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Vitamin A Sources and Functions in the body
Vitamin A covers a pre-formed, retinol, and a pro-vitamin, beta-carotene, some of which is converted to retinol in the intestinal mucosa.
Sources of Vitamin A: Vitamin A is widely distributed in animal and plant foods –in animal foods is preformed vitamin A (retinol), and in plant foods are pro-vitamins (carotenes).
(a) Animal Foods: foods rich in retinol area liver, eggs, butter, cheese, whole milk, fish and meat. Fish liver oils are the richest natural sources of retinol (Table 4), but they are generally used as nutritional supplements rather than as food sources.
(b) Plant foods: The cheapest source of Vitamin A is green leafy vegetables such as spinach amaranth which are found in great abundance in nature throughout the year. The darker the green leaves, the higher its carotene content. Vitamin A also occurs in most green yellow fruits and vegetables (e.g. papaya, mango, pumpkin) and in some roots (e.g., carrots). The most important carotenoid is beta-carotene, which has the highest vitamin A activity. Carotenes are converted to vitamin A in the small intestine. This action is poorly accomplished in malnourished children and those suffering from diarrhoea.
(c) Fortified foods: Foods fortified with vitamin A (e.g. vanaspathi, margarine, and milk) can be an important source.
Functions of Vitamin A: Vitamin A participates in many bodily functions:
(a) It is indispensable for normal vision. It contributes to the production of retinal pigments, which are needed for vision in dim light.
(b) It is necessary for maintaining the integrity and the normal functioning of glandular and epithelial tissue which lines intestinal, respiratory and urinary tracts as well as the skin and eyes
(c) It supports growth, especially skeletal growth
(d) It is anti-infective; there is increased susceptibility to infection and lowered immune response in Vitamin A deficiency, and
(e) It may protect against some epithelial cancers such as bronchial cancers, but the data are not fully consistent. However, the role of Vitamin A at the molecular level is not yet known.
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