Nutrition Science and Everyday Application - beta v 0.1

488 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
cells die, they need to be replaced with new cells. If the body is deficient in vitamin A, those
new cells don’t differentiate normally, resulting in dry eyes, a condition called xerophthalmia.
Instead of producing mucus, these dysfunctional cells produce a protein called keratin.
Keratin is a hard, structural protein that is found in nails, hair, and the outer layer of skin, and
you can imagine the problems it causes when it accumulates in the eye. Instead of a moist,
well-lubricated eye, keratin makes the eye hard and dry, resulting in clouded vision.
Figure 8.21. A child with xerophthalmia caused by vitamin A deficiency, demonstrating
cloudiness on the surface of the eye.
A deficiency in vitamin A can thus impair vision in two ways:
1. Development of night blindness due to a lack of the pigment rhodopsin
2. Development of xerophthalmia, or dry eyes, caused by abnormal cellular
differentiation
Night blindness is usually the first sign of a vitamin A deficiency, followed by xerophthalmia
and clouded vision. If the deficiency persists, the damage from keratin in the lining of the eye
can cause permanent blindness.
Vitamin A’s role in cellular differentiation also makes it critical to cells around the body
involved in normal growth, development, reproduction, and immune function. All of these
processes require cells to develop in specific ways at specific times, and vitamin A helps to
orchestrate these processes. For example, embryonic development requires stem cells to
differentiate into specific types of cells to form new organs, and timing is critical.
Vitamin A also helps the immune system produce different types of immune cells, and
without adequate vitamin A, a person is more susceptible to infections. The common
occurrence of severe xerophthalmia in children who died from infectious diseases led
scientists to hypothesize that supplementing vitamin A in the diets of children with
xerophthalmia might reduce disease-related mortality. In Asia in the late 1980s, researchers
administered vitamin A supplements to targeted populations of children, and their death
rates from measles and diarrhea declined by up to 50%. Vitamin A supplementation in these