficult position. Do you encourage your daughter to exercise regularly, eat her five-a-day and drink her green juice? Or will this trigger an obsession? Will she pick up on your own dietary habits? Will your body dissatisfaction damage her for life?
Now in my thirties, I am physically "recovered". But it has taken more than a decade to get to this point: gaining weight was the hardest fight of my life. If I was in my teens now, desperately trying to beat an eating disorder in today's febrile online environment, I wonder how I would have coped. Goaded or triggered or simply surrounded by these images of super-healthiness, fitness, thinness, other anorexics chewing ice cubes, and my role models existing on wheatgrass shots: would I ever have recovered? I'm not sure.
