vitamin A is in the diet, the effects can be seen in many
different systems.
VISION.The first function of vitamin A to be well
understood was its role in maintaining good vision.
Much of the research that explained how vitamin A
was critical to vision was done by Harvard scientist
George Wald (1906–1997), who won the Nobel Prize
in 1967 for his work. When light enters the eye, it is
absorbed by cells lining the retina at the back of the
eye. This activates a chain of events that results in
vision. Vitamin A (in the form of retinol) is part of a
pigment in the retina called rhodopsin that absorbs the
light. Without enough vitamin A, the eye does not
detect low levels of light. People with this deficiency
develop night blindness. They can see well in bright
light, but cannot see in dim light. Night blindness was
known in early Egyptian, Chinese, and Greek cultures,
all of whom discovered independently that eating liver
(an excellent source of retinol) would cure the disor-
der. Night blindness disappears almost immediately
when vitamin A is added to the diet. If left untreated,
however, dry eye (xeropthalmia) and permanent
blindness can occur because of damage to the cornea,
the clear covering of the eye.
SKIN. Vitamin A helps skin (epithelial) cells to
remain healthy. Skin disorders such as acne can be
treated by prescription drugs such as tretinoin
(Avita, Renova, Retina-A) and isotretinoin (Accu-
tane) that contain synthetic Vitamin A. Vitamin A
supplements are also often given to burn victims to
help them grow large amounts of new skin.
RESISTANCE TO INFECTION.Vitamin A is necessary
for proper functioning of the immune system. The cells
that line the throat, lungs, intestine, bladder, and other
internal cavities are the first line of defense against
bacteria and viruses entering the body. These cells
need vitamin A to grow normally and form a contin-
uous barrier against invading microorganisms. When
these cells br eak down, it is easier for bacteria and
viruses to infect the body. In addition, vitamin A is
needed for the proper development white blood cells
that fight infection. However vitamin A taken in
excess of recommended amounts does not appears to
benefit the immune system.
CANCER PREVENTION.There are mixed results
from research on whether Vitamin A can help prevent
cancer. The prescription drug All-Trans-Retinoic Acid
(ATRA, Vesanoid) has been proved successful in
increasing survival time for individuals with acute pro-
myelocytic leukemia. This drug contains retinoic acid, a
derivative of retinol. Research results on whether vita-
min A is helpful in preventing or treating skin cancer
and breast cancer are unclear. Clinical trials are under-
way to determine safety and effectiveness of vitamin A
in a variety of situations. Individuals interested in par-
ticipating in a clinical trial at no charge can find a list of
open trials at<http://www.clinicaltrials.gov>.Normal vitamin A requirements
The United States Institute of Medicine (IOM) of
the National Academy of Sciences has developed val-
ues calledDietary Reference Intakes(DRIs) for vita-
mins andminerals. The DRIs consist of three sets of
values. The Recommended Dietary Allowance (RDA)
defines the average daily amount of the nutrient
needed to meet the health needs of 97–98% of the
population. The Adequate Intake (AI) is an estimate
set when there is not enough information to determine
an RDA. The Tolerable Upper Intake Level (UL) is
the average maximum amount that can be taken daily
without risking negative side effects. The DRIs are
calculated for children, adult men, adult women, preg-
nant women, andbreastfeedingwomen.
RDAs for vitamin A are measured in both weight
(micrograms) and international units (IU). The IU
measurement is what is used on dietary supplement
labels. Vitamin A comes in two different forms, pre-
formed retinol from animal sources and provitamin A
carotenoid from plant sources. These forms have dif-
ferent activity levels. To adjust for this, dietitians have
developed an equivalency measure called the Retinol
Activity Equivalent. This allows a direct comparison
between the two sources of vitamin A.
For vitamin A from food:
1 RAE = 1 mcg retinol
1 RAE = 12 mcg beta-carotene
1 RAE = 24 mcg any other provitamin A carotenoid
1 RAE = about 3 IUKEY TERMS
Cell differentiation—The process by which stem
cells develop into different types of specialized
cells such as skin, heart, muscle, and blood cells.
Fat-soluble vitamin—A vitamin that dissolves in
and can be stored in body fat or the liver.
Provitamin—A substance that the body can con-
vert into a vitamin.
Vitamin—A nutrient that the body needs in small
amounts to remain healthy but that the body cannot
manufacture for itself and must acquire through
diet.Vitamin A