Vitamin A refers to a group of fat-soluble vitamins. There are two forms of vitamin A available in our diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids.
Preformed vitamin A is from animal sources, including meat, fish, and dairy products, while carotenoids are from plant sources, such as sweet potatoes, carrots, spinach, and apricots. The most important carotenoid is beta-carotene.
Provitamin A carotenoids are converted into vitamin A in the body, and both preformed and provitamin A are metabolized further within the cells into the active forms of vitamin A (retinol and retinoic acid).
Vitamin A is most well known for its important function in vision, but is also important for immune function, reproduction, cellular communication, and the normal formation and maintenance of the heart, lungs, kidneys, and other organs.
Vitamin A deficiency is rare in the United States, but is common in many developing countries, where it typically begins during infancy. The most common symptom is xerophthalmia (abnormal dryness of the conjunctiva and cornea of the eye), which first appears as night blindness. Vitamin A deficiency also increases the severity and mortality of infections, particularly diarrhea and measles.
Vitamin A toxicity is also possible, as vitamin A is a fat-soluble vitamin so can accumulate in fat tissues. Symptoms can include headaches, vomiting, confusion, joint pain, and yellowish skin. Other health complications can include negative effects on bone density, liver health, and increase the risk of congenital birth defects.