Dr Laura Wyness, a senior scientist at the British Nutrition Foundation, said: "Popular media has a large influence on young people's body image, placing a great deal of pressure on obtaining the 'ideal' body shape. This often leads to young girls adopting unhealthy practices.
"These include smoking, skipping meals, especially breakfast, severely limiting foods perceived as fattening, such as red meat and dairy produce, which are important sources of protein, iron, zinc and calcium, and adopting very low energy, and therefore nutrient, diets."
Lizzie Pollard, 19, from Southampton, studies medicine at King's College, London. She was 12 when she developed an eating disorder.
"For me it wasn't about self-image," she said.
"It was a control thing. There's a lot of pressure on young people to look a certain way, but I think there's something very different between people dieting to look thin and an eating disorder."
She adds: "I was bullied, and felt very out of control, and wanted control. [Anorexia] is lonely. It took over my own life and takes everything away from you .... I was admitted to hospital when I was about 14. I hadn't eaten in four days. My heart was getting weaker and weaker and became too weak for me to move. I was put on permanent bed rest, and couldn't even get out to shower.
"After that I was in an in-patient hospital for six, seven months, and then I was supported by my local NHS community team, in Southampton."
Ms Pollard also said: "I think it's important that there are always people they can talk to about this. There are messageboards, and family and friends. And keep focus on your future. The thing about anorexia is it strips everything away from you. You need to find value in who you are."
There are around 2.8 million teenage girls in the UK and around 2.7 million boys.
Analysis of Department for Education statistics by the think tank Demos earlier this year revealed the extent to which self- esteem differs between teenage girls and boys.
It concluded that a significantly higher proportion of girls aged 14 to 15 felt "worthless", "unhappy or depressed", or "low in confidence", compared with boys.
Meanwhile, children as young as 12 are drinking the equivalent of 19 glasses of wine a week, according to the Schools Health Education Unit, which found that four per cent of the 12- to 13-year-olds surveyed had drunk 28 or more units of alcohol in the past week.
- INDEPENDENT