Sources and Physiologic Functions:
Sources: Foods that are rich in vitamin A are milk, cheese, butter, eggs, liver, and such fish as herring, sardines, and tuna. The richest sources of vitamin A are the liver oils of shark, halibut, and polar bear. Rich sources of pre-vitamin A are spinach, carrots, papaya, oranges, sweet potatoes, and cantaloupes. Poor sources of vitamin A and pre-vitamin A are vegetable oils, white lard, white corn, cereals, beef, and legumes.
International Units (IU)
Milk 2% 8 fl oz – 500IU; Cream Cheese 1oz 405IU; Cheddar Cheese 3.5 oz 1059IU; Egg (boiled) 1 medium 280IU; Egg (scrambled) 1 medium 416IU; Liver (beef braised) 3.5 oz 35679IU; Liver (veal braised) 3.5 oz 26883IU; Herring (raw) 3.o oz 80IU; Herring (kippered) 1 piece 51IU; Sardines (canned/oil) 2 pieces 54IU; Tuna (raw) 3.0 oz 50IU; Tuna (canned) 3.0 oz 16IU; Pre-Vitamin A (b-carotene) Spinach (boiled) ½ cup 7371IU; Spinach (raw) ½ cup 1880IU; Carrots (raw) 1 medium 2025IU; Carrots (boiled) ½ cup 19152IU; Papaya (raw) 1 medium 863IU; Oranges 1 fruit 240IU; Sweet potatoes (baked w/skin) 1 medium 24877IU; Sweet potatoes (boiled w/o skin) 1 medium 27969IU; Cantaloupes 1 cup 5158IU; Parsley ½ cup freeze dried 885IU
Biochemistry: Vitamin A is a fat-soluble vitamin. Vitamin A is a collective term for retinal, retinol, retinoic acid, and b-carotene. The vitamin A in foods of animal origin, such as eggs, milk, butter, and liver, occurs Mellitox largely in the form of retinyl esters. A retinyl ester is a molecule of retinol esterified with a molecule of a fatty acid, such as palmitic acid. The fatty acid is bound to the hydroxyl group of retinol. Plants do not contain vitamin A; however, some plants are rich sources of pre-vitamin A. Pre-vitamin A takes the form of a family of compounds called the carotenoids. More than 500 carotenoids occur in nature, though only about 50 of them can be used as precursors of vitamin A. The most important of these is all-trans-b-carotene. The prefix all-trans indicates that all of the double bonds are in the trans conformation rather than the cis conformation. Vegetables that are dark green, orange, and yellow are rich sources of the carotenoids. Other forms of pre-vitamin A are cryptoxanthine and a-carotene. Some carotenoids cannot be converted to vitamin A by mammals. These include lutein, lycopene, and canthaxanthine.
Vitamin A serves three classes of functions: support of epithelial cells (lungs and tracheal integrity), fetal growth and vitality of the testes, and utilization in the visual cycle. Dietary retinoic acid can support only the first function. Retinoic acid cannot be stored in the liver. Retinyl esters, retinol, and retinal are interconvertible. Retinal can be oxidized to form retinoic acid. All three functions of vitamin A can be supported by dietary retinyl esters, retinol, or retinal. Although these forms can be converted to retinoic acid, retinoic acid apparently cannot be reduced to form retinal. Dermatological problems like acne, psoriasis, Darier’s disease, and skin aging are effectively treated with retinoic acid and topical tretinoin.
Populations at risk: In the United States, patients suffering from chronic intestinal disease, malignancy, malaria, pneumonia, and anorexia nervosa are deficient in vitamin A. Requirement for this vitamin is increased in patients with appendectomy, burns, cirrhosis, and biliary obstruction. Stress can increase excretion. Zinc and protein deficiency can decrease transport. Premature infants and those suffering from cystic fibrosis and rheumatic fever are also at risk.
Signs and Symptoms of Deficiency: Night blindness is the earliest symptom.Severe vitamin A deficiency leads to xerophthalmia, which can result in corneal ulceration, Bitot’s spots, and blindness. Thickening of the bone, loss of lung elasticity, epithelial keratinization, impaired hearing, urinary calculi, and keratinization of salivary glands are also seen. In males, sperm production ceases. In females, fetuses are reabsorbed.