Wednesday, July 16, 2008

Vitamin A: Deficiency, Daily Needs, Excessive Intake

Vitamin A

Vitamin A was the first vitamin to be discovered. It is a fat soluble vitamin found to be useful in preventing eye disorders. It includes a family of compounds: retinol, and the carotenoids. Retinol can be found in animal foods and these are ready to be used by the body. Carotenoids are a group of fat soluble pigments found in orange, dark yellow and green vegetables and fruits. Some carotentoids like beta carotene can be converted to vitamin A. Vitamin A can only be absorbed in the presence of fat.

Functions Of Vitamin A

Vitamin A is essential for normal eyesight. It combines with a specialized protein in the retina of the eye to prevent night blindness. Vitamin A is also known for its anti-infective property. It functions in the development and maintenance of the body's barrier to infections. It also enhances the activity of the immune system. Vitamin A helps develop and maintain moist, healthy epithelial tissue which lines the body's external and internal surfaces. Vitamin A is, therefore, necessary in the maintenance of the cornea of the eye, all mucous membranes, the digestive tract, urinary tract, reproductive tract, skin and lungs. Adequate vitamin A also
maintains gastric lining which may help to prevent and treat gastric ulcers. Vitamin A plays an important role in growth and formation of bones and soft tissues. The formation of tooth enamel and the proper spacing of teeth are also dependent on adequate amounts of vitamin A.

Vitamin A may help in preventing certain types of cancer, such as breast, stomach, cervical and lung cancers by maintaining healthy epithelial tissues, discouraging the formation of abnormal cells or by strengthening the immune system. Vitamin A also plays a role in smoothing wrinkles and is a major ingredient in anti-wrinkle skin care products.

Vitamin A Deficiency

Prolonged vitamin A deficiency results in changes in the skin, eyes (including eyesight) and teeth. Chronic vitamin A deficiency results in ulceration and distortion of the cornea of the eye and eventually blindness. This vitamin deficiency disease called xerophthalmia is common in third world countries, especially in infants and children, but is seldom found in developed countries. Night blindness is a primary symptom of vitamin A deficiency. Poor intake of vitamin A results in growth retardation, loss of appetite, weight loss and bone deformities. Vitamin A deficiency results in keratosis (hardening) of the taste buds and loss of appetite.

Long term poor intake of vitamin A results in skin problems known as "goose flesh" or
"toad skin" where small, hard bumps are formed. The skin also becomes dry, scaly and rough which is known as xeroderma. The most common locations are the shoulders, neck, back, forearms, thighs and abdomen. People who are at risk of vitamin A deficiency are those with intestinal malabsorption disorders, liver disease; those who are on restrictive diets, do not eat enough vegetables, smokes and abuses alcohol.

Daily Needs Of Vitamin A

The US Recommend Dietary Allowances (1989) for vitamin A is 1000 RE or ug for male
adults and 800 RE for female adults. RE (retinol equivalents) = 1 ug of retinol = 6ug of beta carotene. The need for vitamin A increases with increasing body weight. The RDA for infants are based on the vitamin A content in breastmilk and the RDA for women is about 80% that for men.

Food Sources Of Vitamin A

Dietary sources of vitamin A, which are found in animal foods. Beta carotene which are found in orange, yellow and dark green fruits and vegetables can be converted in the body to vitamin A.

Major Food Sources
Food (100g) Vitamin A content (RE)
Beef liver, cooked 10602
Pork liver, cooked 5399
Chicken liver, cooked 4913
Carrot, raw 2813 (as carotenoid)
Sweet potato, raw 2182 (as carotenoid)
Margarine 800
Spinach, raw 672 (as carotenoid)
Egg yolk 582
Chinese mustard green (kai lan), raw 504 (as carotenoid)
Watercress, raw 470 (as carotenoid)
Mango, raw 389 (as carotenoid)
Pak choy, raw 300 (as carotenoid)
Processed cheese 290
Apricot 261 (as carotenoid)
Broccoli, raw 154 (as carotenoid)
Oysters, cooked 146

Excessive Intake

Vitamin A is not excreted and can accumulate in the body to toxic levels if excessive amounts are taken long term. Prolonged daily intake of 10000RE to 20000RE can cause vomiting, nausea, loss of appetite, weight loss, joint pain, bone deformities, hair loss, liver enlargement, dry and cracked lips. Children can develop toxicity symptoms more easily than adults and can experience the above symptoms and slowed growth even when much smaller doses, such as 5000RE are consumed. Symptoms disappear quickly once supplementation is discontinued. Vitamin A toxicity from dietary sources is rare and individual tolerance to vitamin A varies widely among people.

Overdoses (5 to 10 times of USRDA)of vitamin A during pregnancy can lead to birth defects, such as cleft lip, cleft palate, missing limbs etc.

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