Deficiency and toxicity: A number of difficulties can arise from a deficiency of vitamin A, many of which are based on impairments in the biochemical functions that this important nutrient performs. It is estimated that the diets of approximately 25 percent of the people in the United States are supplying less than the RDA for vitamin A. This commonly occurs in those who avoid the carotene-containing fruits and vegetables or when the diet is filled with processed foods that are depleted of vitamins. The fruits and vegetables are really the most important food groups for intake of the many vitamins and minerals, as well as for fiber and water content. The elderly, teenagers, and alcoholics are the three groups most commonly deficient in vitamin A. Worldwide, vitamin A deficiency is even more common than it is in the United States.

Night blindness, the inability to adapt the eyes to see clearly in the dark, is probably one of the first signs of vitamin A deficiency. Lack of general eye tissue health and vitality may occur, as can decreased vision, irritated, reddened, or dry eyes, and eyes that tire easily; in severe deficiency states, corneal ulcers may develop. Usually, supplemental vitamin A or beta-carotene will correct these problems; the RDA of vitamin A will prevent night blindness and these other eye problems.

Perhaps of greater significance, vitamin A and beta-carotene deficiencies may decrease our protection against infectious agents and the internal process of carcinogenesis. A depletion or deficiency of vitamin A reduces both T lymphocyte (cellular immunity) and B lymphocyte (antibody production) responses; severe vitamin A deficiency has been shown to cause atrophy of the thymus and spleen, both immunologically important organs, and to reduce the number of circulating lymphocytes. Low dietary levels of vitamin A have been associated with an increased risk of many cancers, including breast, cervical, lung, prostate, laryngeal, and stomach cancers, while beta-carotene deficiency has been clearly seen in patients with cancers of the cervix or lungs.

Vitamin A deficiency also affects the skin. Dry, bumpy skin may occur, especially on the backs of the arms. Since vitamin A promotes skin growth, moisture retention, and proper cell differentiation, a deficiency causes decreased skin tone and rapid aging of the skin and a variety of blemishes, acne, or boils. Vitamin A supports the mucous-secreting cells of the internal mucous membranes. Mucus protects these membranes from infection and irritants. When vitamin A is deficient, these internal epithelial cells secrete a protein (keratin) commonly found externally in hair and nails. This keratinization process makes the epithelial cells harder and dryer and thus less protecting.

Vitamin A along with adequate protein intake generates healthy hair. With a lack of A, the hair may lack luster and dandruff is more likely because of the loss of scalp skin moisture. Bone softness or abnormal menstruation may also develop from vitamin A deficiency. Fatigue and insomnia are also possible, as are a decrease in the appetite and some loss of smell and taste. When vitamin A is deficient, vitamin C seems to be lost more rapidly from the body. In addition to the lowered immune function and increased infection rate associated with vitamin A deficiency, periodontal disease, kidney stone formation, ear problems, and acne may occur more frequently.

A number of toxicity symptoms and difficulties may occur when we take too much vitamin A. Since it is stored in the body and is not readily excreted, toxicity may occur from mildly increased doses (say 50,000–100,000 IUs per day) over an extended time, such as a month or two, or from very high doses over a shorter period. Animal liver meats have the highest concentrations of vitamin A; beef liver has about 15,000 IUs per ounce, while polar bear liver has much higher concentrations than this and has been known to cause vitamin A toxicity from one serving. It is highly unlikely that one would get vitamin A toxicity from the diet alone (without lots of liver), since we receive much of it which must then be converted to active vitamin A. The synthetic vitamin A supplements, such as the palmitate or acetate forms, have a greater potential to produce toxic symptoms; high amounts of fish liver oil may produce side effects as well. The levels that cause symptoms vary from person to person, just as the proper optimum dosage does. Under high levels of stress, with illness or trauma, if we smoke or live in a polluted environment, or if we are pregnant or nursing, our vitamin A requirements are higher. When there is depletion or deficiency of vitamin A, higher amounts can be taken for up to a month without the usual risks of toxicity.