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12.64: Vitamin A Functions

Vitamin A has a number of important functions in the body.

Vision

The retina is the inner back lining of the eye that takes visual images and turns them into nerve signals that are sent to the brain to form the images that we "see", as shown in the following link1.

Eye Anatomy. Image used with permission (CC BY-SA 3.0; BruceBlaus).

Inside the retina are the photoreceptor cells, rods and cones. Cones are responsible for color vision, while rods are important for seeing black and white. Within the rods, 11-cis retinal combines with the protein, opsin, to form rhodopsin. When light strikes rhodopsin, the compound splits into opsin and all-trans retinal. This sends a signal to your brain for us to “see”. This process is illustrated in the figure below1.

Figure 12.641 Vitamin A in the rod

Most all-trans retinal is converted back to 11-cis retinal through a series of steps so it can continue to be used to form rhodopsin. However, this recycling is not 100% efficient. Vitamin A stores, or continued intake, is required to provide the 11-cis retinal needed to continue to form rhodopsin. Normally, our eyes adapt to darkness by increasing the amount of rhodopsin available so we can see under reduced light conditions1. If a person does not have enough rhodopsin he/she will become night blind, meaning his/her eyes do not adjust, or adjust very slowly, preventing he/she from seeing under limited light conditions. The picture below is an example of what night blindness looks like.

Figure 12.642 The left is normal vision, the right is what someone with night blindness would see2.

Cell Differentiation

Vitamin A, in particular, retinoic acid, is important for cell differentiation, or the ability of stem cells to develop into specialized cells.

Figure 12.643 Cellular differentiation3

Other functions that vitamin A is important for include:

  • Growth and development
  • Reproduction
  • Immune function

References & Links

  1. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
  2. https://commons.wikimedia.org/wiki/F...nachtblind.jpg
  3. http://www.wikipremed.com/image.php?...mage_id=334000