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Vitamin A and beta-carotene are both fat-soluble substances that must be broken down by pancreatic digestive enzymes in the intestine in order to be absorbed.
Good food sources of vitamin A include butter, cream, egg yolk, fortified skim milk, kidney, liver, and whole milk. Good food sources of beta-carotene include yellow and dark leafy green vegetables such as carrots, collards, spinach, sweet potatoes, squash and apricots, peaches, cantaloupe. Cod liver oil and halibut fish oil also contain high levels of vitamin A.
Vitamin A was the first fat-soluble vitamin to be isolated. It was discovered as a result of its ability to prevent night blindness and drying and hardening of the mucous membrane that lines the eyelids. Beta-carotene was discovered to be the precursor to vitamin A. Vitamin A belongs to a class of compounds called retinoids, which only occur in animal products. Vitamin A requires fats as well as minerals in order to be properly absorbed from the digestive tract. Large amounts of vitamin A are stored in the liver, and therefore, it does not need to be supplied in the diet on a daily basis. Beta-carotene is found exclusively in fruit and vegetable sources. Beta-carotene consists of two molecules of vitamin A linked head to head. Enzymes in the intestinal tract split beta-carotene into two molecules of vitamin A whenever the body needs it.
Toxicities & Precautions
Vitamin A is fat soluble, excesses levels can accumulate in fatty tissues and reach toxic levels. Signs of vitamin A toxicity include abnormal liver function, anorexia, bone pain, brittle nails, diarrhea, dry itchy skin, enlarged liver, fatigue, gingivitis, hair loss, headaches, increased infections, muscle and joint pains. Symptoms are fully reversible when vitamin A is discontinued.
Caution during pregnancy, doses greater than 10,000 IU may cause birth defects. Women who could potentially become pregnant should limit their daily vitamin A levels to less than 10,000 IU daily.
Functions in the Body
Eyes and Vision
Required for night vision. It is also essential for the lubricant from tear glands that prevent drying of the cornea.
Growth and Bone Development
It is essential for the growth of bone and soft tissue, and also for the formation of tooth enamel. A deficiency of Vitamin A has been shown to contribute to bone loss associated with osteoporosis.
Helps maintain healthy cell surfaces within many glands, organs, and skin. Vitamin A also helps to facilitate effective barriers to infections.
Symptoms & Causes of Deficiency
Vitamin A deficiency can be caused by inadequate dietary intake or metabolic dysfunction that interferes with absorption, transport or storage of the vitamin.Deficiency of vitamin A is associated with the development and promotion of surface tissue cell cancers in various glands and organs throughout the body. Vision problems may occur including the following; night blindness, is the classic vision problem resulting from vitamin A deficiency. A drying or hardening of the epithelial cell membranes in the eye can also develop. Long-term vitamin A deficiency causes a condition in which the skin becomes dry, scaly, and rough. Small hard bumps develop on the skin because hair follicles plug up with a hard protein called keratin, ker-uh-tin.