Vegetable Vitamins Fact
Vitamin A is a vitamin which is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal. This molecule is absolutely necessary for both scotopic and color vision. Vitamin A also functions in a very different role, as an irreversibly oxidized form retinoic acid, 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 an alcohol (retinol) 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. The associated acid (retinoic acid), a metabolite which can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina or some essential parts of the reproductive system.
All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group . This structure is 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 vitamin A in herbivores and omnivore animals, which possess the enzyme required to convert these compounds to retinal. Carnivores in general are poor converters of ionine-containg carotenoids, and pure carnivores such as cats and ferets lack beta-carotene 15,15'-monooxygenase and cannot convert any carotenoids to retinal (resulting in none of the carotenoids being forms of vitamin A for these species).
History
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. Since "water-soluble factor B" (Vitamin B) had recently been discovered, the researchers chose the name "fat-soluble factor A" (vitamin A). Vitamin A was first synthesized in 1947 by two Dutch chemists, David Adriaan van Dorp and Jozef Ferdinand Arens.
Equivalencies of retinoids and carotenoids (IU)
As some carotenoids can be converted into vitamin A, attempts have been made to determine how much of them in the diet is equivalent to a particular amount of retinol, so that comparisons can be made of the benefit of different foods. Unfortunately the situation is confusing because the accepted equivalences have changed. For many years, a system of equivalencies was used in which an international unit (IU) was equal to 0.3 μg of retinol, 0.6 μg of β-carotene, or 1.2 μg of other provitamin-A carotenoids. Later, a unit called retinol equivalent (RE) was introduced. 1 RE corresponded to 1 μg retinol, 2 μg β-carotene dissolved in oil (it is only partly dissolved in most supplement pills, due to very poor solubility in any medium), 6 μg β-carotene in normal food (because it is not absorbed as well as when in oils), and 12 μg of either α-carotene, γ-carotene, or β-cryptoxanthin in food (these molecules only provide 50% of the retinol as β-carotene, due to only half the molecule being convertible to usable vitamin).
Newer research has shown that the absorption of provitamin-A carotenoids is only half as much as previously thought, so in 2001 the US Institute of Medicine recommended a new unit, the retinol activity equivalent (RAE). 1 μg RAE corresponds to 1 μg retinol, 2 μg of β-carotene in oil, 12 μg of "dietary" beta-carotene, or 24 μg of the three other dietary provitamin-A carotenoids.
Because the production of retinol from provitamins by the human body is regulated by the amount of retinol available to the body, the conversions apply strictly only for vitamin A deficient humans. The absorption of provitamins also depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin.
The conclusion that can be drawn from the newer research is that fruits and vegetables are not as useful for obtaining vitamin A as was thought; in other words, the IU's that these foods were reported to contain were worth much less than the same number of IU's of fat-dissolved oils and (to some extent) supplements. This is important for vegetarians. (Night blindness is prevalent in countries where little meat or vitamin A-fortified foods are available.)
A sample vegan diet for one day that provides sufficient vitamin A has been published by the Food and Nutrition Board (page 120). On the other hand, reference values for retinol or its equivalents, provided by the National Academy of Sciences, have decreased. The RDA (for men) of 1968 was 5000 IU (1500 μg retinol). In 1974, the RDA was set to 1000 RE (1000 μg retinol), whereas now the Dietary Reference Intake is 900 RAE (900 μg or 3000 IU retinol). This is equivalent to 1800 μg of β-carotene supplement (3000 IU) or 10800 μg of β-carotene in food (18000 IU).
Recommended daily intake
Vitamin A
Dietary Reference Intake:
(Note that the limit refers to synthetic and natural retinoid forms of vitamin A. Carotene forms from dietary sources are not toxic.)
According to the Institute of Medicine of the National Academies, "RDAs are set to meet the needs of almost all (97 to 98 percent) individuals in a group. For healthy breastfed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover the needs of all individuals in the group, but lack of data prevent being able to specify with confidence the percentage of individuals covered by this intake."
Sources
Vitamin A is found naturally in many foods:
- liver (beef, pork, chicken, turkey, fish) (6500 μg 722%)
- carrot (835 μg 93%)
- broccoli leaf (800 μg 89%) - According to USDA database broccoli florets have much less.
- sweet potato (709 μg 79%)
- butter (684 μg 76%)
- kale (681 μg 76%)
- spinach (469 μg 52%)
- pumpkin (400 μg 41%)
- collard greens (333 μg 37%)
- Cheddar cheese (265 μg 29%)
- cantaloupe melon (169 μg 19%)
- egg (140 μg 16%)
- apricot (96 μg 11%)
- papaya (55 μg 6%)
- mango (38 μg 4%)
- pea (38 μg 4%)
- broccoli (31 μg 3%)
- milk (28 μg 3%)
Note: data taken from USDA database bracketed values are retinol activity equivalences (RAEs) and percentage of the adult male RDA per 100g.
Conversion of carotene to retinol varies from person to person and bioavailability of carotene in food varies.
Metabolic functions
Vitamin A plays a role in a variety of functions throughout the body, such as:
- Vision
- Gene transcription
- Immune function
- Embryonic development and reproduction
- Bone metabolism
- Haematopoiesis
- Skin health
- Antioxidant Activity
Vision
The role of vitamin A in the vision cycle is specifically related to the retinal form. Within the eye, 11- cis -retinal is bound to rhodopsin (rods) and iodopsin (cones) at conserved lysine residues. As light enters the eye the 11- cis -retinal is isomerized to the all-"trans" form. The all-"trans" retinal dissociates from the opsin in a series of steps called bleaching. This isomerization induces a nervous signal along the optic nerve to the visual center of the brain. Upon completion of this cycle, the all-"trans"-retinal can be recycled and converted back to the 11-"cis"-retinal form via a series of enzymatic reactions. Additionally, some of the all-"trans" retinal may be converted to all-"trans" retinol form and then transported with an interphotoreceptor retinol-binding protein (IRBP) to the pigment epithelial cells. Further esterification into all-"trans" retinyl esters allow this final form to be stored within the pigment epithelial cells to be reused when needed. The final conversion of 11- cis -retinal will rebind to opsin to reform rhodopsin in the retina. Rhodopsin is needed to see black and white as well as see at night. It is for this reason that a deficiency in vitamin A will inhibit the reformation of rhodopsin and lead to night blindness.
Gene transcription
Vitamin A, in the retinoic acid form, plays an important role in gene transcription. Once retinol has been taken up by a cell, it can be oxidized to retinal (by retinol dehydrogenases) and then retinal
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