Reading this question: Does eating carrots improve your eyesight? - brought to mind a common piece of medical advice in USSR - to eat carrots to prevent your eyesight from going bad (as opposed to that question's claim to improve eyesight).
Is it true that a lack of Vitamin A in your diet can be statistically proven to be linked to worse eyesight?
Any definition of "worse" is acceptable for the purposes of this question, from abnormal eye pressure to nearsightedness to glaucoma to whatever else, though I personally am more interested in near-sightedness.
I don't have a cite for the claim but it was stated repeatedly by different doctors in USSR in my presence.
It sounds at least somewhat plausible in a sense of certain diseases being obviously caused by vitamin deficiencies.