Properties and functions of vitamin A

Vitamin A is an essential micronutrient for humans, meaning that it cannot be biosynthesized in the body and thus must be obtained from dietary sources.

The vitamin exists in three major forms: retinal (the aldehyde isoform), retinol (the alcohol isoform), and retinoic acid (RA), which is the irreversibly oxidized form of retinol.

Retinol empirical formula: C20H30O with molecular weight of 286.45

Retinol is soluble in fats and oils and practically insoluble in water and glycerol. Vitamin A esters are readily soluble in fats, oils, ether, acetone and chloroform. They are soluble in alcohol but insoluble in water.

Dietary vitamin A is absorbed in the small intestine in the form of retinol and transported in blood attached to retinol-binding protein (RBP). Inside the cells, retinol is oxidized into its main biologically active derivatives, first retinaldehyde (retinal), which plays a role in vision, and then retinoic acid (RA), which regulates the expression of multiple target genes.

Among other functions of vitamin A:
* Vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin, the oxygen carrier in red blood cells.

*Together with the protein opsin, the 11-cis isomer of retinol forms the light -sensitive visual pigment rhodopsin located in the rod of the retina. Rod cells with rhodopsin can detect very small amounts of light, making them important for night vision.

*Animal studies shown that vitamin A is required for normal growth and development. Retinol and retinoic acid (RA) are essential for embryonic development. During fetal development, RA functions in limb development and formation of the heart, eyes, and ears.

*Another major function of vitamin A is its role in cell differentiation.

*Vitamin A often called the antireflective vitamin, is protection against infections. The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body's first line of defense against infection. Retinol and its metabolites are required to maintain the integrity and function of these cells.
Properties and functions of vitamin A

Two forms of vitamin A in food

Vitamin A exists in two forms: preformed vitamin A or retinol available only from animal sources, and provitamin A available from plant sources.

Active vitamin A is present in foods of animal origin. The richest sources are liver and fish oil but milk and milk products and other fortified foods such as cereal, to which active vitamin is added can also be good sources. Even better and eggs provide some vitamin A.

An active vitamin A or retinol is also stored in the intestinal walls of fish, in the body fat of eels, and in the eyes of certain species of shrimp.

Plants contain no active vitamin A, but many vegetables and fruits provide the vitamin A precursor, beta-carotene. Beta-carotene is the important because it has the highest vitamin A activity.

Because the body uses both the performed vitamin A and the beta-carotene in foods to make retinol, the amount o vitamin A that comes from foods is usually expressed in retinol activity equivalents (RAE).
Two forms of vitamin A in food

Vitamin A in cow’s milk

All-trans retinol is the main form of vitamin A present in milk, the cis isomer of retinol being sometimes observed in a very low amount in cow’s milk.

Cow’s milk contains about 0.31 mg/l of vitamin A (retinol). The amount of vitamin A in milk varies with the carotene content of the feed. Normally, vitamin A potency is highest when the cow is on succulent pastures in the spring and lowest when the cow is hay-fed during the winter season.

The vitamin A potency of milk can be increased to a level that approaches summer milk by feeding rations high in carotene content.

Concentrations of vitamin A and carotenoids in milk are also dependent on animal species. Indeed caprine milk is richer (30%) in retinol and conversely beta-carotene is 30% higher in cow’s milk than goat’s milk.

Guernsey milk contained more carotene than preformed vitamin A, Jersey approximately equal proportions whereas the Holstein milk the carotene constituted approximately 30 percent and the vitamin A, 70 percent of the total biological active vitamin A.
Vitamin A in cow’s milk

Vitamin A

Vitamin A
Vitamin A (retinol) functions in reproduction, growth, the maintenance of skin and mucous membranes and the visual process.

Vitamin A is normally transported in the blood linked to a specific protein, retinol binding protein (RBP).

Specific proteins on cell surfaces and within cells are also involved with intracellular transport of the vitamin.

Vitamin A is fat soluble and is primarily stored in the liver, where RBP is synthesized. In a well nourished person, vitamin A stores are generally sufficient to last many months on a vitamins A-deficient diet before signs of deficiency appear.

The initial symptoms of vitamin A deficiency are night blindness and keratinization of hair follicles.

Continued deficiency leads to damage to eye tissue and irreversible blindness.

The US recommended Daily Allowance (RDA) of vitamin A for adults is 5000 IU (1000 retinol equivalents).

Rich dietary sources of retinol (preformed vitamin A) include dairy products, eggs and organ meats.

Some carotenoids (found in deep-yellow and dark green vegetables) can be converted to vitamin A during digestion.

In the US diet, approximately half of the vitamin A activity is derived from B-carotene and other carotenoids.
Vitamin A