Vitamin A was the first vitamin to be discovered and officially named, hence its letter A. Vitamin A is not a single substance, but a group of nutrients that include retinol, retinal, and the carotenoids. Retinol and retinal are both known as preformed vitamin A and are found in a variety of animal foods, especially liver. Butter, cream, egg yolk, fish oils, and whole and fortified nonfat milk are all good sources of preformed vitamin A. Orange fruits and green, leafy, and yellow vegetables are all rich sources of various precursor carotenoids, particularly beta-carotene, which the body converts to vitamin A.

Vitamin A is important for a variety of body functions, including eyesight, healthy teeth and skin, bone growth, cell differentiation, and tissue repair. Vitamin A also plays an important role in maintaining proper function of the cornea, lungs, mucus membranes, the lining of the gastrointestinal tract, and the bladder and urinary tract. It also acts as an antioxidant, helps prevent infectious disease, and is needed for the production of various anti-tumor compounds in the body.

Vitamin A stores are diminished by both stress and illness, as well as alcohol consumption, which also interferes with its absorption. When of the first signs of vitamin A deficiency is night blindness. Other signs of deficiency include supoptimum bone and tooth formation, eye inflammation, impaired immune response, weight loss, and keratinosis, a condition resulting in hardened pigmented deposits around hair follicles and the body's upper and lower extremities.