Don't miss the piglix.com special BONUS offer during our Beta-test period. The next 100 new Registered Users (from a unique IP address), to post at least five (5) piglix, will receive 1,000 extra sign-up points (eventually exchangeable for crypto-currency)!

* * * * *    Free piglix.com Launch Promotions    * * * * *

  • Free Ads! if you are a small business with annual revenues of less than $1M - piglix.com will place your ads free of charge for up to one year! ... read more

  • $2,000 in free prizes! piglix.com is giving away ten (10) Meccano Erector sets, retail at $200 each, that build a motorized Ferris Wheel (or one of 22 other models) ... see details

Vitamin A


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


Substance and its chemical environment Micrograms of retinol equivalent
per microgram of the substance
retinol 1
beta-carotene, dissolved in oil 1/2
beta-carotene, common dietary 1/12
alpha-carotene, common dietary 1/24
gamma-carotene, common dietary 1/24
beta-cryptoxanthin, common dietary 1/24
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

  • 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).
  • Vision
  • Gene transcription
  • Immune function
  • Embryonic development and reproduction
  • Bone metabolism
  • Haematopoiesis
  • Skin and cellular health
  • Antioxidant activity
  • Teeth>
  • Mucous membrane
...
Wikipedia

1,000 EXTRA POINTS!

Don't forget! that as one of our early users, you are eligible to receive the 1,000 point bonus as soon as you have created five (5) acceptable piglix.

...