Vitamin "A" For Health

Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is absolutely necessary for both low-light (isotopic vision) and color vision. Vitamin A also functions in a very different role, as an irreversibly oxidized form of retinal known as 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 the 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. The associated acid (retinoic acid), a metabolite that can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina for the visual cycle.

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.[1] 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 carotene 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. In general, carnivores are poor converters of ionine-containing carotenoids, and pure carnivores such as cats and ferrets 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).

 

Recommended daily intake

Life stage group	RDA           Adequate intakes (AI*) μg/day	Upper limit                         μg/day
Infants 0–6 months 7–12 months	400*                      500*	600                              600
Children 1–3 years 4–8 years	300                      400	600                              900
Males 9–13 years 14–18 years 19 – >70 years	600                      900                      900	1700                              2800                              3000
Females 9–13 years 14–18 years 19 – >70 years	600                      700                      700	1700                              2800                              3000
Pregnancy <19 years 19 – >50 years	750                      770	2800                              3000
Lactation <19 years 19 – >50 years	1200                     1300	2800                              3000

(Note that the limit refers to synthetic and natural retinol ester 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%) 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 prevents being able to specify with confidence the percentage of individuals covered by this intake.To reduce the possible risk of bone fracture and osteoporosis in postmenopausal women, an upper limit intake of 1500 μg RE/d has been recommended.

Vitamin A is found naturally in many foods:

• liver (beef, pork, chicken, turkey, fish) (6500 μg 722%), including cod liver oil
• 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 100 grams of the foodstuff (average).
Conversion of carotene to retinol varies from person to person and bioavailability of carotene in food varies.