Vitamin a

Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta-carotene). Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision. Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone-like growth factor for epithelial and other cells.

Functions

In the retina, retinal is combined with the protein opsin to form rhodopsin, the light-absorbing molecule necessary for color vision and low-light vision. Retinoic acid, a derivative of vitamin A, is used in the growth of epithelial cells and other cells.

Dietary Recommendations

The Recommended Dietary Allowance (RDA) for vitamin A is 900 micrograms (mcg) for men and 700 mcg for women. For children and adolescents, the RDA ranges from 300 to 600 mcg. Pregnant women should consume 770 mcg, and lactating women should consume 1,300 mcg.

Deficiency

Vitamin A deficiency is estimated to affect approximately one third of children under the age of five around the world. It is estimated to claim the lives of 670,000 children under five annually. Approximately 250,000–500,000 children in developing countries become blind each year owing to vitamin A deficiency, with the highest prevalence in Southeast Asia and Africa.

Toxicity

Excessive vitamin A consumption can lead to nausea, irritability, anorexia (reduced appetite), vomiting, blurry vision, headaches, hair loss, muscle and abdominal pain and weakness, drowsiness, and altered mental status.

Sources

Vitamin A is found in many foods, including liver, sweet potatoes, carrots, broccoli, kale, spinach, and eggs.

See Also

• Vitamin
• Retinol
• Carotenoid
• Vitamin A Deficiency
• Vitamin A Toxicity

Vitamin a Resources
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