Vitamin A is a fat soluble vitamin and a very important vitamin for human body. Vitamin A is essential for vision (due to the production of rodopsin) and gene transcription. It also helps in overall growth and development and the maintenance of the immune system.
Vitamin A is actually not just a single molecule. A group of fat soluble unsaturated hydrocarbons including retinol, retinal, retinoic acid, and several provitamins like alpha-carotene, beta-carotene and beta-cryptoxanthin are collectively called vitamin A. All forms of vitamin A contain a retinyl functional group, that are essential for vitamin activity.
According to most sources, vitamin A is divided into two types, one is retinoids and the other is carotenoids. The source of retinoids are from animal, non-vegetable foods, where as source of carotenoids are vegetables. Not only the source of these two forms are different but also they take part in different physiological functions.
Different forms of retinoids and carotenoids are given below:
The two principal forms of vitamin A are Retinol and a provitamin ( precursor of vitamin A) called beta-carotene.
The molecular formula of retinol is C20H30O and the structural formula is
The molecular formula of beta carotene is C40H56 and the structural formula is
Under certain conditions some of the carotenoids may convert into retinoids. In human body one molecule of beta carotene is converted into two molecules of retinal or vitamin A, in the presence of the enzyme beta,beta-carotene 15,15′-monooxygenase.
Functions of vitamin A : Vitamin A plays important roles in multiple physiological metabolism like maintaining good vision, gene transcription, embryonic development, immune function, haematopoiesis, skin and cellular health and antioxidant activity.
Vision : The most important role of vitamin A is helping the maintenance of vision. The retinal form of vitamin A act as a chromophore, the molecule that absorbs certain colors and reflects others. Retinal bound with a protein ‘opsin’ to form rhodopsin and iodopsin, the two essential pigments for vision. Rhodopsin is the pigment necessary for low light and night vision. Rhodopsin present in the rod cells of human eye. Iodopsin is the pigment necessary for colored vision and is present in the cone cells of human eye.
Dermatology : Vitamin A, and more specifically, retinoid forms of vitamin A differentiate the immature skin cells or keratinocytes into mature epidermal cells. It is also essential for the correct functioning of the epithelial cells. The result is better skin, including a more even dispersement of skin cells and distribution of melanin on the skin.
Embryology : Retinoic acid influences the process of cell differentiation thus helps in the overall growth and development of the embryos.
Immunology : Maintaining intact epithelial tissues is very important because it act as a physical barrier to infection. Vitamin A is essential to maintain intact epithelial tissues.
Gene Transcription : Retinoic acid regulates gene transcription by binding to the nuclear receptors known as retinoic acid receptors and activate the transcription process for cell replication.
Haematopoiesis : Vitamin A also helps in Hematopoiesis or the formation of blood cellular components.
Antioxidant : Beta-carotene act as an antioxidant.
Sources of Vitamin A: The animal sources of vitamin A are cheese, cream, eggs, meat, liver, kidney, cod liver oil etc. The plant sources of vitamin A are mainly bright yellow and orange colored (due to the presence of beta carotene) fruits and vegetables like carrots, pumpkins, mangoes, papaya etc. Amounts of Vitamin A per 100 gm of food source are:
- cod liver oil (30000 microgram)
- liver (turkey) (8058 microgram)
- liver (beef, pork, fish) (6500 microgram )
- liver (chicken) (3296 microgram)
- carrot (835 microgram )
- broccoli leaf (800 microgram )
- sweet potato (709 microgram)
- butter (684 microgram )
- kale (681 microgram )
- spinach (469 microgram)
- pumpkin (400 microgram)
- collard greens (333 microgram)
- Cheddar cheese (265 microgram)
- cantaloupe melon (169 microgram)
- egg (140 microgram)
- apricot (96 microgram)
- papaya (55 microgram)
- mango (38 microgram)
- pea (38 microgram)
- broccoli (31 microgram)
- milk (28 microgram)
Daily requirement of vitamin A:
Infants (average intake)
0 – 6 months: 400 mcg/day
7 – 12 months: 500 mcg/day
1 – 3 years: 300 mcg/day
4 – 8 years: 400 mcg/day
9 – 13 years: 600 mcg/day
Adolescents and Adults (RDA)
Males age 14 and older: 900 mcg/day
Females age 14 and older: 700 mcg/day
RDA = Recommended Dietary Allowance
Deficiency of vitamin A : The most common vitamin A deficiency symptoms are vision problems. Vitamin A deficiency is the most common cause of blindness in developing countries.
Night blindness or Nyctalopia : Retinal is an essential part of rhodopsin, the photoreceptor pigment responsible for low light or dim light vision. Deficiency in vitamin A inhibits the production of rhodopsin, since human body cannot produce rhodopsin in sufficient amount so lack of vitamin A leads to night blindness.
Xerophthalmia : Vitamin A deficiency also causes the loss of goblet cells. Goblet cells present in the conjunctiva (the protective outer membrane of eye) of the eye and are responsible for the secretion of mucus to keep the eye moist. The eye fails to produce tears due to the loss of goblet cells and the eyes became dry. Further more the dead epithelial cells accumulate on the conjunctiva that may get infected resulting blindness.
Keratomalacia : Vitamin A is necessary for the maintenance of the specialized epithelial cells like cornea and conjunctiva. Deficiency of vitamin A causes atrophic changes in these epithelial cells and normal epithelium is replaced by a keratinized stratified squamous epithelium. For this reason the cornea gradually became opaque leads to loss of vision or blindness.
Supplementation: Adequate ingestion of a combination of meats and vegetables may be sufficient for maintaining adequate levels of Vitamin A in the body. However, if one is not certain, supplementation with pills is recommended in the minimum doses listed above.