Vitamin A is involved in ocular health and function in two distinct ways. First, in the form retinaldehyde, it is an essential component of rhodopsin and is necessary for maintaining vision (Wahlqvist et al 1997). Deficiency states initially cause a reversible night-blindness that can progress to complete blindness due to photoreceptor degeneration (McCaffery & Drager 1993). Second, as retinoic acid it maintains normal differentiation of cells in the conjunctiva, cornea and other ocular structures, with deficiency resulting in xerophthalmia (dry eye) and corneal ulceration. In xerophthalmia, the cells lining the cornea lose their ability to produce mucus, and therefore lubrication of the eye becomes compromised. Dirt particles that eventually enter the eye are more easily able to scratch the surface, increasing the risk of infection and, ultimately, blindness.

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