Just after Emma Cope finished her A levels she went on holiday to Newquay with a group of girlfriends. She drank a lot of vodka, ate very little and didn’t once check her blood-sugar level or inject herself with insulin, even though she had recently learnt that she had type 1 diabetes. By the end of the week she reckons she had lost 1½st (9.5kg). The next day she was admitted to hospital.

“I was in severe pain. I couldn’t move. I thought I was dying,” she recalls. “Obviously my parents were angry, but I’d lost all this weight and thought that was brilliant.”

Two years later Cope has come to understand that her health is more important than being a size 8. After more than 18 months of sometimes misusing insulin, at 20 she is learning to eat sensibly, exercise and take her medication. “I drink but I wouldn’t get excessive ever again because I’m too scared of the consequences,” she says. “I haven’t missed any insulin for four months.”

Yet, confronted by slender peers and even skinnier role models, this intelligent and attractive young woman still wants to be thin and can’t quite forget the thrill of the rapid weight loss that came from not taking the insulin her body needs.

This is why Diabetes UK, the care and research charity, believes that an estimated 5,000, or one diabetic woman in three, under the age of 30, skips insulin injections. Up to 3,000 of them are thought to be teenage girls. The practice is called diabulimia and it puts them at risk of coma, blindness, heart disease and kidney and nerve damage. And this is why Cope, the daughter of a civil engineer from Monmouthshire, wants to talk publicly about her condition and the pressure it puts on a young person who wants to be the same as her friends, who badly wants to be skinny like so many of them, yet who knows that missing insulin could have serious consequences for her health.

Usually, type 1 diabetes develops before the age of 40, when the body is unable to produce insulin, a hormone produced by the pancreas that helps glucose to enter the cells where it is used as fuel by the body. Without it, blood-glucose, or sugar, levels rise because the body cannot use the glucose properly. If an injection is skipped, the sugar continues to circulate in the blood and its absence from cells starves the body of energy. This forces it to get energy from fat reserves and muscle – and leads to rapid weight loss.

Cope was at a sixth-form boarding school in Surrey, surrounded by skinny and beautiful daughters of the rich and famous, when she started to lose weight dramatically. She also found herself drinking vast quantities of water – even during the night – and urinating every half hour, sometimes in pain. She slept a lot too and had blurred vision and thrush – the classic symptoms of type 1 diabetes – but was so delighted by the weight loss that she refused to admit that she might be ill.

“I was obviously getting a lot of attention for losing weight because it was such an image-conscious school. Within six weeks I lost 3st and I was this skinny little thing and it made me so much more confident even though I was in a lot of pain. I was so happy, I was getting all this attention from boys and all my friends were saying, ‘how amazing’.” Her lowest weight was 7½st; she is 5ft 7in.

Her condition was diagnosed when, six weeks after the symptoms began, she saw a doctor and was admitted to hospital with a blood-sugar count of 45; it should not be above eight. “I remember this woman saying, ‘Forty-five – you shouldn’t be here’,” Cope says. During her ten-day stay in hospital she was introduced to a diabetic nurse and a dietician. “The first question I asked was, will I put on weight? The answer was, ‘Yes, you’re going to go back to how you were, if not more’.”

Initially, Cope injected herself with insulin up to four times a day, as instructed; it was the Newquay holiday that was her undoing. “You don’t want to inject, you want to be seen as normal, you don’t want to put on weight and every young person thinks, I’m invincible, nothing’s ever going to happen to me, it only happens when you’re older,” she explains. “Having lost all that weight I thought, I can look that good and might as well carry on doing it, it’s the long-term effects that are serious, not the short term.”

Over the past two years since Newquay Cope’s weight has gone up and down, but she now understands the risks that she faces if she doesn’t try to control her blood-sugar levels. “I’m in a routine now where I’m looking after myself because I’m so scared about blindness,” she says. “To think that just by being stupid and trying to be normal, you’re going to lose something that’s so precious. If you start missing insulin at 15 by the time you’re 30 everything’s damaged – and all just to stay like Victoria Beckham. There’s so much hype about being skinny and, for diabetics, it can kill you.”

Cope is also learning that living with diabetes is not just a physical matter, but an emotional one that requires her to accept her condition, and this means learning to ignore peer pressure. Last year she left a course at Edinburgh University when she became ill, but she is now living independently in London and has started a course in drama and theatre at Royal Holloway, University of London.

“I hate that I’ve got it,” she says. “I hate that I have to tell people. I told my boyfriend only at Christmas and we’d been together for 18 months. I was living with him and having a hypo [hypoglycaemia or a blood-sugar low] at three in the morning. It’s like when you haven’t eaten for too long and feel faint – well, times that by 20: your heart’s racing, you’re sweating, your speech starts to slur and your pupils dilate and it’s horrible and embarrassing. I’d have to go to the lounge shaking and have some Fruit Pastilles to get myself back to a normal level and then go back to bed and not tell him. When I did tell him he was so sweet – I’m very lucky.

“So many people are ignorant about diabetes. At school there were people who didn’t speak to me again because they thought they were going to catch something. They think it’s because you’ve eaten too many chocolates, that you inject with an 8in needle and that you survive on four Mars bars a day. That’s what embarrasses me, but it’s not like that.

“It doesn’t stop me doing anything but, as a 19 or 20-year-old, going out can be difficult. I have to be so cautious if someone buys me a drink and make sure it’s a Diet Coke because Coke makes me high and I feel ill. If I drank Red Bull, God knows where I’d be. But then, before I had diabetes, I was ignorant too.”

She is keen to explain that the injections are delivered by a tiny pen-like device with a 4mm needle in the tummy, side, thigh or bottom. The injections are not painful, though checking her blood sugar by pricking a finger is uncomfortable, she says. “Exercise is the best way to control your blood sugar because it means you have to take less insulin and it makes you more sensitive to it. It makes me feel brighter and happier too.

“It’s been a struggle and it still is. Obviously I want to eat silly things, but when you see the high blood count afterwards, it’s not worth it. I know I have probably damaged my long-term health by missing insulin and I won’t do that any more. If that makes me boring, if that makes me not cool, or whatever people have called me, fair enough. But it’s hurtful, because I just want to be normal.”