Derived from dietary sources and vitamin A precursors such as beta-carotene, alpha-carotene, and cryptoxanthin, vitamin A is taken by patients to treat and prevent cancer. Vitamin A is also used for eye conditions, kidney stones, acne, and to improve immune function and growth and development in children. Vitamin A is necessary for normal differentiation of corneal, conjunctival, and retinal membranes, growth and development, and immune activation.
Clinical data suggest that vitamin A may be effective against growth retardation, acne, eczema (1), and hepatitis C (2). In children, vitamin A may also reduce recurring urinary tract infection (3), parasitic infections (21) and along with zinc, may reduce malaria-related morbidity (4). In developing countries, vitamin A supplementation reduced mortality in children between the ages of 6 months and 5 years (5) (22); however, its effects on infant mortality are conflicting (6) (7) (8). Maternal supplementation does not affect neonatal mortality (9). Vitamin A supplementation may also affect immune response to specific vaccines in children (11) (12); it was shown beneficial and safe in HIV-infected children (23). Studies to determine the proper dosage of vitamin A supplementation for children are still underway (10).
Vitamin A derivatives are in use and are being studied as prescription chemotherapeutic agents for cancer.
A review of 14 clinical trials showed that supplementation with antioxidants beta-carotene, vitamins A, C, and E does not seem to prevent gastrointestinal cancer and may actually increase overall mortality (13). Higher serum retinol was also associated with elevated risk of prostate cancer, with prolonged high exposure resulting in increasing the risk (14).
Adverse effects from chronic vitamin A supplementation include nausea, vomiting, headache, stomatitis, blurred vision, muscular discoordination, and hepatotoxicity. Signs and symptoms of toxicity are non-specific and may include diplopia, headache, insomnia, microcytic anemia, neutropenia, coagulation abnormalities, and bone and skin changes. Pregnant women should not consume vitamin A supplements because chronic consumption of 5,000 International units or greater may cause teratogenic effects. Supplementation with doses greater than the recommended daily allowance may result in toxicity and patients should be monitored accordingly.