A doctor in the U.K. just published a case study on a 17-year-old kid who suffered permanent vision loss after several years of ONLY eating junk food. He ended up with a severe vitamin deficiency, because his entire diet consisted of French fries, Pringles, white bread, and processed meat. He started having symptoms around age 14. His weight was normal, but he was really tired all the time. So they told him to change his diet. And his parents TRIED to make him, but he wouldn't.
Then three years later, he showed back up at the eye doctor. And his vision was permanently blurry. Plus, he's partially colorblind. And it messed with his hearing. The vision issues were caused by something called "nutritional optic neuropathy." Which basically means a lack of vitamins damaged his optic nerves. The good news is it's rare, and only happens after years of eating nothing but junk food. But his doctors hope it's a wake-up call for the rest of us to pay attention to what we're putting in our bodies.