A teenager who was described as a "fussy eater" lost his sight after living off a diet consisting entirely of junk food.

It's a rare occurrence, but the case showcases more of the negative effects of junk food outside of obesity and heart disease that many people are more familiar with.

It turns out, a junk food diet is also extremely damaging to the nervous system and could cause unexpected conditions, such as blindness.

Going Blind From Junk Food

According to a case study published in the journal Annals of Internal Medicine, the boy initially visited his family doctor when he was 14 years old due to tiredness.

Despite being a fussy eater, the physician noted that he looked fine. Tests revealed he was anemic with low levels of vitamin B12, so the teen was given shots of the vitamin and sent home with instructions to improve his diet.

Unfortunately, the teenager developed vision and hearing issues by the age of 15 with doctors being unable to spot the cause of the problems with MRI scans and eye tests. His vision continued to deteriorate, until he turned 17 when his vision was measured as 20/200 in both eyes, which classifies him as legally blind in the United States.

Further tests revealed that his body still had low levels of vitamin B12 as well as copper, selenium, and vitamin D.

When asked about his diet by doctors, the teenage boy said that he doesn't eat food with certain textures. In fact, pretty much the only things he ate were fries, chips, white bread, processed ham, and sausage.

Doctors diagnosed the teen with nutritional optic neuropathy, which is a condition where the optic nerve is damaged due to nutritional deficiencies. While it is reversible when discovered during its early stages, the damage to the boy's optic nerves was already irreversible by the time it was caught.

Instead, he was given nutritional supplements to prevent his vision from getting worse.

He was also recommended to mental health services for an eating disorder known as avoidant-restrictive food intake disorder.

A Surprising Nutritional Deficiency

While the boy exhibited a normal height and weight, it turned out he was extremely malnourished.

"Nutrition does not just depend on how much you eat but what you eat and this case illustrates that fact," Dr. Denize Atan, a study coauthor from Bristol Eye Hospital, explained to Newsweek, pointing out that the boy even ate enough calories regularly. "But he restricted his food to crisps and chips [fries] and a bit of processed pork. In other words, energy-dense foods of little nutritional value. The case illustrates the fact that calorie intake and BMI are not reliable indicators of nutritional status."

She told BBC that the teenager lost minerals from his bone, which is shocking for a young boy.