This story deals with an extreme situation, but it shows what a junk food diet can do to even young people:
https://www.ctvnews.ca/health/teen-went-partially-blind-after-eating-only-pringles-fries-ham-and-sausage-case-study-1.4574787
“A teenager in the United Kingdom described as a “fussy eater” partially lost his vision due to a strict diet of Pringles, French fries, white bread, processed ham and sausage. A case study published Monday in Annals of Internal Medicine, shows the boy suffered from nutritional optic neuropathy, a dysfunction of the optic nerve cause by a diet low in nutrients required for nerve fibres in the eye to function. The condition can lead to permanent vision loss if left untreated. Researchers at the Bristol Eye Hospital in the U.K. said the boy first visited his family doctor as a 14-year-old with a normal body mass index, but complained of tiredness. His doctor gave him some vitamin B12 injections to treat low levels of the vitamin and suggested some dietary changes.”
This is a dramatic illustration of the all maxim, you are what you eat. Literally, it seems:
https://annals.org/aim/fullarticle/2749497/blindness-caused-junk-food-diet