Vitamin A (retinoids) refers to three preformed compounds that exhibit metabolic activity: the alcohol (retinol), the aldehyde (retinal or retinaldehyde), and the acid (retinoic acid). Stored retinol is often esterified to a fatty acid, usually retinylpalmitate, which is usually found complexed with food proteins. The active forms of vitamin A exist only in animal products.
In addition to preformed vitamin A found in animal products, plants contain a group of compounds known as carotenoids, which can yield retinoids when metabolized in the body. Although several hundred carotenoids exist in foods naturally as antioxidants, only a few have significant vitamin A activity. The most important of these is βcarotene. The amount of vitamin A available from dietary carotenoids depends on how well they are absorbed and how efficiently they are converted to retinol.
Vitamin A has essential but separate roles in vision and various systemic functions, including;
- normal cell differentiation and cell surface function (e.g., cell recognition), growth and development, immune functions, and reproduction.
- Retinal is a structural component of the visual pigments of the rod and cone cells of the retina and is essential to photoreception.
- Vitamin A (retinol) is also essential for normal reproduction, bone development and function, and immune system function, although its actions in these roles are currently unclear.
- β Carotene can act as an antioxidant. Its other properties include retinoiddependent signaling, gap junction communications, regulation of cell growth, and induction of enzymes.
Dietary reference intakes (DRIs) have been determined for vitamin A and are expressed in micrograms per day(mcg/day). The AI for infants is based on the amount of retinol in human milk. The DRIs for adults are based on levels that provide adequate blood levels and liver stores and are adjusted for differences in average body size. Increased amounts of the vitamin during pregnancy and lactation allow for fetal storage and the vitamin A in breast milk.
Preformed vitamin A exists only in foods of animal origin, either in storage areas such as the liver or in the fat of milk and eggs. Very high concentrations of vitamin A are found in cod and halibut liver oils. A carotenoids are found in dark green, leafy and yellow-orange vegetables and fruit; deeper colors are associated with higher carotenoid levels. Carrots, greens, spinach, orange juice, sweet potatoes, and cantaloupe are rich sources of provitamin A.
An early sign of vitamin A deficiency is impaired vision from the loss of visual pigments. This manifests clinically as night blindness, or nyctalopia. Subsequent vitamin A deficiency leads to impaired
embryonic development or spermatogenesis, spontaneous abortion, anemia, impaired immunocompetence (reduced numbers and mitogenic responsiveness of T lymphocytes), and fewer osteoclasts in bone. Vitamin A deficiency also leads to the keratinization of the mucous membranes that line the respiratory tract, alimentary canal, urinary tract, skin, and epithelium of the eye.
In fact, vitamin A deficiency is the most significant cause of blindness in the developing world, and an estimated 250 million children are at risk. Between 250,000 and 500,000 cases of blindness from vitamin A deficiency occur annually. Vitamin A deficiency produces characteristic changes in skin texture involving follicular hyperkeratosis (phrynoderma).
Persistent large doses of vitamin A (>100 times the required amount) overcome the capacity of the liver to store the vitamin, produce intoxication, and eventually lead to liver disease. Dry lips (cheilitis) are a common initial sign, followed by dryness of the nasal mucosa and eyes.