Derived from dietary sources and vitamin A precursors such as beta-carotene, alpha-carotene, and cryptoxanthin, vitamin A supplements are taken for eye conditions or acne, to improve immune function and growth and development in children, and to treat and prevent cancer. Vitamin A is necessary for normal differentiation of corneal, conjunctival, and retinal membranes, growth, 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 (4), and along with zinc, may reduce malaria-related morbidity (5). In developing countries, vitamin A supplementation reduced mortality in children (6) (7); however, its effects on infant mortality are conflicting (8) (9) (10), and maternal supplementation does not affect neonatal mortality (11). Vitamin A supplementation may also affect immune response to specific vaccines in children (12) (13), and was shown to be beneficial and safe in HIV-infected children (14). Studies to determine the proper dosage of vitamin A supplementation for children of varying demographics are still incomplete (15) (16) (17). Adults who may be malnourished, such as patients with Crohn’s disease, may also be vitamin A-deficient, and symptoms such as night vision problems should be assessed by physicians (18).
Although vitamin A derivatives are used as chemotherapeutic agents for cancer, a review of 14 clinical trials showed that supplementation with antioxidants beta-carotene, vitamin A, vitamin C, and vitamin E does not seem to prevent gastrointestinal cancer and may actually increase overall mortality (19). Higher serum retinol was also associated with elevated risk of prostate cancer, with prolonged high exposure resulting in increased risk (20) (21). Other large clinical trials have shown that vitamin A supplementation does not help to prolong survival for melanoma patients (22), prevent recurrence of head and neck cancer (23), or reduce the risk of non-small cell lung cancer (24), although a reduction in melanoma risk was associated with supplemental retinol but not carotenoid intake, particularly for women, in a large epidemiological study (25). At the same time, higher retinol intakes are associated with significant direct trends for risk of oral and pharyngeal cancers (26) and may also negatively impact high-dose vitamin D protection against colon cancer risk (27) as well as vitamin D involvement in calcium uptake in the bone (28), increasing the risk for hip fractures (29).
In assessments of nutritional status among in cancer patients, one study noted significantly lower serum retinol and beta-carotene levels in stage III versus earlier-stage breast cancer patients and also in patients after radiotherapy (30). Another study found human papillomavirus positive (HPV+) head and neck cancer patients who have diets richer in micronutrients including vitamin A may have better immune functioning and prognosis (31).
Deficiencies of vitamin A are rare in developed countries (29). Supplementation of vitamin A with doses greater than the recommended Daily Value of 5,000 IU or in conjunction with certain medications or pre-existing conditions may result in adverse effects or toxicity (32) (33). Vitamin A is therefore best obtained from a variety of dietary sources.