Vitamin A
Dietary Supplement Fact Sheet: Vitamin A and
Carotenoids
Office of Dietary Supplements • NIH Clinical Center • National Institutes
of Health
Vitamin A: What is it?
Vitamin A is a family of fat-soluble compounds that play an important role
in vision, bone growth, reproduction, cell division, and cell
differentiation (in which a cell becomes part of the brain, muscle, lungs,
etc.) [1-5]. Vitamin A helps regulate the immune system, which helps
prevent or fight off infections by making white blood cells that destroy
harmful bacteria and viruses [1,6-10]. Vitamin A also may help
lymphocytes, a type of white blood cell, fight infections more
effectively.
Vitamin A promotes healthy surface linings of the eyes and the
respiratory, urinary, and intestinal tracts [8]. When those linings break
down, it becomes easier for bacteria to enter the body and cause
infection. Vitamin A also helps maintain the integrity of skin and mucous
membranes, which also function as a barrier to bacteria and viruses
[9-11].
Retinol is one of the most active, or usable, forms of vitamin A, and is
found in animal foods such as liver and whole milk and in some fortified
food products. Retinol is also called preformed vitamin A. It can be
converted to retinal and retinoic acid, other active forms of the vitamin
A family [1].
Provitamin A carotenoids are darkly colored pigments found in plant foods
that can be converted to vitamin A. In the United States, approximately
26% and 34% of vitamin A consumed by men and women, respectively, is
provided by provitamin A carotenoids [1]. Common carotenoids found in
foods are beta-carotene, alpha-carotene, lutein, zeaxanthin, lycopene, and
cryptoxanthin [11]. Of the 563 identified carotenoids, fewer than 10% are
precursors for vitamin A [12]. Among these, beta-carotene is most
efficiently converted to retinol [1,13-15]. Alpha-carotene and beta-cryptoxanthin
are also converted to vitamin A, but only half as efficiently as
beta-carotene [1]. Lycopene, lutein, and zeaxanthin are carotenoids that
do not have vitamin A activity but have other health promoting properties
[1]. The Institute of Medicine (IOM) encourages consumption of carotenoid-rich
fruits and vegetables for their health-promoting benefits.
Some carotenoids, in addition to serving as sources of vitamin A, have
been shown to function as antioxidants in laboratory tests. However, this
role has not been consistently demonstrated in humans [1]. Antioxidants
protect cells from free radicals, which are potentially damaging
by-products of oxygen metabolism that may contribute to the development of
some chronic diseases [3,14-15].
What foods provide vitamin A?
Retinol is found in animal foods such as whole eggs, milk, and liver. Most
fat-free milk and dried nonfat milk solids sold in the United States are
fortified with vitamin A to replace the amount lost when the fat is
removed [16]. Fortified foods such as fortified breakfast cereals also
provide vitamin A. Provitamin A carotenoids are abundant in darkly colored
fruits and vegetables. The 2000 National Health and Nutrition Examination
Survey (NHANES) indicated that major dietary contributors of retinol are
milk, margarine, eggs, beef liver and fortified ready-to-eat cereals,
whereas major contributors of provitamin A carotenoids are carrots,
cantaloupes, sweet potatoes, and spinach [17].
Animal sources of vitamin A are well absorbed and used efficiently by the
body. Plant sources of vitamin A are not as well absorbed as animal
sources.
Selected
Fruits & Vegetables Compared by Vitamin A Content
Source and more info at
Vitamin and Mineral Supplement Fact Sheets
from the Office
of Dietary Supplements
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