Swipe through photo after photo of legs as thin as matchsticks. Read the comments underneath: “I wish that was me” or “They’re so perfect I could cry”. Feel shock at seeing how an apparently affluent girl’s hipbones stick out from her emaciated belly.

This is one of the dark regions of Instagram, where girls and young women glorify their eating disorders, emulating one another and egging each other on with the most extreme of photos of their skinny selves.

It’s called pro-ana – short for pro-anorexia. Pro-ana is an online movement where anorexic or bulimic women encourage each other to lose more weight. The rule is that you can never be too thin. They elevate their own peculiar ideal of beauty to a commandment.

The community treats their illness not as a disorder but as a brave and enviable state, the more extreme the better.

Pro-ana sites are a space of their own for these women to interact with each other – and it’s happening on Instagram more and more. The internet and apps mean that accessing a network of like-minded women is easy.

That young women post selfies that present the best possible version of themselves has often been commented on. Andreas Schnebel, a German who works on ways to fight an epidemic of anorexia, says such selfies imply that there’s only one physical appearance that is acceptable.

Selfies create a vicious circle which makes everyone want the same body type. The “likes” and followers fuel this beauty addiction.

This internet ecosystem is especially dangerous to young women with eating disorders because the validation they get from other users spurs them on towards sickness and makes them resist pressure from parents and mentors to get back to health.

Instagram brought in new rules a few years ago to stop young women using the portal as a platform to glorify eating disorders. The search function can no longer be used to find hashtags like #probulimia or #proanorexia.

A spokesman for the company explained: “Hashtags promoting eating disorders will be deleted without warning.” For other hashtags which don’t specifically endorse anorexia, a warning is issued before the results are shown – like #ana.

Instagram defends this as useful for some: “For many Instagrammers who are struggling with eating disorders, it’s a great help to be able to talk to others while they’re in the process of getting better.”

They say it is part of a holistic approach to be able to find these hashtags using the search function.

But a lot of the time, those who are looking for pro-ana sites won’t be deterred by warnings or blocks.

The hashtags are often slightly altered to get around this problem – so #bulimia becomes #bulima or #thin is changed to #thynn. And using these hashtags you can find extreme, sometimes disturbing, pictures of emaciated bodies.

There’s one Instagram account taking on the pro-anas.

Amalie Lees was 17 when she developed an eating disorder.

The 21-year-old Briton is thin – but she used to be much thinner. In old photos you can see the matchstick legs she used to have, skinny and frail. Amalie uses Instagram to document her body, her food and her return to health.

“It feels good to find a place online where you can be open about your fight against your eating disorder,” she says. Through Instagram she was connected with people who were going through the same thing.

But this is also where the danger lies – it’s a double-edged sword, says Amalie.

“People with eating disorders can be competitive,” she explains. “They feel like they aren’t sick enough to earn their recovery, if others are sicker than they are.”

And there are some accounts which start off being about recovery – but end up promoting an unrealistic body image after all.

Today Lee says she now doesn’t count calories, weigh herself or go on any diets.

Because defining yourself in numbers is a waste of time.