Vitamin A: Sources, Physiologic Functions and Deficiency

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In this article, we discuss the sources, functions, and deficiency manifestations of vitamin A

Vitamin A

Vitamin A is a group of fat-soluble compounds that can be differentiated into two categories, depending on whether the food source is an animal or a plant:

  • Vitamin A found in foods that come from animals is called preformed vitamin A or retinol;
  • Vitamin A found in fruits and vegetables is called pro-vitamin A carotenoid, which can be cleaved into retinol in the body; the carotenoid beta-carotene is most efficiently converted into retinol, making it animportant vitamin A source.

Nomenclature

Provitamin A: β-Carotene

Vitamin A1: Retinol ( Vitamin A alcohol)

Vitamin A2: 3 – Dehydro-retinol

Vitamin A aldehyde : Retinal

Vitamin A acid : Retinoic acid

Vitamin A ester : Retinyl ester

Neo vitamin A : Stereoisomer of Vitamin A1, has 70 – 80% of biological activity of Vitamin A1

Sources

The richest food source of preformed vitamin A is liver, with considerable amounts also found in egg yolk, whole milk, butter and cheese. Pro-vitamin A carotenoids (e.g., beta-carotene) are found in carrots, yellow and dark green leafy vegetables (e.g., spinach, cale, broccoli), various fruits, pumpkin, apricots, melon, and palm oil.

Physiologic and Heath Functions of Vitamin A

1. Vitamin A and Visual Cycle

Vitamin A plays a crucial role in the visual cycle also called Rhodopsin Cycle. Vitamin A is a component of visual pigments of rods & cones found in the eyes.

Rhodopsin is a visual pigment of rod cells in retina. Rhodopsin, when exposed to light undergoes a series of photo-chemical changes (isomerizations)

These changes result in bleaching of visual pigment and release of all- trans-retinal & opsin. This process triggers nerve impulse transmitted by optic nerve to the visual centre’ of brain.

Regeneration of rhodopsin requires isomerisation of all- trans-retinal to 11- cis-retinal. It combines spontaneously with Opsin in the dark to form rhodopsin.

When light strikes on retina – a number of biochemical changes occur leading to membrane hyper-polarization and the generation of nerve impulse.

Hyper-polarisation is brought about by a visual cascade involving cGMP. When a photon is absorbed by rhodopsin, metarhodopsin II is formed leading to the activation of protein transducin, involving GTP.

Activated transducin further activates enzyme ‘cGMP phosphodiesterase’ which degrades 3’5’-GMP to 5’-GMP.

Rapid decrease in cGMP closes Na+ channels in the membranes of rods which results in hyper-polarisation; an excitatory response transmitted as nerve impulse to brain.

2. Visual Cycle in Color Vision

Cones are specialised in bright & colour vision. Cones are responsible for color vision.

Colour vision is governed by 3 colour sensitive pigments:

– Cyanopsin (Blue)

– Iodopsin (Green)

– Porphyropsin (Red)

All these are retinal-opsin complexes.

When bright light strikes the retina, one or more of these pigments are bleached, depending on the colour of light pigment (s) dissociating into all-trans-retinal & opsin.

Nerve impulse generated by visual cascade causes perception of a specific colour in brain.

3. Vitamin and Maintenance of Epithelial Cells

Vitamin A plays a crucial role in normal growth & differentiation.

This vitamin involves in the following processes:

– Metabolism of intracellular structures

– Glycoprotein synthesis

– Metabolism of mucopolysaccharides

– Prevention of keratin synthesis

4. Reproduction

Both retinol & retinal are essential for Reproduction. It plays an important role in steroid hormone formation (synthesis).

– Supports spermatogenesis in males

– Prevents foetal miscarriages in females

5. Vitamin A and Growth

Plays a role in growth & development process.

– Sulfation of heteropolysaccharides in the matrix of bone & cartilage

6. Synthetic Functions of Vitamin A

Vitamin A is involved in synthesis of various compounds:

– Transferrin, the iron transfer protein

– Cholesterol

– Glucocorticoids

7. Immune Functions

Vitamin A plays an important role in the maintenance of immune system

Summary of Functions of Vitamin A

Adequate amounts of vitamin A (retinol) intake is essential for

  • the process of vision (especially night vision)
  • growth and development it is involved in the genetic regulation of cell and tissue formation, programming, and communication needed for reproduction and for the proper development of the embryo in the womb
  • immune function it helps to protect against infections by ensuring the effectiveness of mechanical barriers (e.g., skin, mucosa), and increasing the production and efficacy of protective cells (e.g., lymphocytes)
  • male and female reproductive organs.

Mechanism of Action

Retinyl esters is hydrolysed to Retinol and is transported to the target tissues. It is attached to plasma retinol binding protein’ (PRBP), an α1- globulin which in turn is bound to pre-albumin.

Retinol – PRBP complex attaches itself to the specific receptors on the cell surface, releasing retinol, which enters into the cells.

In the target tissues retinol combines with ‘cellular retinol binding protein’ (CRBP) present in the cytosol retinol-CRBP attaches to the receptors present on the nucleus.

As it enters the nucleus, it is converted to retinoic acid.

Retinoic acid binds to the inactive nuclear receptor and converts it into active receptor complex.

This activated complex causes gene activation, synthesis of m-RNA and specific proteins which results in cell differentiation.

Mechanism of action: similar to steroid hormone action

Vitamin A Deficiency

Vitamin A deficiency usually results from inadequate intake of foods high in vitamin A or beta-carotene, a precursor of vitamin A.

The earliest symptom of vitamin A deficiency is night blindness.

Individual at risk for insufficient vitamin A supply are mainly pregnant and breast-feeding women, newborns, children with frequent infections, the elderly and people who avoid animal-derived foods, especially vegans.

Vitamin A Toxicity (Hypervitaminosis A)

Vitamin A toxicity results from excessive intake for prolonged periods. This can occur following consumption of > 7.5 mg of retinol/ day

Hypervitaminosis A is commonly seen in children supplemented by vitamin A; eskimos, who consume polar bear liver.

Signs & Symptoms include:

– Skin – dry, with coarse sparse hairs

– Liver – enlarged and can become cirrhotic

– Bones – painful because of enlargement

– Nervous system – headache, nausea and vomiting due to increased intracranial pressure

– Pregnancy – congenital malformations in the developing foetus

Treatment: Hypervitaminosis is treated by avoiding Vitamin A rich foods.

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