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Why does vitamin A deficiency cause night blindness?

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details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

Vitamin A deficiency causes night blindness because it leads to low levels of 11-cis-retinaldehyde, a vital component of the visual pigment rhodopsin.

In the retina of the eye, rod cells are specialized for vision in dim light. These cells rely on rhodopsin to convert light into electrical signals. Rhodopsin is composed of a protein called opsin and 11-cis-retinal, which is derived from vitamin A. When light hits the eye, the 11-cis-retinal configuration changes, triggering a signal to the brain and causing the molecule to dissociate from the opsin, a process known as bleaching.12

For the eye to adapt to darkness, rhodopsin must be regenerated by recycling the retinal back into its 11-cis form. When vitamin A levels are insufficient, the body cannot produce enough 11-cis-retinaldehyde to maintain adequate rhodopsin levels. Consequently, the rod cells lose their sensitivity to low-light conditions, resulting in the inability to see clearly in the dark.134