Fat-Soluble Vitamins; Vitamin A

 Vitamin A

Vitamin A; most essential vitamin

Vitamin A refers to the performed retinoids and provitamin A carotenoids that can be converted to vitamin A activity. Retinoids is a collective term for the biologically active forms of vitamin A. They are called performed vitamin A because, unlikely carotenoids, they do not need to be converted in the body to become biologically active. Retinoids exist in 3 forms; retinol, retinal and retinoic acid. The tail segment of the vitamin A structure terminates in 1 of these 3  chemical groups and determines the name or classification. To some extent, these can be interconverted . However, retinoic acid cannot be converted back to the other forms. the ability to interconvert forms helps maintain adequate amounts of each retinol form for its unique functions. 

Carotenoids are yellow-orange pigmented materials in fruits and vegetables, some of which are provitamins __that is, they can be converted into vitamin A. Of the 600 or more known carotenoids, only alpha-carotene, beta-carotene, and beta-cryptoxanthin can be converted  to biologically active forms of vitamin A. Other carotenoids, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A and thus, do not have vitamin A activity in humans.

Vitamin A in Foods:

Food sources of vitamin A

The richest sources of the retinoids are food derived from animals _liver, fish liver oils, milk and milk products, cheese and eggs. Margarine is usually fortified to provide the same amount of vitamin A as butter.

Plants contain no retinoids, but many vegetables and some fruits contain vitamin A precursors _the carotenoids. Only a few carotenoids have vitamin A activity, the carotenoid with the greatest vitamin A activity is beta-carotene. Beta-carotene is a rich, deep yellow, almost orange compound. The beta carotene in dark green, leafy vegetables is abundant, but masked by large amounts of the green pigment chlorophyll. Attractive meals that include colorful fruits and vegetables rich in beta-carotene are likely to provide vitamin A.

Colors of Vitamin A in Foods:

Dark leafy greens (like broccoli and spinach _not celery or cabbage) and rich yellow or deep orange vegetables and fruits (such as cantaloupe, carrots, and sweet potatoes _not corn or  bananas) help people meet their vitamin A needs. A diet including several servings of such carotene-rich sources helps ensure a sufficient intake.

Bright color is not always a sign of vitamin A activity, however Beets and corn, for example,  derive their colors from the red and yellow xanthopylls, which have no vitamin A activity. As for white plant foods such as potatoes, cauliflower, pasta, and rice, they also offer little or no vitamin A. 

Vitamin A-Rich Liver:

People sometimes wonder if eating liver too frequently can cause vitamin A toxicity. Liver is a rich source because vitamin A is stored in the livers of animals, just as in humans. Liver offers many nutrients, and eating it periodically may improve a person's nutrition status, but caution is warranted not to eat too much too often especially for pregnant women. With 1 ounce of beef liver providing more than three times the RDA for vitamin A, intakes can increase quickly.

Golden Rice and Biofortified Foods:

 In the developing regions of the world, fruits and vegetables are a scarcity, and rice, which contains no beta-carotene or vitamin A, is the staple food. Commonly called golden rice because of its yellow tinge, this rice offers a promising solution to world malnutrition, but it also raises questions about potential risks to the environment.

Functions in the Body:

Vitamin A is a versatile vitamin, known to regulate the expression of several hundred genes. Its major roles include:

Promoting Vision:

Vitamin A play two indispensable roles in the eye: it helps maintain a crystal-clear outer window, the cornea, and it participates in the conversion of light energy into nerve impulses at the retina. Some of the photosensitive cells of the retina contain pigment molecules called rhodopsin. Each rhodopsin molecule is composed of a protein called opsin bonded to a molecule of retinal, which plays a central role in vision. When light passes through the cornea of the eye and strikes the retina, rhodopsin responds. As it does, opsin is released and retinal shifts from a cis to a trans configuration, just as fatty acids do hydrogenation. These changes generate an electrical impulse that conveys the message to the brain. Much of the retinal is then converted back to its active cis form and combined with the opsin protein to regenerate rhodopsin. Some retinal, may be oxidized to retinoic acid, a biochemical dead end for the visual process. Visual activity leads to repeated small losses of  retinal, necessitating its constant replenishment either directly from foods or indirectly from retinol stores.

Protein Synthesis and Cell Differentiation:

Only one-thousandth of the body's vitamin A is in the retina. Much more is in the cells lining the body's surfaces. There, the vitamin participates in protein synthesis and cell differentiation, a process by which each type of cell develops to perform a specific function. All body surfaces, both inside and outside, are covered by layers of cell known as epithelial cells. The epithelial tissue on the outside of the body is, of course, the skin _and vitamin A and beta carotene help protect against skin damage from sunlight. The epithelial tissues that line the inside of the body are the mucous membranes: the lining of the mouth, stomach, and intestines; the linings of the lungs and the passage leading to them; the lining of the urinary bladder and urethra; the linings of the uterus and vagina; and the lining of the eyelids and sinus passageways. Within the body, the mucous membranes line an area larger than a tennis court, and vitamin A helps to maintain their integrity.

Vitamin A promotes differentiation of epithelial cells and goblet cells, one-celled glands that synthesize and secrete mucus. Mucus coats and protects the epithelial cells from invasive microorganisms and other potentially damaging substances, such as gastric juices.

Regulating  Growth and Reproduction:

Vitamin A also supports reproduction and regulates growth. In men, retinol participates in sperm development, and in women, vitamin A supports normal fetal development during pregnancy. Children lacking vitamin A fails to grow; given vitamin A supplements, these children gain weight and grow taller. 

The growth of bones illustrates the growth is a complex phenomenon of remodeling. To convert a small bone into a large bone, some bone cells must "undo" parts of the bone before other cells can build new bone, and vitamin A participates in the dismantling. The cells that break down bone contain acid and enzymes that dissolve the minerals and digest the matrix. With the help of vitamin A , these bone dismantling cells destroys selected sites in the bone, removing the parts that are not needed. After completing their work, the bone-dismantling cells die, leaving their excavation site to be rebuilt by the bone bone-building cells. 

Beta-Carotene as a Precursor & an Antioxidant:

Beta-carotene plays two primary roles in the body. First, it serves as a vitamin A precursor, illustrates. Second, some beta-carotene acts as an antioxidant capable of protecting the body against disease.

 Vitamin A Recommendation:

Because the body can derive vitamin A from both retinoids and carotenoids, its content in foods and its recommendations are expressed as retinol activity equivalents (RAE). One microgram of retinol counts as 1 RAE, as does 12 micrograms of dietary beta-carotene. This difference recognizes that beta-carotene's absorption and conversion are significantly less efficient than those of the retinoids. Food and supplement labels sometimes report vitamin A contents using International Units, a measure of vitamin activity used before direct chemical analysis was possible.  

Vitamin A Deficiency:

Vitamin A status depends mostly on the adequacy of vitamin A stores, 90 percent of which are in the liver. Vitamin A status also depends on a person's protein status because retinol-binding protein serves as the vitamin's transport carrier inside the body.

If a person were to stop eating vitamin A-containing foods, deficiency symptoms would not begin to appear until after stores were depleted _1 to 2 years for a healthy adult but much sooner for a growing child. Then the consequences would be profound and severe. Vitamin A deficiency is uncommon in the United States, but it is a major nutrition problem in many developing countries, responsible for a million or more unnecessary deaths and cases of blindness each year. Many countries around the world nourish their people with biofortified corn, rice, and cassava to protect against vitamin A deficiencies.

Infectious Diseases:

Vitamin A supports immune function and inhibits replication of the measles virus. In developing countries around the world, measles is a devastating infectious disease, killing 367 children each day. The severity of the illness often correlates with the degree of vitamin A deficiency; deaths are usually due to related infections such as pneumonia and severe diarrhea. Providing vitamin A to children diagnosed with measles reduces the risk of dying by half.

The World Health Organization (WHO) and United Nations International Children's Emergency Fund UNICEF) recommend two doses of vitamin A supplements, give 24 hours apart, for all children with measles. In the United States, the American Academy of Pediatrics recommends vitamin A supplements for certain groups of measles-infected infants and children. Vitamin A supplements also protect against blindness and the complications of other life-threatening infections, including malaria, lung diseases, and HIV (human immunodeficiency virus, the virus that causes AIDS).

Night Blindness:

Night blindness is one of the first detectable signs of vitamin A deficiency and permits early diagnosis. In night blindness, the person loves the ability to recover promptly from the temporary blinding that follows a flash of bright light at night or to see after dark. In many parts of the world, after the sun goes own, vitamin A-deficiency people become night-blind. They often cling to others or sit still, afraid that they may trip and fall or lose their way if they try to walk alone. Treatment with vitamin A rapidly corrects night blindness.

Blindness (Xerophthalmia):

Beyond night blindness is total blindness _failure to see at all. Night blindness is caused by a lack of vitamin A at the back of the eye, the retina; total blindness is caused by a lack of vitamin A at the front of the eye, the cornea. Severe vitamin A deficiency is the leading cause of preventable blindness in the world, causing as many as half a million children to lose their sight each year; an estimated half of them die within a year of losing their sight. 

 Blindness due to vitamin A deficiency, known as xerophthalmia, develops in stages. At first, the cornea becomes dry and hard because of inadequate mucus production _a condition known as xerosis. Then xerosis quickly progresses to keratomalacia, the softening of the cornea that leads to irreversible blindness. For this reason, prompt correction of vitamin A deficiency is essential to preserving eyesight.

Keratinization:

Vitamin A also affects other body surfaces. On the body's outer surface, the epithelial cells of the skin change shape and begin to secrete the protein keratin _the same hard, inflexible protein commonly found in their hair and nails. The skin becomes dry, rough and scaly as lumps of keratin accumulate. Without vitamin A, the goblet cells in the GI tract diminish in number and activity, limiting their secretion of mucus. With less mucus, normal digestion and absorption of nutrients falter and this in turn, worsens malnutrition by limiting the absorption of whatever nutrients the diet may deliver. Similar changes in the cells of other epithelial tissues weaken defenses, making infections of the respiratory tract, the GI tract, the urinary tract, the vagina, and inner ear likely. 

  Vitamin A Toxicity:

Symptoms of toxicity begin to develop when all the binding proteins are loaded, and vitamin A is free to damage cells. Such effects are unlikely when a person depends on a balanced diet for nutrients, but toxicity is a real possibility when concentrated amounts of performed vitamin A in foods derived from animals, fortified foods, or supplements are consumed. Children are most vulnerable to toxicity because they need less vitamin A and are most sensitive to overdoses. An Upper Level (UL) of 3000 micrograms has been set for adults and applies only to the performed vitamin. Multivitamin supplements typically provide 1200 to 1500 micrograms _much more vitamin A than most people need. 

Beta-carotene, which is found in a wide variety of fruits and vegetables, is not converted efficiently enough in the body to cause vitamin A toxicity; instead, its is stored in the fat just under the skin. Although consumption of beta-carotene from foods may turn the skin yellow, this is not harmful. In contrast, overconsumption of beta-carotene from supplements may be quite harmful. Adverse effects of beta-carotene supplements are most evident in people who drink alcohol and smoke cigarettes.

Bone Defects:

Excessive intakes of performed vitamin A may weaken the bones and contribute to fractures and osteoporosis. Vitamin A suppresses bone building activity, stimulates bone-dismantling activity, and it interferes with vitamin D's ability to maintain normal blood calcium.

Birth Defects:

Excessive vitamin A during pregnancy leads to abnormal cell death in the spinal cord, which increases the risk of birth defects such as spinal bifida and cleft palate. In such cases, vitamin A is considered a teratogen. High intakes before the seventh week of pregnancy appear to be the most damaging. For this reason, vitamin  is not given as a supplement in the first trimester of pregnancy without specific evidence of deficiency, which is rare.

Not for Acne:

Adolescents need to know that massive doses of vitamin A have no beneficial effect on acne. The prescription medicine Accutane is made from vitamin A but is chemically different. Taken orally, Accutane is effective against the deep lesions of cystic acne. It is highly toxic, however, especially during growth, and has caused birth defects in infants when women have taken it during pregnancy. For this reason, women taking Accutane must agree to pregnancy testing and to using two forms of contraception from at least 1 month before taking the drug through at least 1 month after discontinuing its use. Another vitamin A relative, Retin-A, fights acne, the wrinkles of aging, and other skin disorders. Applied topically, this ointment smooths and softens skin; it also lightens skin that has become darkly pigmented after inflammation.