London's "highly competitive" culture is being blamed for people taking dieting to dangerous extremes.
Experts recommend a "balanced diet with room for naughties"
Catherine Collins, chief dietician at St George's Hospital in Tooting, south London, says the craze of "healthy" diets is doing more harm than good.
People are consumed with being the "fittest and the healthiest", she said.
One patient who was admitted to St George's with malnutrition, had been eating more than 50 portions of fruit and vegetables a day.
Mrs Collins said people are cutting out key foods, such as dairy, wheat and protein, which the body needs.
She said the reasons people take on these diets include greater awareness of diets and nutrition, to alleviate chronic health complaints and a fear of disease.
People have a positive association with being fit and healthy and it also promotes self esteem, she added.
Mrs Collins said "healthy" dieting tended to be confined to cities as in the countryside there was less of a preoccupation about food.
"Taken to the extreme, 'healthy' eating can make people feel unwell by excluding the foods they need," she said.
She recommended a balanced diet which has "room for naughties".
Mrs Collins also blamed a booming unregulated alternative health industry for the increase in such diets.