Vitamin A helps the retina function properly, which is essential for good vision and the prevention of night-blindness. It also lessens the risk of age-related macular degeneration and cataracts.
Zinc deficiency can interfere with vitamin A metabolism in several ways:
(1) Zinc deficiency results in decreased synthesis of retinol-binding protein which transports retinol through the circulation to tissues (eg the retina) and also protects against potential toxicity of retinol;
(2) Zinc deficiency results in decreased activity of the enzyme that releases retinol from its storage form, retinyl palmitate, in the liver;
(3) Zinc is required for the enzyme that converts retinol into retinal, as needed for sight.
The cornea, the outer lining, also needs Vitamin A to keep its cells healthy. Lack of Vitamin A causes dryness, ulceration, scarring to the cornea and eventually blindness. So dry eyes and dryness to any body linings (dry mouth, dry skin etc) are all due to lack of Vitamin A. Without Vitamin A, mucous-forming cells in the cornea deteriorate. The eye can no longer produce enough tears or mucous needed to lubricate the eye and wash away bacteria.