7/31/2019 What is Vitamin A

1/5

What is Vitamin A?

inShare

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 bothscotopic and color vision. Vitamin A also functions in a very different role, as an irreversibly

oxidized form retinoic acid, which is an importanthormone-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 thesmall intestine. The retinol form functions as astorage 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 inhumans.

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, functionas 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

and cannot convert any carotenoids to retinal (resulting in ''none'' of the carotenoids beingforms of vitamin A for these species).

The discovery ofvitamin Amay have stemmed from research dating back to 1906,indicating that factors other thancarbohydrates, proteins, and fats were necessary to keep

http://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/Metabolites-What-are-Metabolites.aspxhttp://www.news-medical.net/health/Metabolites-What-are-Metabolites.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspxhttp://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspxhttp://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-are-Carbohydrates.aspxhttp://www.news-medical.net/health/What-are-Carbohydrates.aspxhttp://www.news-medical.net/health/What-are-Carbohydrates.aspxhttp://www.news-medical.net/health/What-are-Carbohydrates.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/What-Does-the-Small-Intestine-Do.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/Metabolites-What-are-Metabolites.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspx

7/31/2019 What is Vitamin A

2/5

cattle healthy. By 1917 one of these substances was independently discovered by Elmer

McCollum at the University of WisconsinMadison, and Lafayette Mendel and ThomasBurr 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). Later, a unit called retinol equivalent (RE) was introduced. 1 RE corresponded to 1

g retinol, 2 g-carotenedissolved in oil (it is only partly dissolved in most supplementpills, due to very poor solubility in any medium), 6 g -carotene in normal food

(because it is not absorbed as well as when in oils), and 12 g of either -carotene, -

carotene, or -cryptoxanthin in food (these molecules only provide 50% of the retinol as

-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 asmuch as previously thought, so in 2001 the US Institute of Medicine recommended a new

unit, the retinol activity equivalent (RAE). 1 g RAE corresponds to 1 g retinol, 2 g of

-carotene in oil, 12 g of "dietary"beta-carotene, or 24 g of the three other dietaryprovitamin-A carotenoids.

Substance and its chemicalenvironment

Micrograms of retinol equivalent per microgram ofthe 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

Because the production of retinol from provitamins by the human body is regulated bythe 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 theamount oflipidsingested with the provitamin; lipids increase the uptake of the

provitamin.

The conclusion that can be drawn from the newer research is that fruits and vegetablesare 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 foodsare available.)

A sample vegan diet for one day that provides sufficientvitamin Ahas been published bythe Food and Nutrition Board (page 120:

Life Stage GroupRecommended Dietary Allowances (RDA)

Adequate Intakes (AI*)

Upper Limit

g/day

http://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/What-are-Lipids.aspxhttp://www.news-medical.net/health/What-are-Lipids.aspxhttp://www.news-medical.net/health/What-are-Lipids.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-are-Lipids.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspx

7/31/2019 What is Vitamin A

3/5

g/day

Infants06 months712 months

400*

500*

600

600

Children13 years

48 years300400

600900

Males913 years1418 years

19 - >70 years

600

900

900

1700

2800

3000

Females913 years

1418 years19 - >70 years

600700

700

17002800

3000

Pregnancy50 years750

770

2800

3000

Lactation50 years1200

1300

2800

3000

(Note that the limit refers to synthetic and natural retinoid forms of vitamin A. Caroteneforms from dietary sources are not toxic.)

According to the Institute of Medicine of the National Academies, "RDAs are set to meetthe needs of almost all (97 to 98 percent) individuals in a group. For healthybreastfed

infants, the AI is the mean intake. The AI for other life stage and gender groups isbelieved 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."

Vitamin A Toxicity

inShare

Sincevitamin Ais fat-soluble, disposing of any excesses taken in through diet is much harder

than with water-soluble vitamins B and C, thus vitamin A toxicity may result. This can lead to

http://www.news-medical.net/health/Breastfeeding.aspxhttp://www.news-medical.net/health/Breastfeeding.aspxhttp://www.news-medical.net/health/Breastfeeding.aspxhttp://www.news-medical.net/health/Vitamin-A-Toxicity.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/Vitamin-A-Toxicity.aspxhttp://www.news-medical.net/health/Breastfeeding.aspx

7/31/2019 What is Vitamin A

4/5

nausea,jaundice, irritability, anorexia (not to be confused with anorexia nervosa, theeating

disorder),vomiting, blurry vision, headaches, hairloss, muscle andabdominal painandweakness, drowsiness and altered mental status.

Acute toxicity generally occurs at doses of 25,000 IU/kg of body weight, with chronic toxicity

occurring at 4,000 IU/kg of body weight daily for 615 months. However, liver toxicities canoccur at levels as low as 15,000 IU per day to 1.4 million IU per day, with an average daily toxic

dose of 120,000 IU per day. In people withrenal failure4000 IU can cause substantial damage.Additionally, excessive alcohol intake can increase toxicity. Children can reach toxic levels at

1,500 IU/kg of body weight.

It has been estimated that 75% of people may be ingesting more than the RDA for vitamin A on

a regular basis in developed nations. Intake of twice the RDA of preformed vitamin A

chronically may be associated withosteoporosisand hip fractures. This may be due to the fact

that an excess of vitamin A can block the expression of certain proteins that are dependent onvitamin K. This could hypothetically reduce theefficacyofvitamin D, which has a proven role

in the prevention of osteoporosis and also depends on vitamin K for proper utilization.

Highvitamin Aintake has been associated with spontaneous bone fractures in animals. Cell

culture studies have linked increased bone resorption and decreased bone formation with high

vitamin A intakes. This interaction may occur because vitamins A and D may compete for thesame receptor and then interact with parathyroidhormonewhich regulatescalcium. Indeed, a

study by Forsmo ''et al.'' shows a correlation between low bone mineral density and too high

intake of vitamin A.

Toxic effects of vitamin A have been shown to significantly affect developing fetuses.

Therapeutic doses used foracnetreatment have been shown to disrupt cephalic neural cell

activity. The fetus is particularly sensitive to vitamin A toxicity during the period oforganogenesis.

These toxicities only occur with preformed (retinoid)vitamin A(such as from liver). The

carotenoid forms (such asbeta-caroteneas found in carrots), give no such symptoms, but

excessive dietary intake of beta-carotene can lead to carotenodermia, which causes orange-yellow discoloration of the skin.

Researchers have succeeded in creating water-soluble forms of vitamin A, which they believedcould reduce the potential for toxicity. However, a 2003 study found that water-soluble vitamin

A was approximately 10 times as toxic as fat-soluble vitamin.

A 2006 study found that children given water-soluble vitamin A and D, which are typically fat-

soluble, suffer fromasthmatwice as much as a control group supplemented with the fat-soluble

vitamins.

Chronically high doses of Vitamin A can produce the syndrome of "pseudotumor cerebri". This

syndrome includesheadache, blurring of vision and confusion. It is associated with increasedintracerebral pressure.

http://www.news-medical.net/health/What-is-Nausea.aspxhttp://www.news-medical.net/health/What-is-Nausea.aspxhttp://www.news-medical.net/health/Jaundice-What-is-Jaundice.aspxhttp://www.news-medical.net/health/Jaundice-What-is-Jaundice.aspxhttp://www.news-medical.net/health/Jaundice-What-is-Jaundice.aspxhttp://www.news-medical.net/health/What-is-an-Eating-Disorder.aspxhttp://www.news-medical.net/health/What-is-an-Eating-Disorder.aspxhttp://www.news-medical.net/health/What-is-an-Eating-Disorder.aspxhttp://www.news-medical.net/health/What-is-an-Eating-Disorder.aspxhttp://www.news-medical.net/health/What-is-Vomiting.aspxhttp://www.news-medical.net/health/What-is-Vomiting.aspxhttp://www.news-medical.net/health/What-is-Vomiting.aspxhttp://www.news-medical.net/health/What-is-Abdominal-Pain.aspxhttp://www.news-medical.net/health/What-is-Abdominal-Pain.aspxhttp://www.news-medical.net/health/What-is-Abdominal-Pain.aspxhttp://www.news-medical.net/health/Renal-Failure-What-is-Renal-Failure.aspxhttp://www.news-medical.net/health/Renal-Failure-What-is-Renal-Failure.aspxhttp://www.news-medical.net/health/Renal-Failure-What-is-Renal-Failure.aspxhttp://www.news-medical.net/health/What-is-Osteoporosis.aspxhttp://www.news-medical.net/health/What-is-Osteoporosis.aspxhttp://www.news-medical.net/health/What-is-Osteoporosis.aspxhttp://www.news-medical.net/health/What-is-Vitamin-K.aspxhttp://www.news-medical.net/health/What-is-Vitamin-K.aspxhttp://www.news-medical.net/health/Efficacy-What-Does-Efficacy-Mean.aspxhttp://www.news-medical.net/health/Efficacy-What-Does-Efficacy-Mean.aspxhttp://www.news-medical.net/health/Efficacy-What-Does-Efficacy-Mean.aspxhttp://www.news-medical.net/health/What-is-Vitamin-D.aspxhttp://www.news-medical.net/health/What-is-Vitamin-D.aspxhttp://www.news-medical.net/health/What-is-Vitamin-D.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/Calcium-What-is-Calcium.aspxhttp://www.news-medical.net/health/Calcium-What-is-Calcium.aspxhttp://www.news-medical.net/health/Calcium-What-is-Calcium.aspxhttp://www.news-medical.net/health/Acne.aspxhttp://www.news-medical.net/health/Acne.aspxhttp://www.news-medical.net/health/Acne.aspxhttp://www.news-medical.net/health/Embryogenesis-Organogenesis.aspxhttp://www.news-medical.net/health/Embryogenesis-Organogenesis.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/Asthma.aspxhttp://www.news-medical.net/health/Asthma.aspxhttp://www.news-medical.net/health/Asthma.aspxhttp://www.news-medical.net/health/What-is-a-Headache.aspxhttp://www.news-medical.net/health/What-is-a-Headache.aspxhttp://www.news-medical.net/health/What-is-a-Headache.aspxhttp://www.news-medical.net/health/What-is-a-Headache.aspxhttp://www.news-medical.net/health/Asthma.aspxhttp://www.news-medical.net/health/Beta-Carotene-What-is-Beta-Carotene.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/Embryogenesis-Organogenesis.aspxhttp://www.news-medical.net/health/Acne.aspxhttp://www.news-medical.net/health/Calcium-What-is-Calcium.aspxhttp://www.news-medical.net/health/What-are-Hormones.aspxhttp://www.news-medical.net/health/What-is-Vitamin-A.aspxhttp://www.news-medical.net/health/What-is-Vitamin-D.aspxhttp://www.news-medical.net/health/Efficacy-What-Does-Efficacy-Mean.aspxhttp://www.news-medical.net/health/What-is-Vitamin-K.aspxhttp://www.news-medical.net/health/What-is-Osteoporosis.aspxhttp://www.news-medical.net/health/Renal-Failure-What-is-Renal-Failure.aspxhttp://www.news-medical.net/health/What-is-Abdominal-Pain.aspxhttp://www.news-medical.net/health/What-is-Vomiting.aspxhttp://www.news-medical.net/health/What-is-an-Eating-Disorder.aspxhttp://www.news-medical.net/health/What-is-an-Eating-Disorder.aspxhttp://www.news-medical.net/health/Jaundice-What-is-Jaundice.aspxhttp://www.news-medical.net/health/What-is-Nausea.aspx

7/31/2019 What is Vitamin A

5/5