Clinical Information

The level of vitamin A in the plasma or serum is a reflection of the quantities of vitamin A and carotene (provitamin A) ingested and absorbed by the intestine (carotene is converted to vitamin A by intestinal absorptive cells and hepatocytes).

Vitamin A plays an essential role in the function of the retina (adaptation to dim light), is necessary for growth and differentiation of epithelial tissue, and is required for growth of bone, reproduction, and embryonic development. Together with certain carotenoids, vitamin A also plays a critical role in immune function, with deficiency associated with increased susceptibility and severity of some infectious diseases.

Degenerative changes in eyes and skin are commonly observed in vitamin A deficiency. In developing countries, vitamin A deficiency is the principal preventable cause of blindness. Poor adaptation of vision to darkness (nyctalopia, night blindness) is an early symptom that may be followed by degenerative changes in the retina. Severe or prolonged deficiency leads to xerophthalmia, which can result in dry eye, corneal ulcers, Bitot spots, keratomalacia, and ultimately blindness. Skin changes such as dry skin, generalized xerosis, and phrynoderma are commonly observed in conjunction with vision disorders caused by vitamin A deficiency.

Vitamin A in excess can be toxic. In particular, chronic vitamin A intoxication is a concern in normal adults who ingest more than 15 mg per day and children who ingest more than 6 mg per day of vitamin A over a period of several months. Manifestations are various and include dry skin, cheilosis, glossitis, vomiting, alopecia, bone demineralization and pain, hypercalcemia, lymph node enlargement, hyperlipidemia, amenorrhea, and features of pseudotumor cerebri with increased intracranial pressure and papilledema. Liver fibrosis with portal hypertension may also result. Congenital malformations, like spontaneous abortions, craniofacial abnormalities, and valvular heart disease have been described in pregnant women taking vitamin A in excess. Consequently, in pregnancy, the daily dose of vitamin A should not exceed 3 mg.