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.