The Multifunctional Vitamin A
The nutrients present in our food are basically classified into two major groups -macronutrients and micronutrients. The body requires macronutrients in large quantities for healthy functioning and micronutrients are required in small quantities or in trace amounts. Vitamins and minerals are categorized under micronutrients and play a central role in various metabolic reactions. Vitamins have been further classified into fat and water soluble vitamins. Vitamin A is a fat soluble vitamin and is a common name given to a group of retinoids. Generally vitamin A is mostly associated with vision, but apart from acting as a light-absorbing molecule in the retina, it performs multifarious functions in our body.
Functions of Vitamin A
- Role of vitamin A in promoting normal vision –
Vitamin A is a vital component of the protein rhodopsin, present in the retina of the eye. It absorbs light and visionary signals are sent to brain making the objects around us visible. It also promotes the differentiation and functioning of other tissues of the eye like cornea and conjunctival membranes
- Role of vitamin A in immunity –
Vitamin A stimulates and activates the cells of the immune system (WBCs) which fight with various microorganisms.
- Vitamin A as an antioxidant –
Due to its antioxidant properties it nullifies the harmful effects of free radicals. The free radicals are produced as an outcome of normal metabolic reactions or due to exposure to radiations and they can damage healthy cells, leading to diseases like cancer.
- Vitamin A facilitates the growth and normal functioning of different organs like kidneys heart, lungs, etc.
- Vitamin A is involved in the remodeling of the bone, hence making it important for the general health of bones.
- Vitamin A maintains the health of the mucous membranes which covers the interior surfaces.
- Vitamin A keeps skin healthy and promotes tooth growth.
- Vitamin A promotes cell growth and differentiation.
After discovering various vital functions of vitamin A, let us find out the sources of this vitamin A.
Rich Sources of Vitamin A
Vitamin A is present in two forms in human diet which are, preformed vitamin A and provitamin A carotenoids. Preformed vitamin A like retinol, retinyl esters, retinal is present in animal foods. Provitamin A carotenoids like beta-carotene, beta-cryptoxanthin are plant pigments which get converted to vitamin A in our body. Some of the rich animal sources of vitamin A are liver and fish oils. Egg and milk contain vitamin A in considerable quantities. Plant sources of vitamin A include, green leafy vegetables (spinach, broccoli), yellow or orange fruits and vegetables (carrot, mango, cantaloupe, sweet potato, papaya etc) some dry fruits (dried apricots) and tomato juice. Bright orange colored sweet potatoes are the richest source of beta-carotene. Depending upon the availability and dietary habits, one can acquire vitamin A from any of these sources.
How is vitamin A absorbed by the body?
Retinal and retinoic acid are the active forms of vitamin A and both preformed vitamin A and provitamin A gets metabolically converted into these active forms. As it is a fat soluble vitamin, it gets dissolved in fats in the presence of bile acids. Vitamin A gets absorbed along with fats, through duodenal mucosal cells of small intestines. Liver stores the excess of vitamin A in the active form of retinyl esters.
Recommended Dietary Allowances for Vitamin A
Institute of Medicine recommends men to consume 900 micrograms of retinol (equivalent to 3,000 IU) and women to consume 700 micrograms of retinol (equivalent to 2,333 IU). Deficiency of vitamin A can lead to many eye problems. As the retinyl group performs unique function of absorption of light, its deficiency leads to night blindness. However if the deficiency is unrecognized and not treated it may lead to complete blindness and xerophthalmia (drying of the conjuctive). Vitamin A deficiency also affects the absorption of lipids and impairs the bile production. Pregnant and breast feeding women should be very careful regarding the dietary intake of vitamin A as it effects the infants. In cases of acute deficiency, this vitamin is also given in the form of supplements.
Excessive consumption of this vitamin is also harmful and the upper level of intake has been suggested to be 3,000 micrograms. An excessive intake hinders the positive effects of vitamin D and increases the chances of hip fracture. Some other symptoms generally noticed during excess intake are, irritation of skin, dizziness, joint pains, headache etc.
A diet rich in fruits and vegetables and animal products can keep us away from many deficiency related problem. However in case of any health issues one should seek the advice of doctor and consume vitamin A supplements in adequate quantities.
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