What is Vitamin A?

Vitamin A is a group of 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, blood, or other specialized tissue). 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. Vitamin A also may help lymphocytes (a type of white blood cell) fight infections more efficiently.

The most familiar function of vitamin A is in eyesight. Vitamin A plays indispensable roles in two areas: in perception of light at the retina and in the maintenance of a healthy, crystal-clear outer window, the cornea.

Vitamin A is needed by all epithelial tissue (external skin and internal linings), not just by the cornea. The skin and all of the protective linings of the lungs, intestines, vagina, urinary tract, and bladder serve as barriers to infection by bacteria and to damage from other sources. Vitamin A works behind the scenes at the genetic level to promote the process of cell differentiation, in which each type of cell develops to perform a different specific function.

Vitamin A also assists in bone growth. Normal children’s bones grow longer, and the children grow taller, by remodeling each old bone into a new, bigger version. To do so, the body dismantles the old bone structures and replaces them with new, larger bone parts. Growth cannot take place just by adding on to the original small bone; vitamin A is needed in the critical dismantling steps.

What are the sources of Vitamin A?

In general, there are two categories of vitamin A, depending on whether the food source is an animal or a plant.

Vitamin A found in foods that come from animals is called preformed vitamin A. It is absorbed in the form of retinol, one of the most usable (active) forms of vitamin A. Sources include liver, whole milk, and some fortified food products.

Vitamin A that is found in colorful fruits and vegetables is called provitamin A carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids. Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Among these, beta-carotene is most efficiently made into retinol.

The following chart displays the Vitamin A content of various foods.

The following table lists foods that provide large quantities of Vitamin A, ranked by micrograms Retinol Activity Equivalents (RAE) per standard amount.

Food, Standard Amount	Vitamin A (µg RAE)	% Daily Value	Calories
Organ meats (liver, giblets), various, 3 oz	1490-9126	165-1014	134-235
Carrot Juice, 3/4 cup	1692	188	71
Sweet Potato, baked, 1 medium	1096	122	103
Pumpkin, canned, 1/2 cup	953	106	42
Carrots, cooked, 1/2 cup	671	75	27
Spinach, cooked, 1/2 cup	573	64	30
Collards, cooked, 1/2 cup	489	54	31
Kale, cooked, 1/2 cup	478	53	20
Mixed Vegetables, canned, 1/2 cup	478	53	20
Turnip Greens, cooked, 1/2 cup	441	49	24
Instant cereals, fortefied, prepared, 1 packet	285-376	32-42	75-97
Various cereals, with added Vit A, 1 oz	180-376	20-42	100-117
Carrot, raw, 1 small	301	33	20
Beet Greens, cooked, 1/2 cup	276	31	19
Winter Squash, cooked, 1/2 cup	268	30	38
Dandelion Greens, cooked, 1/2 cup	260	29	18
Cantaloupe, raw, 1/4 medium melon	233	26	46
Mustard Greens, cooked, 1/2 cup	221	25	11
Pickled Herring, 3 oz	219	24	222
Red Sweet Pepper, cooked, 1/2 cup	186	21	19
Chinese cabbage, cooked, 1/2 cup	180	20	10

Vitamin A as an antioxidant

Some provitamin A carotenoids have been shown to function as antioxidants in laboratory studies; however, this role has not been consistently demonstrated in humans. 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.

Vitamin A Under/Over

The following table describes some of the symptoms that signify a lack of Vitamin A (Deficiency) and an overabundance of Vitamin A (Toxicity).

Deficiency Symptions		Toxicity Symptons
Blood/Circulatory System
Anemia		Red blood cell breakage, cessation of menstruation, nosebleeds
Bones/Teeth
Cessation of bone growth, painful joints; impaired enamel formation, cracks in teeth, tendency toward tooth decay		Bone pain; growth retardation; increased pressure inside skull; headaches; possible bone mineral loss
Digestive System
Diarrhea, changes in intestinal and other body linings		Abdominal pain, nausea, vomiting, diarrhea, weight loss
Immune System
Depression, frequent respiratory, digestive, bladder, vaginal, and other infections		Overreactivity
Nervous/Muscular System
Night blindness (retinal)		Blurred vision, muscle weakness, fatigue, irritability, loss of appetite
Skin and Cornea
Keratinization, corneal degeneration leading to blindness, rashes		Dry skin, rashes, loss of hair; cracking and bleeding lips, brittle nails
Other
Kidney stones, impaired growth		Liver enlargement and liver damage; birth defects