Vitamin A

What can high-vitamin A foods do for you?
  • Preserve and improve your eyesight
  • Help you fight off viral infections 
 What events can indicate a need for more high-vitamin A foods?
  • Frequent viral infections
  • Night blindness
  • Goose bump-like appearance of the skin

Although vitamin A is found only in foods of animal origin, some fruits and vegetables contain compounds, called carotenoids, which can be converted into vitamin A by your body.

What is vitamin A?

When researchers first discovered vitamin A during the early 1900’s, they didn’t discover it by doing lab chemistry, but by studying the benefits of certain foods. Interestingly, what scientists discovered was the ability of extracts from butter and eggs to restore normal growth to rats which had stopped growing normally after being placed on experimental diets. Eventually, scientists determined that the missing component in the experimental diets – and the component provided by butter and eggs – was vitamin A. Researchers were initially struck by the fat-soluble nature of vitamin A, and today we still classify vitamin A as a fat-soluble vitamin. But in the early days of research, scientists focused exclusively on one category of vitamin A compounds called retinoids. The retinoids – including retinol, retinal, and retinoic acid – were forms of vitamin A found in animal foods (like eggs and butter). It took several decades before scientists discovered that the vitamin A family included a second category of compounds found in plant foods called carotenoids. Today we recognize the presence of several hundred different carotenoids in plant foods, including alpha-carotene, beta-carotene, gamma-carotene, lycopene, lutein, and zeaxanthin. Both carotenoids and retinoids provide us with unique health benefits. Our vision (especially our night vision), development of white blood cells in our immune system, fetal development, differentiation of stem cells into red blood cells, and regulation of gene expression are examples of processes that depend on the presence of retinoids. Prevention of oxidative stress and free radical damage to blood vessels and other body components, prevention of excessive inflammation, and maintenance of healthy intercellular communication are examples of processes requiring the presence of carotenoids. Because some carotenoids (including lycopene, lutein, alpha-carotene, beta-carotene, beta-cryptoxanthin and zeaxanthin) can be converted into retinoids (primarily retinol), carotenoids are often referred to as “previtamin A” or “provitamin A” to point out that they have not yet been converted into retinoids. Similarly, retinol is often referred to as “preformed vitamin A” since it already exists in retinoid form. As important as these terms are for understanding what people are talking about when they talk about vitamin A, they are also somewhat misleading because they suggest that carotenoids do not become fully valuable unless they are converted into retinoids. This conclusion is not correct, since both carotenoids and retionoids bring unique health benefits that our bodies depend on in an everyday way.

What is the function of vitamin A?

While vitamin A is best known for its vital role in vision, this vitamin also participates in physiological activities related to the immune system, maintenance of epithelial and mucosal tissues, growth, reproduction, and bone development. In food, vitamin A typically occurs as a fat compound called retinyl palmitate. The body converts retinyl palmitate to three metabolically active forms of the vitamin: retinol, retinal, and retinoic acid.

Vision support

The human retina contains four kinds of photopigments that store vitamin A compounds. One of these pigments, called rhodopsin, is located in the rod cells of the retina. Rhodopsin allows the rod cells to detect small amounts of light, and, thus, plays a fundamental role in the adaptation of the eye to low-light conditions and night visión. Retinal, the aldehyde form of the vitamin, participates in the synthesis of rhodopsin, and in the series of chemical reactions that causes visual excitation, which is triggered by light striking the rod cells. The remaining three pigments, collectively known as iodopsins, are found in the cone cells of the retina and are responsible for day visión.

Immune function support

Vitamin A stimulates several immune system activities, possibly by promoting the growth, and preventing the stress-induced shrinkage, of the thymus gland. Vitamin A is known to enhance the function of white blood cells, increase the response of antibodies to antigens, and to have anti-viral activity. In addition, retinoic acid is needed to maintain the normal structure and function of epithelial and mucosal tissues, which are found in the lungs, trachea, skin, oral cavity, and gastrointestinal tract. These tissues, when healthy and intact, serve as the first line of defense for the immune system, providing a protective barrier that disease-causing microorganisms cannot penetrate.

Cell growth support

Vitamin A is also necessary for normal cell growth and development. Although the mechanisms by which vitamin A promotes cell growth and development are not yet fully understood, it is known that retinoic acid is necessary for the synthesis of many glycoproteins, which control cellular adhesion (the ability of cells to attach to one another), cell growth and cell differentiation.

Other roles for vitamin A

It is also known that vitamin A is essential for reproductive processes in both males and females and plays a role in normal bone metabolism. In addition, some of the most cutting-edge research in the field of genetics has been examining the role of vitamin A (in the form of retinoic acid) in regulating genetic events.

What are deficiency symptoms for vitamin A?

Dietary deficiency of vitamin A is quite common in developing countries, and is associated with the high incidence of blindness, viral infections, and child mortality that occurs in impoverished populations. Vitamin A deficiency primarily affects the health of the skin, hair, eyes, and immune system, though loss of appetite, bone abnormalities, and growth retardation are also associated with inadequate intake of this vitamin.¨A tell-tale sign of vitamin A deficiency is hyperkeratosis, a goose bump-like appearance of the skin caused by excessive production of keratin (a protein found in skin) that blocks hair follicles. In initial stages, hyperkeratosis is found on the forearms and thighs, where the skin becomes dry, scaly, and rough. In advances stages, hyperkeratosis affects the whole body, causing hair loss. Due to the important role of vitamin A in supporting the functions of the immune system, individuals with insufficient intake of this vitamin often experience increased susceptibility to viral infections, most notably measles, chicken pox, pneumonia, and respiratory syncytial virus (RSV). Prolonged vitamin A deficiency can lead to night blindness, due to impaired production of rhodopsin, the compound in the retina responsible for detecting small amounts of light. Xerophthalmia, a condition characterized by changes to the conjunctiva and cornea of the eye, also results from prolonged vitamin A deficiency, and is a major cause of blindness in developing nations.

What are toxicity symptoms for vitamin A?

Vitamin A can cause side effects when taken in excessive amounts. Most causes of vitamin A toxicity are due to accidental ingestion of doses exceeding 660,000 IU (200 mg of retinol equivalents) and 330,000 IU (100 mg of retinol equivalents) by adults and children, respectively. Adverse reactions to acute ingestion are usually temporary and include loss of appetite, irritability, fatigue, weakness and vomiting. Chronic vitamin A toxicity may occur following many months of daily intake of the vitamin in amounts exceeding 14,000 IU (4.2 mg or retinol equivalents) in children and 25,000 IU in adults, and is most likely to develop in individuals taking high doses of vitamin A compounds to treat skin disorders, or in those with poor liver function. The symptoms of chronic toxicity in infants include growth retardation of the long bones (for example, the femur bone in the leg) and premature epiphyseal bone closing. In adults, vitamin A toxicity causes a variety of health conditions, including dry and itchy skin, dry and brittle fingernails, hair loss, headaches, visual changes, bone and muscle pain, fatigue, irritability, depression, fever, liver damage, anemia, and/or loss of appetite. In most cases, these maladies begin to disappear as soon as vitamin A intake is decreased. For all of the above reasons, in 2000 the National Academy of Sciences set Tolerable Upper Intake Levels (ULs) for preformed vitamin A as follows:

    • Children 3 years or younger, 600 micrograms (2,000 IU) of preformed vitamin A (retinol) per day
    • Children 4-8 years, 900 micrograms (3,000 IU) of preformed vitamin A (retinol) per day
    • Children 9-14 years, 1,700 micrograms (5,666 IU) of preformed vitamin A (retinol) per day
    • Teenagers 14-18 years, 2,800 mcg (9,333 IU) of preformed vitamin A (retinol) per day
    • Adults 19 years and older, 3,000 mcg (10,000 IU) of preformed vitamin A (retinol) per day
    • Pregnant or lactating women 18 years or younger, 2,800 mcg (9,333 IU) of preformed vitamin A (retinol) per day
    • Pregnant or lactating women 19 years or older, 3,000 mcg (10,000 IU) of preformed vitamin A (retinol) per day

 

It is important to emphasize that although excessive intake of preformed vitamin A (retinol) by children and adults does not usually cause irreversible damage, very large amounts of preformed vitamin A (retinol) can be toxic to the fetus and can cause severe birth defects, including cleft palate and spina bifida. In these circumstances, “very large amounts” means doses of preformed vitamin A (retinol) exceeding 3,000 mcg (10,000 IU) by pregnant women. It is recommended, therefore, that women who are pregnant or may become pregnant limit their intake of supplemental preformed vitamin A (retinol) to no more than 1,500 mcg (5,000 IU) per day.

What factors might contribute to a deficiency of vitamin A?

Since vitamin A is a fat-soluble vitamin, vitamin A deficiency may be caused by a diet that is extremely low in fat and/or the presence of medical conditions that cause a reduction in the ability to absorb dietary fat, such as pancreatic enzyme deficiency, Crohn’s disease, celiac sprue, cystic fibrosis, surgical removal of part or all of the stomach, gall bladder disease, and liver disease. In addition, chronic diarrhea caused by gastrointestinal infections and/or intestinal parasites may contribute to vitamin A deficiency. Viral infections, specifically the measles, decrease vitamin A status. In addition, exposure to certain toxic chemicals (for example, polybrominated biphenyls and dioxin) enhances the breakdown of vitamin A by the liver.Inadequate intake of protein contributes to vitamin A deficiency (see further explanation in the section on Nutrient Interactions).

What health conditions require special emphasis on vitamin A?

Vitamin A may play a role in the prevention and/or treatment of the following health conditions:

  • Acne
  • AIDS
  • Alcoholism
  • Atopic dermatitis
  • Cataracts
  • Cervical dysplasia
  • Diabetes
  • Dry Eyes
  • Fibrocycstic breast disease
  • Hyperkeratosis
  • Inflammaotory bowel disease
  • Kaposi’s sarcoma
  • Leukoplakia
  • Osteoarthritis
  • Otitis media (ear infection)
  • Poor vision
  • Psoriasis
  • Thyroid disorders
  • Ulcers
  • Vaginitis
  • Varicose veins
  • Viral infections
What are current public health recommendations for vitamin A?

Vitamin A was originally measured in International Units, which still appear on food and supplement labels. However, it is now common practice to also measure the amount of vitamin A activity available from a given food or supplement, as expressed in micrograms (mcg) of retinol equivalents (RE), which allows consumers to easily determine the vitamin A activity of foods containing the provitamain A carotenoids. When converting between these two units of measure, 1 retinol equivalent (or RE) is equal to 3.33 International Units (or IU) of preformed vitamin A.In 2000, the National Academy of Sciences established the following Adequate Intake (AI) levels for consumption of vitamin A by infants:

    • Males and females 0-6 months: 400 micrograms (1,333 IU)
    • Males and females 7-12 months: 500 micrograms (1,666 IU)

 

In 2000, the National Academy of Sciences established the following Estimated Average Requirements (EARs) for consumption of vitamin A by pregnant and lactating women:

    • Pregnant women less than 18 years of age: 2,500 IU (750 mcg RAE)
    • Pregnant women 19 years and older 2,567 IU (770 mcg RAE)
    • Lactating women, 18 years or younger: 4,000 IU (1,200 mcg RAE)
    • Lactating women, 19 years or older: 4,333 IU (1,300 mcg RAE)

 

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