Vitamin A: Essential Nutrient for Vision and Epithelial Health

Vitamins are organic compounds required by the body in trace amounts to perform specific functions. Most vitamins cannot be synthesized in the human body and must be supplied through diet. Among these essential nutrients, vitamin A plays crucial roles in maintaining healthy vision, epithelial tissues, and supporting growth and development.

Understanding Vitamins

Vitamins are classified into two main groups based on their solubility:

1. Water-soluble vitamins: Include vitamin C and B-complex vitamins
2. Fat-soluble vitamins: Include vitamins A, D, E, and K (easily remembered as “ADEK”)

Unlike macronutrients (carbohydrates, proteins, and fats), vitamins don’t provide energy but help in the metabolism of these macronutrients. They regulate the synthesis of many body compounds including bone, skin, nerves, brain, and blood cells. Vitamins prevent nutritional deficiency diseases and support optimal health throughout life.

Fat-Soluble Vitamins

Fat-soluble vitamins, including vitamin A, share several characteristics:

• They are hydrophobic molecules
• They are absorbed and transported with dietary fat
• Deficiency can occur in cases of fat malabsorption
• They are not readily excreted in urine
• Excess amounts are stored in the liver and can cause toxicity

Forms of Vitamin A

Vitamin A exists in several forms:

Biologically Active Forms:

1. Retinol: An alcohol found in animal tissues as retinyl ester with long-chain fatty acids
2. Retinal: An aldehyde derived from oxidation of retinol (retinol and retinal can be interconverted)
3. Retinoic acid: An acid derived from oxidation of retinal (cannot be reduced back to retinal)

Provitamin A:

• Beta-carotene: The plant precursor of vitamin A
• Consists of two retinal molecules linked at their aldehydes
• In humans, beta-carotene has only about one-sixth the biological activity of retinol

Sources of Vitamin A

Vitamin A can be obtained from both animal and plant sources:

• Animal sources (preformed vitamin A): Liver, milk fat, and eggs
• Plant sources (provitamin A): Dark green vegetables and orange vegetables and fruits

Beta-carotene is stored in adipose tissues, and excessive consumption can cause a yellow coloration of the skin, which is harmless.

Absorption and Storage

Vitamin A follows a specific pathway in the body:

1. It is absorbed with fat in the intestine
2. Secreted into chylomicrons that reach the bloodstream
3. Taken up by the liver where it is stored
4. When needed, it is released and transported to tissues by plasma retinol-binding protein (RBP)

Functions of Vitamin A

Epithelial Health

Retinol and retinoic acid are required for the growth, differentiation, and maintenance of epithelial cells. They bind to intracellular receptors and regulate transcription through specific response elements. This function helps maintain healthy skin and internal organ linings.

Vision

Retinal is a critical component of the visual pigment rhodopsin found in the rod cells of the retina. Rod cells are responsible for vision in dim light (night vision). The visual cycle involves:

1. Light converting 11-cis-retinal to all-trans-retinal
2. This change triggers nerve impulses
3. The impulses are transmitted by the optic nerve to the brain

Antioxidant Properties

Beta-carotene acts as an antioxidant, protecting tissues from the toxic effects of certain oxidants.

Other Functions

Vitamin A is also involved in reproduction and growth.

Mechanism of Action

Inside the cell, retinol is oxidized to retinoic acid, which:

1. Binds to intra-nuclear receptors forming a complex
2. This complex binds to DNA
3. Activates transcription of specific genes (such as keratin genes)
4. Produces effects similar to hormones by influencing gene transcription

Recommended Dietary Allowance

The conversion factors for vitamin A are:

• 1 μg of retinol = 6 μg of beta-carotene = 5 IU
• The daily requirement is approximately 5,000 IU of retinol
• Daily intake of 15 times the RDA can cause toxicity

Vitamin A Deficiency

Insufficient vitamin A can lead to various health issues:

Eye Problems

• Night blindness
• Xerophthalmia (dryness and roughness of the cornea)

Skin and Mucous Membrane Issues

• Roughness of skin and mucous membranes
• Increased susceptibility to infections

Other Effects

• Growth retardation
• Hydrocephalus and increased intracranial pressure
• Degeneration of testes and abortion (observed in animals)

Therapeutic Uses

• Treatment of vitamin A deficiency
• All-trans retinoic acid for treating psoriasis and leukemia
• 13-cis retinoic acid for treatment of severe acne

Vitamin A Toxicity

Excessive intake of vitamin A can cause:

• Headache
• Nausea
• Bone pain
• Hair loss
• Dermatitis and skin itching

High doses of vitamin A (>15,000 IU/day) during the first two months of pregnancy can be teratogenic (causing birth defects). Unlike vitamin A, beta-carotene is not toxic even at high doses.

Vitamin A is an essential fat-soluble vitamin with critical roles in vision, epithelial health, and growth. Maintaining appropriate levels through a balanced diet rich in both animal and plant sources helps prevent deficiency while avoiding toxicity. For those with specific conditions, therapeutic forms of vitamin A derivatives can provide targeted treatment options.