Facts Importance of Vitamin A Top Sources to get the Vitamin A Know Everything About Vitamin A

Vitamin A is stable to heat but if exposed to continuous high temperatures over 100 degrees centigrade , it is destroyed.

In 1912, Funk coined the term vitamine, from the Latin vita for "life" and amine, for the prominent chemical reactive group.

Polish-born American biochemist Casimir Funk. While studying beriberi, a disease that causes depression, fatigue, and nerve damage, Funk discovered an organic compound in rice husks that prevents the illness.

He named the compound vitamine, derived from the chemical name amine and the Latin word vita, "life," because vitamins are required for life and were originally thought to be amines.

Funk's compound is now known as vitamin B1, or thiamine.

His research and discovery led him, along with English biochemist Sir Frederick Gowland Hopkins, to propose the vitamin hypothesis of deficiency, which stated that certain diseases, such as scurvy or rickets, are caused by dietary deficiencies and can be avoided by taking vitamins.

Further research allowed scientists to isolate and identify the vitamins that we know today to be essential for human health.

Vitamins include A, C, D, E, K, thiamine, riboflavin, niacin, B6, B12, folic acid, biotin, and pantothenic acid. Vitamins are distinguished from minerals, such as calcium, iron, and magnesium, which are also essential for optimum health.

Vitamins are organic compounds, essential nutrients that are required by our body to carry out all the important functions in our body.

Some vitamins cannot be synthesized by our body, and therefore, our body obtains them from our diet.

Vitamins participate in many complex biochemical reactions inside our body.

Vitamins allow our body to carry out all the tasks necessary to maintain life.

Most vitamins cannot be synthesized by the body, but are found naturally in foods obtained from plants and animals.

Vitamins are either water-soluble or fat-soluble.

Most water-soluble vitamins, such as the vitamin B complex, act as catalysts and coenzymes in metabolic processes and energy transfer and are excreted fairly rapidly.

Fat-soluble vitamins, such as vitamins A, D, and E are necessary for the function or structural integrity of specific body tissues and membranes and are retained in the body.

Vitamin A –

The discovery of vitamin A may have stemmed from research dating back to 1906, indicating that factors other than carbohydrates, proteins, and fats were necessary to keep cattle healthy. By 1917, one of these substances was independently discovered by Elmer McCollum at the University of Wisconsin–Madison, and Lafayette Mendel and Thomas Burr Osborne at Yale University.

Vitamin A is the name of a group of fat-soluble retinoids, including retinol, retinal, retinoic acid, and retinyl esters

Vitamin A is involved in immune function, vision, reproduction, and cellular communication

Vitamin A is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea

Vitamin A also supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs

Two forms of vitamin A are available in the human diet:

1-preformed vitamin A (retinol and its esterified form, retinyl ester)

2- provitamin A carotenoids

Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver). By far the most important provitamin A carotenoid is beta-carotene; other provitamin

A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A. Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin's important biological functions

Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A.

The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells

Both retinyl esters and provitamin A carotenoids are converted to retinol, which is oxidized to retinal and then to retinoic acid

Most of the body's vitamin A is stored in the liver in the form of retinyl esters.

Recommended intake of Vitamin A –

RDAs for vitamin A are given as mcg of retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids

Because the body converts all dietary sources of vitamin A into retinol, 1 mcg of physiologically available retinol is equivalent to the following amounts from dietary sources:

1 mcg of retinol, 12 mcg of beta-carotene, and 24 mcg of alpha-carotene or beta-cryptoxanthin. From dietary supplements, the body converts 2 mcg of beta-carotene to 1 mcg of retinol.

Currently, vitamin A is listed on food and supplement labels in international units (IUs) even though nutrition scientists rarely use this measure. Conversion rates between mcg RAE and IU are as follows

a) 1 IU retinol = 0.3 mcg RAE

b) 1 IU beta-carotene from dietary supplements = 0.15 mcg RAE

c) 1 IU beta-carotene from food = 0.05 mcg RAE

d) 1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE

An RAE cannot be directly converted into an IU without knowing the source(s) of vitamin A. For example, the RDA of 900 mcg RAE for adolescent and adult men is equivalent to 3,000 IU if the food or supplement source is preformed vitamin A (retinol).

However, this RDA is also equivalent to 6,000 IU of beta-carotene from supplements, 18,000 IU of beta-carotene from food, or 36,000 IU of alpha-carotene or beta-cryptoxanthin from food.

So a mixed diet containing 900 mcg RAE provides between 3,000 and 36,000 IU of vitamin A, depending on the foods consumed.

Table 1: Recommended Dietary Allowances (RDAs) for Vitamin A
Age	Male	Female	Pregnancy	Lactation
0–6 months*	400 mcg RAE	400 mcg RAE
7–12 months*	500 mcg RAE	500 mcg RAE
1–3 years	300 mcg RAE	300 mcg RAE
4–8 years	400 mcg RAE	400 mcg RAE
9–13 years	600 mcg RAE	600 mcg RAE
14–18 years	900 mcg RAE	700 mcg RAE	750 mcg RAE	1,200 mcg RAE
19–50 years	900 mcg RAE	700 mcg RAE	770 mcg RAE	1,300 mcg RAE
51+ years	900 mcg RAE	700 mcg RAE

* Adequate Intake (AI), equivalent to the mean intake of vitamin A in healthy, breastfed infants.

Sources of Vitamin A

Food
Concentrations of preformed vitamin A are highest in liver and fish oils

Other sources of preformed vitamin A are milk and eggs, which also include some provitamin A Most dietary provitamin A comes from leafy green vegetables, orange and yellow vegetables, tomato products, fruits, and some vegetable oils

The top food sources of vitamin A in the U.S. diet include dairy products, liver, fish, and fortified cereals; the top sources of provitamin A include carrots, broccoli, cantaloupe, and squash

Table 2 suggests many dietary sources of vitamin A.

The foods from animal sources in Table 2 contain primarily preformed vitamin A, the plant-based foods have provitamin A, and the foods with a mixture of ingredients from animals and plants contain both preformed vitamin A and provitamin A.

Table 2: Selected Food Sources of Vitamin A
Food	mcg RAE per serving	IU per serving	Percent DV*
Sweet potato, baked in skin, 1 whole	1,403	28,058	561
Beef liver, pan fried, 3 ounces	6,582	22,175	444
Spinach, frozen, boiled, ½ cup	573	11,458	229
Carrots, raw, ½ cup	459	9,189	184
Pumpkin pie, commercially prepared, 1 piece	488	3,743	249
Cantaloupe, raw, ½ cup	135	2,706	54
Peppers, sweet, red, raw, ½ cup	117	2,332	47
Mangos, raw, 1 whole	112	2,240	45
Black-eyed peas (cowpeas), boiled, 1 cup	66	1,305	26
Apricots, dried, sulfured, 10 halves	63	1,261	25
Broccoli, boiled, ½ cup	60	1,208	24
Ice cream, French vanilla, soft serve, 1 cup	278	1,014	20
Cheese, ricotta, part skim, 1 cup	263	945	19
Tomato juice, canned, ¾ cup	42	821	16
Herring, Atlantic, pickled, 3 ounces	219	731	15
Ready-to-eat cereal, fortified with 10% of the DV for vitamin A, ¾–1 cup (more heavily fortified cereals might provide more of the DV)	127–149	500	10
Milk, fat-free or skim, with added vitamin A and vitamin D, 1 cup	149	500	10
Baked beans, canned, plain or vegetarian, 1 cup	13	274	5
Egg, hard boiled, 1 large	75	260	5
Summer squash, all varieties, boiled, ½ cup	10	191	4
Salmon, sockeye, cooked, 3 ounces	59	176	4
Yogurt, plain, low fat, 1 cup	32	116	2
Pistachio nuts, dry roasted, 1 ounce	4	73	1
Tuna, light, canned in oil, drained solids, 3 ounces	20	65	1
Chicken, breast meat and skin, roasted, ½ breast	5	18	0

*DV = Daily Value.

DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet.

The DV for vitamin A is 5,000 IU for adults and children age 4 and older. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.

Dietary supplements

Vitamin A is available in multivitamins and as a stand-alone supplement, often in the form of retinyl acetate or retinyl palmitate

A portion of the vitamin A in some supplements is in the form of beta-carotene and the remainder is preformed vitamin A; others contain only preformed vitamin A or only beta-carotene. Supplement labels usually indicate the percentage of each form of the vitamin. The amounts of vitamin A in stand-alone supplements range widely

Multivitamin supplements typically contain 2,500–10,000 IU vitamin A, often in the form of both retinol and beta-carotene.

Vitamin A Deficiency

vitamin A deficiency is rare in the United States.

However, vitamin A deficiency is common in many developing countries, often because residents have limited access to foods containing preformed vitamin A from animal-based food sources and they do not commonly consume available foods containing beta-carotene due to poverty

According to the World Health Organization, 190 million preschool-aged children and 19.1 million pregnant women around the world have a serum retinol concentration below 0.70 micromoles/L

In these countries, low vitamin A intake is most strongly associated with health consequences during periods of high nutritional demand, such as during infancy, childhood, pregnancy, and lactation.

In developing countries, vitamin A deficiency typically begins during infancy, when infants do not receive adequate supplies of colostrum or breast milk

Chronic diarrhea also leads to excessive loss of vitamin A in young children, and vitamin A deficiency increases the risk of diarrhea

The most common symptom of vitamin A deficiency in young children and pregnant women is xerophthalmia.

One of the early signs of xerophthalmia is night blindness, or the inability to see in low light or darkness

Vitamin A deficiency is one of the top causes of preventable blindness in children

People with vitamin A deficiency (and, often, xerophthalmia with its characteristic Bitot's spots) tend to have low iron status, which can lead to anemia

Vitamin A deficiency also increases the severity and mortality risk of infections (particularly diarrhea and measles) even before the onset of xerophthalmia

Pregnant women need extra vitamin A for fetal growth and tissue maintenance and for supporting their own metabolism

The World Health Organization estimates that 9.8 million pregnant women around the world have xerophthalmia as a result of vitamin A deficiency

Other effects of vitamin A deficiency in pregnant and lactating women include increased maternal and infant morbidity and mortality, increased anemia risk, and slower infant growth and development.

Vitamin A and Health

This section focuses on three diseases and disorders in which vitamin A might play a role: cancer, age-related macular degeneration (AMD), and measles.

Cancer -Cancer Protection

(Food Sources Only) - Studies suggest beta-carotene and vitamin A lower risk of many types of cancer.10

This effect could mainly be from a diet high in vegetables and not from supplements.

Vitamin A supplements have been shown to increase risk of cancer.

Because of the role vitamin A plays in regulating cell growth and differentiation, several studies have examined the association between vitamin A and various types of cancer. However, the relationship between serum vitamin A levels or vitamin A supplementation and cancer risk is unclear.

Several prospective and retrospective observational studies in current and former smokers, as well as in people who have never smoked, found that higher intakes of carotenoids, fruits and vegetables, or both are associated with a lower risk of lung cancer

However, clinical trials have not shown that supplemental beta-carotene and/or vitamin A helps prevent lung cancer.

Health Risks from Excessive Vitamin A

Because vitamin A is fat soluble, the body stores excess amounts, primarily in the liver, and these levels can accumulate. Although excess preformed vitamin A can have significant toxicity (known as hypervitaminosis A), large amounts of beta-carotene and other provitamin A carotenoids are not associated with major adverse effects

The manifestations of hypervitaminosis A depend on the size and rapidity of the excess intake. The symptoms of hypervitaminosis A following sudden, massive intakes of vitamin A, as with Arctic explorers who ate polar bear liver, are acute

Chronic intakes of excess vitamin A lead to increased intracranial pressure (pseudotumor cerebri), dizziness, nausea, headaches, skin irritation, pain in joints and bones, coma, and even death

Although hypervitaminosis A can be due to excessive dietary intakes, the condition is usually a result of consuming too much preformed vitamin A from supplements or therapeutic retinoids

When people consume too much vitamin A, their tissue levels take a long time to fall after they discontinue their intake, and the resulting liver damage is not always reversible.

Observational studies have suggested an association between high intakes of preformed vitamin A (more than 1,500 mcg daily—only slightly higher than the RDA), reduced bone mineral density, and increased fracture risk

However, the results of studies on this risk have been mixed, so the safe retinol intake level for this association is unknown.

Total intakes of preformed vitamin A that exceed the UL and some synthetic retinoids used as topical therapies (such as isotretinoin and etretinate) can cause congenital birth defects

These birth defects can include malformations of the eye, skull, lungs, and heart

Women who might be pregnant should not take high doses of vitamin A supplements [2].

Unlike preformed vitamin A, beta-carotene is not known to be teratogenic or lead to reproductive toxicity

And even large supplemental doses (20–30 mg/day) of beta-carotene or diets with high levels of carotenoid-rich food for long periods are not associated with toxicity.

The most significant effect of long-term, excess beta-carotene is carotenodermia, a harmless condition in which the skin becomes yellow-orange

This condition can be reversed by discontinuing beta-carotene ingestion.

Supplementation with beta-carotene, with or without retinyl palmitate, for 5–8 years has been associated with an increased risk of lung cancer and cardiovascular disease in current and former male and female smokers and in male current and former smokers occupationally exposed to asbestos [24,38]. In the ATBC study, beta-carotene supplements (20 mg daily) were also associated with increased mortality, mainly due to lung cancer and ischemic heart disease [24]. The CARET study ended early, after the investigators found that daily beta-carotene (30 mg) and retinyl palmitate (25,000 IU) supplements increased the risk of lung cancer and cardiovascular disease mortality [38].

The FNB has established ULs for preformed vitamin A that apply to both food and supplement intakes [5]. The FNB based these ULs on the amounts associated with an increased risk of liver abnormalities in men and women, teratogenic effects, and a range of toxic effects in infants and children. The FNB also considered levels of preformed vitamin A associated with decreased bone mineral density, but did not use these data as the basis for its ULs because the evidence was conflicting. The FNB has not established ULs for beta-carotene and other provitamin A carotenoids [22]. The FNB advises against beta-carotene supplements for the general population, except as a provitamin A source to prevent vitamin A deficiency.
Table 3: Tolerable Upper Intake Levels (ULs) for Preformed Vitamin A [5]* Age Male    Female             Pregnancy             Lactation
0–12 months   600 mcg RAE
(2,000 IU)       600 mcg RAE
(2,000 IU)
1–3 years         600 mcg RAE
(2,000 IU)       600 mcg RAE
(2,000 IU)
4–8 years         900 mcg RAE
(3,000 IU)       900 mcg RAE
(3,000 IU)
9–13 years       1,700 mcg RAE
(5,667 IU)       1,700 mcg RAE
(5,667 IU)
14–18 years     2,800 mcg RAE
(9,333 IU)       2,800 mcg RAE
(9,333 IU)       2,800 mcg RAE
(9,333 IU)       2,800 mcg RAE
(9,333 IU)
19+ years         3,000 mcg RAE
(10,000 IU)     3,000 mcg RAE
(10,000 IU)     3,000 mcg RAE
(10,000 IU)     3,000 mcg RAE
(10,000 IU)

* These ULs, expressed in mcg and in IUs (where 1 mcg = 3.33 IU), only apply to products from animal sources and supplements whose vitamin A comes entirely from retinol or ester forms, such as retinyl palmitate. However, many dietary supplements (such as multivitamins) do not provide all of their vitamin A as retinol or its ester forms. For example, the vitamin A in some supplements consists partly or entirely of beta-carotene or other provitamin A carotenoids. In such cases, the percentage of retinol or retinyl ester in the supplement should be used to determine whether an individual's vitamin A intake exceeds the UL. For example, a supplement labeled as containing 10,000 IU of vitamin A with 60% from beta-carotene (and therefore 40% from retinol or retinyl ester) provides 4,000 IU of preformed vitamin A. That amount is above the UL for children from birth to 13 years but below the UL for adolescents and adults.
Interactions with Medications

Vitamin A can interact with certain medications, and some medications can have an adverse effect on vitamin A levels. A few examples are provided below. Individuals taking these and other medications on a regular basis should discuss their vitamin A status with their health care providers.

Orlistat

Orlistat (Alli®, Xenical®), a weight-loss treatment, can decrease the absorption of vitamin A, other fat-soluble vitamins, and beta-carotene, causing low plasma levels in some patients [39]. The manufacturers of Alli and Xenical recommend encouraging patients on orlistat to take a multivitamin supplement containing vitamin A and beta-carotene, as well as other fat-soluble vitamins [40,41].

Retinoids

Several synthetic retinoids derived from vitamin A are used orally as prescription medicines. Examples include the psoriasis treatment acitretin (Soriatane®) and bexarotene (Targretin®), used to treat the skin effects of T-cell lymphoma. Retinoids can increase the risk of hypervitaminosis A when taken in combination with vitamin A supplements [39].
Vitamin A and Healthful Diets

The federal government's 2010 Dietary Guidelines for Americans notes that "nutrients should come primarily from foods. Foods in nutrient-dense, mostly intact forms contain not only the essential vitamins and minerals that are often contained in nutrient supplements, but also dietary fiber and other naturally occurring substances that may have positive health effects. ...Dietary supplements...may be advantageous in specific situations to increase intake of a specific vitamin or mineral."

The Dietary Guidelines for Americans describes a healthy diet as one that:

Emphasizes a variety of fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products.

Many fruits, vegetables, and dairy products are good sources of vitamin A. Some ready-to-eat breakfast cereals are fortified with vitamin A.

Includes lean meats, poultry, seafood, beans and peas, eggs, and nuts and seeds.

Beef liver contains high amounts of vitamin A. Other sources of the nutrient include some fish, beans, and nuts.

Limits solid fats (saturated fats and trans fats), cholesterol, salt (sodium), added sugars, and refined grains.

Stays within your calorie needs.

Source - http://ods.od.nih.gov

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Thursday, April 11, 2013

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