Vitamin A Benefits
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.) . 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 . 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.
Retinol is one of the most active, or usable, forms of vitamin A,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 .
Provitamin A carotenoids are darkly colored pigments found in plant foods that can be converted to vitamin A.Common carotenoids found in foods are beta-carotene, alpha-carotene, lutein, zeaxanthin, lycopene, and cryptoxanthin . Of the 563 identified carotenoids, fewer than 10% are precursors for vitamin A . Among these, beta-carotene is most efficiently converted to retinol . Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties .
Some carotenoids, in addition to serving as sources of vitamin A, have been shown to function as antioxidants in laboratory tests. But, 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.
Symptoms and Signs
The severity of the effects of vitamin A deficiency is inversely related to age. Growth retardation is a common sign in children. Inadequate intake or utilization of vitamin A can cause impaired dark adaptation and night blindness; xerosis of the conjunctiva and cornea; xerophthalmia and keratomalacia; keratinization of lung, GI tract, and urinary tract epithelia; increased susceptibility to infections; and sometimes death. Follicular hyperkeratosis of the skin is common.
Pathognomonic changes are confined to the eye. The earliest change, rod dysfunction, can be detected by dark adaptometry, rod scotometry, or electroretinography (these tests require cooperative subjects). Dysfunction of the retina is followed by changes in the structure and function of epithelial cells. Keratomalacia rapidly supervenes with liquefaction of part or all of the cornea, leading to rupture, with extrusion of the eye contents and subsequent shrinking of the globe (phthisis bulbi), or to anterior bulging (corneal ectasia and anterior staphyloma) and blindness.
Retinol is found in animal foods such as whole eggs, milk, and liver, most fat-free milk Fortified foods such as fortified breakfast cereals also provide vitamin A. Provitamin A carotenoids are abundant in darkly colored fruits and vegetables. The 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.
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.
Table 1: Selected animal sources of vitamin A
Table 2: Selected plant sources of vitamin A (from beta-carotene)
||Vitamin A (IU)*
|Liver, beef, cooked, 3 ounces
|Liver, chicken, cooked, 3 ounces
|Milk, fortified skim, 1 cup
|Cheese, cheddar, 1 ounce
|Milk, whole (3.25% fat), 1 cup
|Egg substitute, ¼ cup
* IU = International Units.
||Vitamin A (IU)*
|Carrot juice, canned, ½ cup
|Carrots, boiled, ½ cup slices
|Spinach, frozen, boiled, ½ cup
|Kale, frozen, boiled, ½ cup
|Carrots, 1 raw (7½ inches)
|Vegetable soup, canned, chunky, ready-to-serve, 1 cup
|Cantaloupe, 1 cup cubes
|Spinach, raw, 1 cup
|Apricots with skin, juice pack, ½ cup
|Apricot nectar, canned, ½ cup
|Papaya, 1 cup cubes
|Mango, 1 cup sliced
|Oatmeal, instant, fortified, plain, prepared with water, 1 cup
|Peas, frozen, boiled, ½ cup
|Tomato juice, canned, 6 ounces
|Peaches, canned, juice pack, ½ cup halves or slices
|Peach, 1 medium
|Pepper, sweet, red, raw, 1 ring (3 inches diameter by ¼ inch thick)
** 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 10-19% 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.
Vitamin A deficiency is common in developing countries. Approximately 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A . Severe zinc deficiency, which is also associated with strict dietary limitations, often accompanies vitamin A deficiency. Zinc is required to synthesize retinol binding protein (RBP) which transports vitamin A. Therefore, a deficiency in zinc limits the body's ability to mobilize vitamin A stores from the liver and transport vitamin A 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 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 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 subclinical forms of vitamin A deficiency, described as low storage levels of vitamin A that do not cause overt deficiency symptoms. Children 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;
· children with diseases of the pancreas, liver, or intestines, or with inadequate fat digestion or absorption.
Excess alcohol intake and diets high in alcohol depletes vitamin A stores. It is very important for people who consume excessive amounts of alcohol to include good sources of vitamin A in their diets.
Who may need extra vitamin A to prevent a deficiency?
Recommend vitamin A administration for all children diagnosed with measles in communities where vitamin A deficiency is a serious problem and where death from measles is greater than 1%.
Fat malabsorption can result in diarrhea and prevent normal absorption of vitamin A. Over time this may result in vitamin A deficiency. Those conditions include:
:Often referred to as sprue, celiac disease is a genetic disorder. People with celiac disease are intolerant to a protein called gluten found in wheat and some other grains. In celiac disease, gluten can trigger damage to the small intestine, where most nutrient absorption occurs.
:This inflammatory bowel disease affects the small intestine. People with Crohn's disease often experience diarrhea, fat malabsorption, and malnutrition .
:These often result in fat malabsorption , since the pancreas secretes enzymes important for fat absorption. Without these enzymes, it is difficult to absorb fat.
Vegetarians who do not consume eggs and dairy foods need provitamin A carotenoids to meet their need for vitamin A. They should include a minimum of five servings of fruits and vegetables in their daily diet and regularly choose dark green leafy vegetables and orange and yellow fruits to consume recommended amounts of vitamin A.
Issues and Controversies
Vitamin A, beta carotene and cancer
Surveys suggest an association between diets rich in beta-carotene and vitamin A and a lower risk of many types of cancer . A higher intake of green and yellow vegetables or other food sources of beta carotene and/or vitamin A may decrease the risk of lung cancer. However, a number of studies that tested the role of beta-carotene supplements in cancer prevention did not find them to be protective.
The IOM states that "beta-carotene supplements are not advisable for the general population," although they also state that this advice "does not pertain to the possible use of supplemental beta-carotene as a provitamin A source for the prevention of vitamin A deficiency in populations with inadequate vitamin A nutriture".
Vitamin A and osteoporosis
Osteoporosis, a disorder characterized by porous and weak bones, is a serious public health problem, majority of them are women. Many factors increase the risk for developing osteoporosis, including being female, thin, inactive, at advanced age, and having a family history of osteoporosis. An inadequate dietary intake of calcium, cigarette smoking, and excessive intake of alcohol also increase the risk.
Researchers are now examining a potential new risk factor for osteoporosis: an excess intake of vitamin A. Animal, human, and laboratory research suggests an association between greater vitamin A intake and weaker bones.
There is no evidence of an association between beta-carotene intake, especially from fruits and vegetables, and increased risk of osteoporosis. Current evidence points to a possible association with vitamin A as retinol only.
Hypervitaminosis A refers to high storage levels of vitamin A in the body that can lead to toxic symptoms.
There are four major adverse effects of hypervitaminosis A
· birth defects,
· liver abnormalities,
· reduced bone mineral density that may result in osteoporosis,
· central-nervous-system disorders .
Toxic symptoms can also arise after consuming very-large amounts of preformed vitamin A over a short period of time. Signs of acute toxicity include nausea and vomiting, headache, dizziness, blurred vision, and muscular uncoordination. Although hypervitaminosis A can occur when large amounts of liver are regularly consumed, most cases result from taking excess amounts of the nutrient in supplements.
Retinoids are compounds that are chemically similar to vitamin A. Over the past 15 years, synthetic retinoids have been prescribed for acne, psoriasis, and other skin disorders . Isotretinoin (Roaccutane® or Accutane®) is considered an effective anti-acne therapy. At very high doses, however, it can be toxic, which is why this medication is usually saved for the most severe forms of acne. The most serious consequence of this medication is birth defects. It is extremely important for sexually active females who may become pregnant and who take these medications to use an effective method of birth control. Women of childbearing age who take these medications are advised to undergo monthly pregnancy tests to make sure they are not pregnant.
Provitamin A carotenoids such as beta-carotene are generally considered safe because they are not associated with specific adverse health effects. Their conversion to vitamin A decreases when body stores are full. A high intake of provitamin A carotenoids can turn the skin yellow, but this is not considered dangerous to health.
Evidence of vitamin A depletion is unobtainable in the preclinical stage, except for a history of inadequate intake. Plasma retinol levels fall after liver stores are exhausted. The normal range is 20 to 80 µg/dL (0.7 to 2.8 µmol/L); 10 to 19 µg/dL (0.35 to 0.66 µmol/L) is low, and < 10 µg/dL (< 0.35 µmol/L) is deficient. Mean plasma retinol binding protein (RBP) is 47 µg/mL for adult males and 42 µg/mL for adult females. Up to the age of 10 yr, the range is 20 to 30 µg/mL. Plasma levels of vitamin A and RBP fall in deficiency states and in acute infections. Other causes of night blindness (eg, retinitis pigmentosa) must be excluded. Secondary infection may complicate the corneal changes. Trial with therapeutic doses of vitamin A assists in the diagnosis.
The cause should be corrected, and vitamin A should be given in therapeutic doses at once. Oral vitamin A palmitate in oil 20,000 µg (60,000 IU) daily for 2 days and once before discharge from the hospital after 7 to 10 days is usually effective. In the presence of vomiting or malabsorption, water-miscible vitamin A must be given IM (oil preparations are not used IM). Thereafter, 3,200 to 8,000 µg (10,000 to 25,000 IU)/day is given orally in three divided doses as cod liver oil, red palm oil, or another concentrate. Prolonged daily administration of large doses, especially to infants, must be avoided because toxicity may result.
During pregnancy and lactation, prophylactic or therapeutic doses should not exceed two times the RDA to avoid possible damage to the fetus.