Vitamin A is an essential human nutrient. It exists not as a single compound, but in several forms.
- In foods of animal origin, the major form of vitamin A is an alcohol (retinol), but can also exist as an aldehyde (retinal), or as an acid (retinoic acid). Precursors to the vitamin (a provitamin) are present in foods of plant origin as some of the members of the carotenoid family of compounds.
- It is actually a family of fat-soluble vitamins. Retinol is one of the most active, or usable, forms of vitamin A, and is found in animal foods such as liver and eggs. It can be converted to retinal and retinoic acid, other active forms of the vitamin A family. Some plant foods contain orange pigments called provitamin A carotenoids that the liver can convert to retinol. Beta-carotene is a provitamin A carotenoid found in many foods. Lycopene, lutein, and zeaxanthin are also carotenoidsfound in food, but your body cannot convert them to vitamin A.
Digestion and absorption of Vitamin A
Retinyl estrs are hydrolysed by the pancreatic and intestinal enzymes to form free Retinol. After absorption the retinol is reesterified and transported to blood.
Carotenes are split in the intestines to form retinaldehyde which is then reduced to retinol. Some carotenes are absorbed intact and later cionverted to vitamin A in the liver or kidney.
Bile is necessary for the absor[ption of vitamin A and carotene.
Functions of vitamin A
Vitamin A plays an important role in
- bone growth,
- cell division and differentiation.
- It maintains the surface linings of your eye and your respiratory, urinary, and intestinal tracts. When those linings break down, bacteria can enter your body and cause infection. The immune system helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses.
- Vitamin A may help lymphocytes, a type of white blood cell that fights infections, function more effectively.
- Vitamin A also may help prevent bacteria and viruses from entering your body by maintaining the integrity of skin and mucous membranes.
THe common severe defeciency are increased suspectibility to microbial infestions, xeropthalmia, and other eye disorders, loss of apppetite and weight and sterlity.
In children severe defeciency can lead to irreversible blindness.
Further consequences of vviatamin A defeciency includes abnormalities of nerves, connective tissues and bones. In severe defeciency the affected epithelial and connective tissue may become the site for infections due to rdeuced resistsnce of cells to bacterial invasion.
Lack of vitamin a causes the epithelial tissues to dry and gradually harden to form scales that slough off. This process is called keratinization.
Eye disorders -First there is dryness of the conjunctiva (xerosis) as the normal lacrimal and mucus secreting epithelium is replaced by a keratinized epithelium. This is followed by the build-up of keratin debris in small opaque plaques (Bitot's spots) and, eventually, erosion of the roughened corneal surface with softening and destruction of the cornea (keratomalacia) and total blindness.
Acute defeciency of vitamin a may lead to blindness.In general due to lack of vitamin A epithelial tissue are more prone to develop i nfections and tissue breakdown.
Sources of vitamin A
Vitamin A is found naturally in many foods. Each of the following contains at least 0.15 mg (which is equal to 150 micrograms -500 IU). See Recommended Daily Intake.) of Vitamin A or beta carotene per 1.75-7 oz. (50-200 g):
(bracketed values are retinol equivalences and percentage of RDI per 100g.)