Experts are warning about the risks of extreme restrictive eating after a teen developed permanent sight loss after living on a diet of chips and crisps.
Eye doctors in Bristol cared for the 17-year-old after his vision had deteriorated to the point of blindness.
Since leaving primary school, the teen had been eating only French fries, Pringles and white bread, as well as an occasional slice of ham or a sausage.
Tests revealed he had severe vitamin deficiencies and malnutrition damage.
The adolescent, who cannot be named, had seen his GP at the age of 14 because he had been feeling tired and unwell. At that time he was diagnosed with vitamin B12 deficiency and put on supplements, but he did not stick with the treatment or improve his poor diet.
Three years later, he was taken to the Bristol Eye Hospital because of progressive sight loss, Annals of Internal Medicine journal reports.
Dr Denize Atan, who treated him at the hospital, said: "His diet was essentially a portion of chips from the local fish and chip shop every day. He also used to snack on crisps - Pringles - and sometimes slices of white bread and occasional slices of ham, and not really any fruit and vegetables.
"He explained this as an aversion to certain textures of food that he really could not tolerate, and so chips and crisps were really the only types of food that he wanted and felt that he could eat."
Dr Atan and her colleagues rechecked the young man's vitamin levels and found he was low in B12 as well as some other important vitamins and minerals - copper, selenium and vitamin D.
He was not over or underweight, but was severely malnourished from his eating disorder - avoidant-restrictive food intake disorder.
"He had lost minerals from his bone, which was really quite shocking for a boy of his age."
He was put on vitamin supplements and referred to a dietitian and a specialist mental health team.
In terms of his sight loss, he met the criteria for being registered blind.
"He had blind spots right in the middle of his vision," said Dr Atan. "That means he can't drive and would find it really difficult to read, watch TV or discern faces.
"He can walk around on his own though because he has got peripheral vision."
Nutritional optic neuropathy - the condition the young man has - is treatable if diagnosed early. Left too long, however, the nerve fibres in the optic nerve die and the damage becomes permanent.
Dr Atan said cases like this are thankfully uncommon, but that parents should be aware of the potential harm that can be caused by picky eating, and seek expert help.
For those who are concerned, she advised: "It's best not to be anxious about picky eating, and instead calmly introduce one or two new foods with every meal."
She said multivitamin tablets can supplement a diet, but are not a substitute for eating healthily.
"It's much better to take on vitamins through a varied and balanced diet," she said, adding that too much of certain vitamins, including vitamin A, can be toxic, "so you don't want to overdo it".
Dr Atan said vegans are also at increased risk of B12 deficiency-related sight problems if they do not replace what they can lack when excluding meat from their diet.
"Nutritional yeast is a way of adding B12 to your diet," she said.
Sources of vitamin B12 for vegans include:
- breakfast cereals fortified with B12
- unsweetened soya drinks fortified with vitamin B12
- yeast extract, such as Marmite, which is fortified with vitamin B12
Rebecca McManamon, consultant dietitian and spokesperson for the British Dietetic Association, said restricted diets might happen for a range of reasons, including eating disorders, allergies and autism, and need specialist assessment.
"It's also worth noting that since 2016 the UK government has recommended daily Vitamin D supplementation (10 microgrammes/400 International Units) for everyone between October and March as we are not likely to get enough from fortified foods.
"Multivitamin supplementation is recommended for all children up to their fifth birthday."