Vitamin A

Vitamin A is key for good vision, a healthy immune system, and cell growth. There are two types of vitamin A. This entry is primarily about the active form of vitamin A — retinoids — that comes from animal products. Beta-carotene is among the second type of vitamin A, which comes from plants.

The American Heart Association recommends obtaining antioxidants, including beta-carotene, by eating a well- balanced diet high in fruits, vegetables, and whole grains rather than from supplements until more is known about the risks and benefits of supplementation.

High doses of antioxidants (including vitamin A) may actually do more harm than good. Vitamin A supplementation alone, or in combination with other antioxidants, is associated with an increased risk of mortality from all causes, according to an analysis of multiple studies.

Is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids, and beta-carotene. Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision.

Is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells.
In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol) in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.

All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are essential for vitamin activity. The orange pigment of carrots (beta-carotene) can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group, which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.


Vitamin A can be found in two principal forms in foods:

Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.
The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin (all of which contain beta-ionone rings), but no other carotenoids, function as provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene 15,15′-dioxygenase which cleaves beta-carotene in the intestinal mucosa and converts it to retinol. In general, carnivores are poor converters of ionone-containing carotenoids, and pure carnivores such as cats and ferrets lack beta-carotene 15,15′-dioxygenase and cannot convert any carotenoids to retinal (resulting in none of the carotenoids being forms of vitamin A for these species).

Vitamin A, also called retinol, helps your eyes adjust to light changes when you come in from outside and also helps keep your eyes, skin and mucous membranes moist. Vitamin A mostly comes from animal foods, but some plant-based foods supply beta-carotene, which your body then converts into Vitamin A. It also has antioxidant properties that neutralize free radicals in the body that cause tissue and cellular damage.
Early information from scientific studies suggests that beta-carotene might help people who already have Coronary Artery Disease (CAD).

Nutritionists categorize vitamins by the materials that a vitamin will dissolve in. There are two categories: water-soluble and fat-soluble vitamins. Fat-soluble are stored in the fat tissues of the body for a few days to up to six months. If you get too much of a fat-soluble vitamin, it can be stored in your liver and may sometimes cause health problems. Vitamin A is a fat-soluble vitamin.

Some people take mega-doses of fat-soluble vitamins, which can lead to toxicity. Eating a normal diet of foods rich in these vitamins won’t cause a problem. Remember, you only need small amounts of any vitamin. In the case of vitamin A, overconsumption has been linked with an increased risk of fractures in postmenopausal women.

Some health problems can make it hard for a person’s body to absorb these vitamins. If you have a chronic health condition, ask your doctor about whether your vitamin absorption will be affected.

 

Eye Health

Vitamin A has many varied functions. Retinol not only creates the pigments in the retina of the eye, according to NLM, but also is integral for good vision, especially night vision, and overall eye health. An age-related eye disease study by the National Eye Institute found that taking high levels of antioxidants, such as vitamin A, along with zinc, may reduce the risk of developing advanced age-related macular degeneration by about 25 percent. Age-related macular degeneration is the most common cause of loss of vision in the older population, said Ross.

A Vitamin A deficiency can lead to a thickening of the cornea and eventually even to blindness. Keratomalacia, a condition that comes from severe deficiency of vitamin A, is a condition that is bilateral, meaning it usually affects both eyes.
This type of deficiency may be dietary, meaning your daily intake of the vitamin, or metabolic, meaning your body’s ability to absorb it. Early symptoms of Keratomalacia may include night blindness and extreme dryness of the eyes.
Your eyesight may be followed by wrinkling, cloudiness, and a softening of the corneas. If the corneas continue to soften, without adequate attention and treatment, this may lead to infected corneas, a rupture, or degenerative tissue changes- all can cause blindness.

Skin

Vitamin A also helps skin grow and repair skin.
Vitamin A helps to keep your body free from free radicals and toxins, which might cause damage to your skin. It helps to keep the skin soft and supple by ensuring moisture retention, thereby preventing dryness, keratinization and skin conditions like psoriasis.

Vitamin A deficiency will lead to the drying, scaling, and follicular thickening of the skin. Keratinization of the skin, when the epithelial cells lose their moisture and become hard and dry, can occur in the mucous membranes of the respiratory, gastrointestinal tract, and urinary tract.

Immune Support

Several immune system functions are dependent on sufficient vitamin A, which is why it is known as an important immune booting vitamin. Genes involved in immune responses are regulated by Vitamin A, which means it is essential for fighting serious conditions like cancer and autoimmune diseases, but also illnesses like the flu or common colds.

Beta-carotene is also a powerful antioxidant that can help boost the immune system and prevent a variety of chronic illnesses. Vitamin A can especially help the immunity of children. A study done in London found that vitamin A supplements reduced child mortality by 24 % in low and middle-income countries. The study also found that vitamin A deficiency in children increased their vulnerability to infections like diarrhea and measles.

Vitamin A enhances the body’s immunity against infections by increasing the lymphocytic responses against disease-causing antigens. It keeps the mucus membranes moist to ensure better immunity and also enhances the activity of white blood cells. It not only prevents the germs from entering your body, but also helps to fight the infections once the germs gain entry into the body system, thereby ensuring a double core of protection.

Fights Inflammation

Vitamin A has antioxidant properties that neutralize free radicals in the body that cause tissue and cellular damage. Vitamin A can prevent the cells from becoming overactive. When the immune system overreacts to food proteins, this is what creates food allergies and eventually inflammation. Vitamin A intake can help to lower the risk of certain types of food allergies because it helps to prevent this dangerous overreaction.
Reduced levels of inflammation are also correlated with a lower risk for neurodegenerative diseases like Alzheimer’s disease and Parkinson’s disease.


Deficiency and dosage

Vitamin A deficiency is rare in the United States, although it is common in many developing countries. “In fact, Vitamin A deficiency is the leading cause of childhood blindness in Southeast Asia,” said Greuner. Around 250,000 to 500,000 children around the world with vitamin A deficiency become blind every year. Half of those children die within 12 months of losing their sight, according to the World Health Organization. Symptoms of a severe deficiency are night blindness, dry eyes, diarrhea and skin problems.

Vitamin A dosage is tricky. Too little can make a person more susceptible to disease and vision problems while too much can create many problems, as well. The recommended dietary intakes for vitamin A depend on age, gender and reproductive status.

The Dietary Reference Intakes (DRI) for adult women is 700 micrograms (mcg) and for adult men it is 900 mcg per day, according to the University of Maryland Medical Center.

Doses over 25,000 international units (IU) per day should be avoided as they are likely to cause side effects, according to the NLM. One IU is the biological equivalent of 0.3 mcg retinol, or of 0.6 mcg beta-carotene, according to the National Institute of Health.

For comparison, a baked sweet potato (half a cup) has 19,218 IU of vitamin A, according to the U.S. Department of Agriculture. A medium cantaloupe has 18,668 IU and a carrot (half a cup, chopped) has 10,692 IU.

Overconsumption

Overdose of Vitamin A is absolutely a plausible scenario given its fat soluble nature, and it has been associated with a diverse set of symptoms ranging from skin and hair loss to neurologic problems, to gastrointestinal complains. In addition, liver injury has been described in situations of long term excess.

Overconsumption of *vitamin A can cause nausea, irritability and blurred vision in its mild form. In addition, the palms of the hands and the bottoms of the feet can turn orange if a person has a high intake of *Vitamin A. *Vitamin A toxicity can cause growth retardation, hair loss and enlarged spleen and liver in its more severe form. *Vitamin A overdose can also cause birth defects and has been linked to increased risk of bone fractures in some people.

Sources of vitamin A:

Beef liver
Egg yolk
Cheddar cheese
Fortified milk

Sources of beta-carotene:

Sweet potato
Carrots
Pumpkin
Cantaloupe
Broccoli
Apricots
Spinach and collard greens
Pink grapefruit
Winter squash
Mangoes
Papaya
Peaches