The active form of vitamin A is found in retinol, retinal and retinoic acid.
Carotenoids are compounds known as pro-vitamin A which an iron-dependant enzyme known as carotene dioxygenase converts to vitamin A.
Conversion of beta-carotene to vitamin A is accelerated by thyroid hormones and vitamin A requires zinc in order to be utilised by the body.
Adequate levels of vitamin A are important to prevent poor night vision and also play an important role in day-light vision.
Vitamin A is fundamental to maintain healthy mucous membranes, such as those found in the nose, lungs and gut, which helps prevent infections by pathogens.
Vitamin A promotes the production of collagen, a protein found in connective tissue such as skin, tendons, bones, cartilages, the gut, etc.
Carotenoids, the vitamin A precursors, have been found to have antioxidant activity and are therefore thought to help scavenge free-radicals and reduce the risk of certain cancers.
Vitamin A deficiency may impair fertility as it is needed for sperm production and the development of the placenta.
Vitamin A in large doses has been linked to teratogenesis, i.e. birth defects so pregnant women should avoid food supplements not specifically formulated for pregnancy or recommended by a health practitioner and eating liver as it is very high in vitamin A. Foods containing beta-carotene are safe in pregnancy.
Research shows that smokers taking beta-carotene supplements may have an increased risk of developing lung cancer as the high levels of free-radicals may transform beta-carotene compounds into harmful substances.
Foods containing pre-formed vitamin A include liver and to a lesser extent oily fish, eggs and dairy.
Beta-carotenes are found in dark coloured fruit and vegetables: broccoli, spinach, watercress, kale, pumpkin, squash, peppers, carrots, sweet potatoes, oranges, apricots, mango.