Comment by Johanx64

Nothing controversial, but it is more likely a correlation, not causation.

If you're vitamin d deficient, that means you're not spending time outdoors (in the sun).

If you're not spending time outdoors, it probably means you're focusing your vision on short-distance objects, like screens and books. Which naturally leads to myopia for most people.

Obviously, vitamin D supplementation is highly unlikely to do anything to improve the condition, because vitamin D deficiency is just a symptom.

Similarly seasonal affective disorder is also very rarely improved by vitamin D supplementation alone.