London: Mental health experts are warning of the risks of the increasingly popular “clean eating” dietary trend, which is leaving a growing number of teenagers very thin and even at risk of dying when taken to extremes.

One nutritionist said she had been contacted by a girl as young as 12 and people had got in touch on social media saying they wanted to be healthier, giving details of their existing diets.

Rhiannon Lambert, a registered associate nutritionist in Harley Street, London, has encountered people who obsess over where food comes from and some clients who will not drink water from a tap, because they normally stick to a brand of bottled water. “They develop particular habits, or won’t eat food when walking, because they think that food can only be processed when they’re sitting down,” she said. “All this interferes with general life and becomes an obsession.”

Lambert, who treats about 180 clients a year with various kinds of eating disorders, says has seen the number of those presenting due to “clean eating” double in the last year. The extreme form of this is a psychological condition known as orthorexia nervosa, the Californian doctor Steven Bratman has said. Experts have described it as a “fixation with righteous eating”.

Clean eating is promoted by some food bloggers, who are increasingly felt by a number of medical experts to be having a negative impact on certain vulnerable young people. “Young people lose sleep over this and cannot afford the lifestyle needed to maintain it,” Lambert said. “Health bloggers can be unqualified and offer dangerous advice. Not all of them want to impose their lifestyle on others, but lots of them do and they often give advice on clean eating with no scientific backing.

“The books come along, the products come along and these people are now role models whose every word will inspire impressionable young people. I have clients who think they have to be vegan to be successful.”

There are no official figures for the number of children and young people following a clean eating regime, because orthorexia is not recognised as a clinical diagnosis. But psychologists and nutritionists have reported a recent surge in the phenomenon among younger clients, especially girls, and believe that it is gaining in popularity.

The eating disorders charity Beat told the Guardian that it had recently seen a rise in the number of calls to its helpline from young people who have experienced problems as a result of following the trend. Ursula Philpot, a dietitian at the British Dietetic Association, said a fixation with eating healthily had been a noticeable route into eating disorders for vulnerable individuals in the past couple of years. She identified social media and the rise of healthy food trends and blogs as key drivers of the trend, but said it is difficult to blame them completely. “If it wasn’t health bloggers, then it could be something else that becomes the inroad, but it seems to be the route in now,” she said. Orthorexia affects girls more than boys, although boys are much more affected than previously, she added.

The range of foods that people worry about eating has changed, Philpot said. “At the top of most people’s lists [of bad foods] is gluten and dairy. When you talk to young people more, you find out about their stringent rules — some will worry all day about eating a biscuit,” she said.

The condition starts out as an innocent attempt to eat more healthily, but those who experience it become fixated on food quality and purity, according to experts. A Beat spokeswoman said: “We are concerned by the rising trend of ‘clean eating’ and the impact it could have on young people vulnerable to the development of an eating disorder. We are aware that contacts to our helpline are raising issues around orthorexia and clean eating.” There may be several reasons for someone to take up clean eating, the spokeswoman said. “Eating disorders are serious mental illnesses and their causes are many and complex. Research is telling us that they may be more biologically based than we previously thought, but social and environmental factors will also play a part in their development,” she said.

“Orthorexia does not have a clinical diagnosis and it would be for clinicians to determine whether it should, which may be helpful, because then it would have a clear clinical pathway of treatment. “There is a view that it may be more closely connected to OCD due to the nature of the illness, although it does also share behavioural traits with anorexia. Anorexia has the highest mortality rate of any mental illness.”

Deanne Jade, the founder of the National Centre for Eating Disorders, a counselling network, said the number of cases of orthorexia that she dealt with had more than doubled over the last five years. She said: “A lot of younger people don’t think they need therapy and that the solution to bulimia and anorexia is to eat clean, but this can become an obsession and there’s now more pressure than ever to be healthy. “There are too many messages in the media and especially social media. What worries me is that a lot of people promoting these ideas have no knowledge of nutrition. “I don’t know what the solution is, but a lot of the time getting people to recover from an eating disorder means getting them to relax their ideas about clean eating.”