1-Overview and activities: Vitamin A is a liposoluble nutrient that occurs in the natural state in two types: preformed vitamin A and provitamin A or precursor of vitamin A. Retinol is the amount of vitamin A used, which is split up and is in its place in the blood. Beta-carotene is also called precursor or provitamin. This substance is found in fruit and vegetables. The carotene must be converted to vitamin A in the body before being absorbed. Vitamin A helps to keep skin soft, smooth and free from diseases. It encourages and promotes nutrition, growth, vital activity of all organic tissues and in particular the epithelial and the mucosal ones. In particular, it helps to protect the mucous membranes of the nose, sinuses, lungs, eyelids, mouth, throat and digestive tract, vagina and uterus, reducing exposure to infection. All the coatings of kidneys and urinary bladder are also protected. The Vitamin A regulates the differentiation of mucosal epithelia and its deficiency results in keratinization of the epithelium with a process called squamous metaplasia. Other important functions of vitamin A include the stability and development of the cell membrane and the correct functioning of the immune system. The production of RNA is strongly increased by vitamin A: this nucleic acid is important in regulating the cell cycle, so that the life, the health and the correct cell functions are maintained; the body must be able to synthesize new RNA or the cells will begin to degenerate.
2-Assimilation and storage: The factors that interfere with the absorption of vitamin A include intense physical activity, intake of mineral oil, excessive alcohol consumption, excessive consumption of iron and the use of cortisone and other drugs. Taking large amounts of vitamin E hinders the absorption of beta-carotene. Diabetics can not convert beta-carotene into vitamin A. About 90% of vitamin A contained in the body is stored in the liver, with small amounts deposited in fat tissue, lungs, kidneys and retina . An adequate intake of zinc is necessary to allow the use of the vitamin A stored in the liver.
3-Dosage and toxicity: The recommended dose of vitamin A can be taken through food, such as calf's liver and carrot juice. However, it can be toxic in excessive doses, because the body accumulates in the fat cells under the skin, which may turn yellow, the skin becomes dry, scaly and itchy , rashes appear. In addition, excessive daily intake of vitamin A can lead to abnormalities of the mucous membrane.
4-Effects of deficiency and symptoms: vitamin A deficiency can occur when the dietary intake is insufficient, when the body is unable to absorb or to store the vitamin or when an illness prevents the conversion of carotene into vitamin A and where there is a rapid body loss of the vitamin. A major symptom is keratomalacia, which is manifested by the softening of the weakening of the tissues. The symptoms at epithelial level arise because there is a degeneration of cells associated with viral and bacterial infections because the barrier function of mucipare cells is missing, which are different from squamous cells. The deficit may occur in general: during the neonatal period, due to ill absorption syndromes or liver diseases. The deficiency is also evident with wrinkled, dry, prematurely aging skin or skin stained with the harder and exfoliated areas.
5-Beneficial effects in diseases: Vitamin A and zinc are beneficial in the treatment of psoriasis. Vitamin A, controls cell differentiation and protects the epithelial tissues of the external and internal coatings, such as skin, and it is also used to treat acne. New forms of vitamin A derivatives have proved more effective with fewer side effects: the Tigason (etretinate) has been successful in the treatment of psoriasis and has healed 80% of patients who took it daily for three or four months.