Everyone knows (or at least should know) that a diet consisting of little more than potato chips and french fries is going to lead to poor health. For one teen in the UK, a refusal to eat anything but carb-heavy junk food ultimately cost him far more than anyone could have guessed.

The boy’s case is described in a new paper published in the journal Annals of Internal Medicine, which notes that the unnamed 14-year-old boy was considered a “fussy eater” when he was first seen by doctors. During that initial check, he was noted as having low vitamin B12 levels and was advised to change his diet. He did not follow that advice.

Three years later, the now-17-year-old boy returned to the doctor with vision problems that had progressed to the point of blindness. He also reported hearing issues and general tiredness. After additional testing, it was discovered that he had very low levels of not just B12 but also vitamin D, copper and selenium, along with high levels of zinc.

The boy’s diet, which reportedly consisted of Pringles chips, french fries, white bread, and occasionally processed meats, was blamed for his condition. Unfortunately for the patient, years and years of eating nothing but refined carbs had caused significant damage to his optical nerves, which doctors say is likely permanent.

“Our vision has such an impact on quality of life, education, employment, social interactions, and mental health,” Dr. Denize Atan, lead author of the study, said in a statement. “This case highlights the impact of diet on visual and physical health, and the fact that calorie intake and BMI are not reliable indicators of nutritional status.”