Vitamin A: What is it?

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 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 the skin and mucous membranes function as a barrier to bacteria and viruses.

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. Retinol can be made into retinal and retinoic acid (other active forms of vitamin A) in the body.

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 . Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene.

Of the 563 identified carotenoids, fewer than 10% can be made into vitamin A in the body . Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties. The Institute of Medicine (IOM) encourages consumption of all carotenoid-rich fruits and vegetables for their health-promoting benefits.

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.

What foods provide vitamin A?

Retinol is found in foods that come from animals 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. 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 breakfast cereals, whereas major contributors of provitamin A carotenoids are carrots, cantaloupes, sweet potatoes, and spinach.

Vitamin A in foods that come from animals is well absorbed and used efficiently by the body. Vitamin A in foods that come from plants is not as well absorbed as animal sources of vitamin A. Tables 1 and 2 suggest many sources of vitamin A and provitamin A carotenoids.

Table 1: Selected animal sources of vitamin A

Food	Vitamin A (IU)*	%DV**
Liver, beef, cooked, 3 ounces	27,185	545
Liver, chicken, cooked, 3 ounces	12,325	245
Milk, fortified skim, 1 cup	500	10
Cheese, cheddar, 1 ounce	284	6
Milk, whole (3.25% fat), 1 cup	249	5
Egg substitute, ¼ cup	226	5

Table 2: Selected plant sources of vitamin A (from beta-carotene)

Food	Vitamin A (IU)*	%DV**
Carrot juice, canned, ½ cup	22,567	450
Carrots, boiled, ½ cup slices	13,418	270
Spinach, frozen, boiled, ½ cup	11,458	230
Kale, frozen, boiled, ½ cup	9,558	190
Carrots, 1 raw (7½ inches)	8,666	175
Vegetable soup, canned, chunky, ready-to-serve, 1 cup	5,820	115
Cantaloupe, 1 cup cubes	5,411	110
Spinach, raw, 1 cup	2,813	55
Apricots with skin, juice pack, ½ cup	2,063	40
Apricot nectar, canned, ½ cup	1,651	35
Papaya, 1 cup cubes	1,532	30
Mango, 1 cup sliced	1,262	25
Oatmeal, instant, fortified, plain, prepared with water, 1 cup	1,252	25
Peas, frozen, boiled, ½ cup	1,050	20
Tomato juice, canned, 6 ounces	819	15
Peaches, canned, juice pack, ½ cup halves or slices	473	10
Peach, 1 medium	319	6
Pepper, sweet, red, raw, 1 ring (3 inches diameter by ¼ inch thick)	313	6

* IU = International Units
** DV = Daily Value. DVs are reference numbers based on the Recommended Dietary Allowances (RDAs). They were developed to help consumers determine if a food contains a lot or a little of a nutrient. The DV for vitamin A is 5,000 IU. Most food labels do not list vitamin A content. The percent DV (%DV) column in the table above indicates the percentage of the DV provided in one serving. A food providing 5% or less of the DV is a low source while a food that provides 10% to 19% of the DV is a good source. A food that provides 20% or more of the DV is high in that nutrient. It is important to remember that foods that provide lower percentages of the DV also contribute to a healthful diet.

What are recommended intakes of vitamin A?

Recommendations for vitamin A are provided in the Dietary Reference Intakes (DRIs) developed by the Institute of Medicine (IOM). DRI is the general term for a set of reference values used for planning and assessing nutrient intake in healthy people. Three important types of reference values included in the DRIs are Recommended Dietary Allowances (RDA), Adequate Intakes (AI), and Tolerable Upper Intake Levels (UL). The RDA recommends the average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97% to 98%) healthy individuals in each age and gender group. An AI is set when there are insufficient scientific data to establish an RDA. AIs meet or exceed the amount needed to maintain nutritional adequacy in nearly all people. The UL, on the other hand, is the maximum daily intake unlikely to result in adverse health effects.

In Table 3, RDAs for vitamin A are listed as micrograms (mcg) of Retinol Activity Equivalents (RAE) to account for the different biological activities of retinol and provitamin A carotenoids. Table 3 also lists RDAs for vitamin A in International Units (IU), which are used on food and supplement labels (1 RAE = 3.3 IU).

Table 3: Recommended Dietary Allowances (RDAs) for vitamin A

Age (years)	Children (mcg RAE)	Males (mcg RAE)	Females (mcg RAE)	Pregnancy (mcg RAE)	Lactation (mcg RAE)
1-3	300 (1,000 IU)
4-8	400 (1,320 IU)
9-13	600 (2,000 IU)
14-18		900 (3,000 IU)	700 (2,310 IU)	750 (2,500 IU)	1,200 (4,000 IU)
19+		900 (3,000 IU)	700 (2,310 IU)	770 (2,565 IU)	1,300 (4,300 IU)

Information is insufficient to establish an RDA for vitamin A for infants. AIs have been established based on the amount of vitamin A consumed by healthy infants fed breast milk (Table 4).

Table 4: Adequate Intakes (AIs) for vitamin A for infants

Age (months)	Males and females (mcg RAE)
0-6	400 (1,320 IU)
7-12	500 (1,650 IU)

The NHANES III survey (1988-1994) found that most Americans consume recommended amounts of vitamin A. More recent NHANES data (1999-2000) show average adult intakes to be about 3,300 IU per day, which also suggests that most Americans get enough vitamin A .

There is no RDA for beta-carotene or other provitamin A carotenoids. The IOM states that consuming 3 mg to 6 mg of beta-carotene daily (equivalent to 833 IU to 1,667 IU vitamin A) will maintain blood levels of beta-carotene in the range associated with a lower risk of chronic diseases . A diet that provides five or more servings of fruits and vegetables per day and includes some dark green and leafy vegetables and deep yellow or orange fruits should provide sufficient beta-carotene and other carotenoids.

When can vitamin A deficiency occur?

Vitamin A deficiency is common in developing countries but rarely seen in the United States. Approximately 250,000 to 500,000 malnourished children in the developing world become blind each year from a deficiency of vitamin A . In the United States, vitamin A deficiency is most often associated with strict dietary restrictions and excess alcohol intake . Severe zinc deficiency, which is also associated with strict dietary limitations, often accompanies vitamin A deficiency. Zinc is required to make retinol binding protein (RBP) which transports vitamin A. Therefore, a deficiency in zinc limits the body's ability to move vitamin A stores from the liver to body tissues.

Night blindness is one of the first signs of vitamin A deficiency. In ancient Egypt, it was known that night blindness could be cured by eating liver, which was later found to be a rich source of the vitamin . Vitamin A deficiency contributes to blindness by making the cornea very dry and damaging the retina and cornea.

Vitamin A deficiency diminishes the ability to fight infections. In countries where such deficiency is common and immunization programs are limited, millions of children die each year from complications of infectious diseases such as measles. In vitamin A-deficient individuals, cells lining the lungs lose their ability to remove disease-causing microorganisms. This may contribute to the pneumonia associated with vitamin A deficiency.

There is increased interest in early forms of vitamin A deficiency, described as low storage levels of vitamin A that do not cause obvious deficiency symptoms. This mild degree of vitamin A deficiency may increase children's risk of developing respiratory and diarrheal infections, decrease growth rate, slow bone development, and decrease likelihood of survival from serious illness. Children in the United States who are considered to be at increased risk for subclinical vitamin A deficiency include:

• toddlers and preschool age children;
• children living at or below the poverty level;
• children with inadequate health care or immunizations;
• children living in areas with known nutritional deficiencies;
• recent immigrants or refugees from developing countries with high incidence of vitamin A deficiency or measles; and
• children with diseases of the pancreas, liver, or intestines, or with inadequate fat digestion or absorption.

A deficiency can occur when vitamin A is lost through chronic diarrhea and through an overall inadequate intake, as is often seen with protein-energy malnutrition. Low blood retinol concentrations indicate depleted levels of vitamin A. This occurs with vitamin A deficiency but also can result from an inadequate intake of protein, calories, and zinc, since these nutrients are needed to make RBP. Iron deficiency can also affect vitamin A metabolism, and iron supplements provided to iron-deficient individuals may improve body stores of vitamin A and iron .

Excess alcohol intake depletes vitamin A stores. Also, diets high in alcohol often do not provide recommended amounts of vitamin A. It is very important for people who consume excessive amounts of alcohol to include good sources of vitamin A in their diets. Vitamin A supplements may not be recommended for individuals who abuse alcohol, however, because their livers may be more susceptible to potential toxicity from high doses of vitamin A . A medical doctor will need to evaluate this situation and determine the need for vitamin A supplements.