Nutrition Science and Everyday Application - beta v 0.1

VITAMINS IMPORTANT FOR VISION 487
Figure 8.19. One molecule of beta-carotene can be enzymatically cleaved to 2 molecules of
vitamin A.
As with other fat-soluble vitamins, vitamin A is packaged into chylomicrons in the
enterocytes of the small intestine and then transported to the liver. The liver stores and
exports vitamin A as needed; it is released into the blood bound to a retinol-binding protein,
which transports it to cells. Beta-carotene can be converted into vitamin A in the intestine, or
it can be absorbed intact, packaged in chylomicrons, and then transported around the body
in lipoproteins. Beta-carotene and other carotenoids that aren’t converted to vitamin A can
also act as powerful antioxidants and have other helpful functions in the body, which may in
part explain the health benefits of a diet rich in fruits and vegetables.
The retinoids are aptly named, as their most notable function is in the retina of the eye.
Retinol that is circulating in the blood is taken up by cells in the retina, where it is converted
to retinal and is used as part of the pigment rhodopsin. Rhodopsin is especially important to
our ability to see in low-light conditions. When light hits rhodopsin in the eye, a nerve signal
is sent to the brain, allowing us to detect that light. A person that is deficient in vitamin A has
less rhodopsin pigment in the eye and is therefore less able to detect low-level light. This
makes it more difficult to see at night, a condition referred to as night blindness, and this
is one of the first signs that a person is deficient in vitamin A.
Figure 8.20. A depiction of the vision of a person suffering from night blindness, in which there
is inadequate rhodopsin for detection of light.
Vitamin A is also required for normal cellular differentiation, the process by which cells
change from stem cells to more specialized cells with specific structure and function. Cellular
differentiation is important in every tissue of the body, but if there is a shortage of vitamin
A, the eye is one of the first areas to be impacted. Specialized cells in the lining of the eyes
produce mucus and tears, which keep eyes moist and lubricated. When the mucus-secreting