Vitamin A (VA) is a group of compounds that are fat soluble in nature. They play an important role in vision, bone growth, reproduction, cell division and cell differentiation (in which a cell is assigned its role and it becomes part of brain or muscles or lungs).
There are two categories of VA, depending on its food source. VA found in animal foods is called preformed Vitamin A. It is absorbed in the form of retinol, one of the most usable forms of VA. Sources include liver, whole milk and its products, cod live oil, kidney, egg, fish, meat and some fortified food products. VA that is found in colorful fruits and vegetables (yellow, orange and dark greem leafy ones like carrot, papaya, tomato, capsicum, mango, apricot, spinach, fenugreek,etc.) is called provitamin A. Provitamin A can be converted to retinol in the body. Most common and most efficiently converted to retinol is beta-carotene. 90% of retinoids can be absorbed onlyl 3% of carotenoids can be absorbed by the body. Few other carotenoids like lycopene, lutein and zeaxanthin do not have VA activity but have other health promoting properties.
Vitamin A functions and health benefits
Vitamin A is one of the most versatile vitamins, with roles in such diverse functions as vision, immune defenses, maintenance of body linings and skin, bone and body growth, normal cell development and reproduction. Thus VA may collectively be important in protecting against conditions related to aging, air pollution, arthritis, cancer, cardiovascular disease, cataracts, diabetes mellitus and infection. Let us discuss a few at length.
Vision – The most important role of this vitamin for eyesight. Retinol is required to start the chemical process that signals the brain that light is striking the eye, which allows the eye to adjust from bright to dim light.
Immune system – VA boosts the immune system by stimulating white blood cell function and increasing the activity of antibodies (the germ fighters in our bodies).
As an Antioxidant: Beta-carotene, lutein and lycopene act as antioxidants which destroy free radicals. Free radicals are believed to be associated with many of the degenerative changes seen with aging, and may contribute to development of cardiovascular diseases and cancers.
Epithelial cell – VA maintains the health of epithelial cells that line internal and external surfaces of lungs, intestines, stomach, vagina, urinary tract, bladder, eyes and skin. These cells act as important barriers to bacteria. Many epithelial cells produce mucus which is necessary to lubricate body surfaces and protect against invading micro-organisms.
Pregnancy – VA is particularly essential for pregnant women because it helps with post delivery tissue repair, as well as maintaining normal vision and helping fight off infections. A lack during pregnancy can cause night blindness in the mother, problems with the placenta and low birth weight of newborns.
In our bodies, 90% of Vitamin A is stored in the liver.
Vitamin A can be lost from foods during preparation, cooking or storage. To retain VA: increase intake of raw fruits and vegetables, keep vegetables and fruits covered, cook vegetables or meat by pan roasting, baking or grilling instead of frying. Overcooking destroys this vitamin.
Absorbtion of Vitamin A is hindered if there is not enough dietary fat. Inadequate intake of protein or presence of tannins also hinders absorption.
Recommended Vitamin A intake: A daily intake of 400 µg (microgram) of retinol for children 7 and 600 µg thereafter. (Nutritive value of Indian foods, 2004)
Deficiency of Vitamin A: leads to delayed dark adaptation (increase in time taken by eyes to adjust when you come from a brightly lit to dimly lighted area), night blindness or poor visionin dark and in extreme cases it leads to complete blindness with complete eye tissue getting converted into a scar. Apart from affecting eyes it also leads to drying of skin and increased susceptibility to respiratory and urinary tract infections
At risk population for Vitamin A deficiency:
- Children: toddlers and preschool age children, children living below the poverty line, children with inadequate health care or immunizations, children with diseases of the pancreas, liver, or intestines or with inadequate fat digestion or absorption.
- Those suffering with chronic diarrhoea. A deficiency can occur when VA is lost through chronic diarrhea.
- Disorders: celiac disease, liver disorders, gall bladder disorders
- Excess alcohol intake depletes VA stores. It is thus very important for people who consume excessive amounts of alcohol to include good sources of VA in their diets.
- Vegetarians who do not consume eggs and dairy foods. They should include a minimum of five servings of fruits and vegetables in their daily diet and regularly choose dark green leafy vegetables and orange and yellow fruits to consume recommended amounts of VA.
Toxicity of Vitamin A or Hypervitaminosis A: refers to high storage levels of VA in the body that can lead to toxic symptoms. There are four major adverse effects of hypervitaminosis A: birth defects, liver abnormalities, reduced bone mineral density that may result in osteoporosis and central nervous system disorders. Symptoms include tiredness, discomfort, upset stomach, decreased appetite, vomiting, slow growth, headache, drying and cracking of lips and skin, hair loss, and yellowing of the skin.
Toxicity can occur when large amounts of liver is regularly consumed or by taking excess amounts of Vitamin A as supplements.
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