Bottom Line: Vitamin A is essential for many bodily functions. It is unclear if taking extra vitamin A can prevent cancer.
Vitamin A must be obtained from diet. Many vegetables and dairy products are rich in vitamin A precursors, such as beta-carotene and cryptoxanthin, which are converted into the active form retinol. Scientists are familiar with many of the normal functions that vitamin A has in the human body, but know less about what effect extra vitamin A supplementation has. Receiving the recommended daily allowance (RDA) of vitamin A is important because this vitamin is essential for a variety of bodily functions, including vision, embryonic development, maintenance of tissue integrity, and proper immune activation.
To treat acne Prescription forms of vitamin A have been shown to improve acne, but there is no proof that non-prescription forms can have the same effect.
To prevent and treat cancer A few large clinical trials show that vitamin A does not help prevent recurrence or prolong survival in patients with resected melanoma, head and neck cancer, or non-small cell lung cancer.
To treat Crohn's disease Clinical evidence does not support this use.
To enhance tissue strength A diet containing adequate amounts of vitamin A is important in the maintenance of tissue strength, but there is no proof from clinical trials that high-dose vitamin A adds any benefit.
To treat eye disorders Clinical trials have not definitively supported this use.
To treat gastrointestinal disorders No scientific evidence supports this use.
To stimulate the immune system No scientific evidence supports this use.
To treat infections No scientific evidence supports this use.
To treat kidney stones No scientific evidence supports this use.
To prevent menorrhagia (excessive bleeding at menstruation) No scientific evidence supports this use.
Melanoma: The ability of high-dose vitamin A to prevent a recurrence of completely resected melanoma was tested in a study of 240 patients. Patients were randomly assigned to receive 100,000 International units of oral vitamin A for 18 months or no treatment for the same amount of time. After eight years of follow-up, patients taking vitamin A had the same average disease-free survival and overall survival. In addition, patients taking vitamin A reported numerous severe side effects, including headaches, nausea, blood clotting problems, dizziness, and emotional disturbances. High-dose oral vitamin A supplementation does not appear to influence recurrence or survival following complete resection of melanoma.
Preventing cancer recurrence: The EUROSCAN clinical trial studied the effects of high-dose vitamin A and N-acetylcysteine on the recurrence rates and survival of 2573 patients with cancer of the oral cavity, laryngeal cancer, or non-small cell lung cancer (NSCLC). The patients were randomly split into four groups and given either: 1) 300,000 IU (international units) of vitamin A daily for 1 year followed by 150,000 IU daily for 1 year, 2) 600 mg of N-acetylcysteine daily for 2 years, 3) both vitamin A and N-acetylcysteine, or 3) two placebo pills. The patients were followed for an average of four years, and it was found that all four groups had similar recurrence rates and overall survival times. Many patients taking vitamin A reported having side effects. This clinical trial suggests that high-dose vitamin A and N-acetylcysteine do not prevent recurrences or extend survival in patients with head and neck cancers or NSCLC.
Gastrointestinal cancer: A recent review of 14 clinical trials shows that supplementation with antioxidants beta-carotene, vitamins A, C, and E does not seem to prevent gastrointestinal cancer and may actually increase overall mortality.
This product is regulated by the FDA as a dietary supplement. Unlike approved drugs, supplements are not required to be manufactured under specific standardized conditions. This product may not contain the labeled amount or may be contaminated. In addition, it may not have been tested for safety or effectiveness.
Stomatitis (swelling of the mucous membranes of the mouth)
Blurred vision
Muscular discoordination
Elevated liver function tests
Chronic toxicity or hypervitaminosis A: This toxicity is usually associated with chronic intake of more than 30,000 IU of vitamin A. Symptoms may include double vision, headache, insomnia, microcytic anemia, neutropenia (abnormally low levels of white blood cells), blood clotting problems, liver damage, bone and skin changes, and other nonspecific adverse effects.
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. Current data are inconsistent regarding its anticancer activity. Vitamin A derivatives are in use and under study as prescription chemotherapeutic agents for cancer. 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 reduces mortality in preschool children (5); however, its effects on infant mortality are conflicting (6)(7)(8). Furthermore, maternal supplementation does not affect neonatal mortality (9). In addition, determining the proper dosage of vitamin A supplementation for children is still underway (10). Vitamin A supplementation may also affect a child's immune response to specific vaccines (11)(12).
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. A recent review of 14 clinical trials shows 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).
Vitamin A is essential for a variety of physiological processes. The retinal form of vitamin A is required for vision in the conversion of light to neuronal signals. Retinoic acid is necessary for maintaining normal differentiation of corneal, conjunctival, and retinal membranes, including the photoreceptor cone and rod cells of the retina. Other biological functions of vitamin A include significant roles in embryonic development, maintenance of epithelial cell integrity in many body tissues, and proper immune activation through cell differentiation and proliferation (14).
Absorption: Absorption efficiency of preformed vitamin A is generally high, ranging from 70 to 90%. Intestinal absorption follows the breakdown of retinyl esters in the gut lumen. Distribution: In the presence of adequate vitamin A intake, more than 90% of total body stores are located in the liver as retinyl esters. Since vitamin A is fat-soluble, it is distributed widely to sites throughout the body including adipose tissue. Metabolism/Excretion: The majority of vitamin A metabolites are excreted in the urine. Retinol also is metabolized in the liver to various products, which can be conjugated prior to excretion in the bile. This additional excretion pathway is postulated to serve as a protective mechanism for decreasing the risk of excess hepatic vitamin A storage. (14)
Reported: Nausea, vomiting, headache, stomatitis, blurred vision, muscular discoordination, elevated liver function tests. Chronic toxicity or hypervitaminosis A: Usually associated with chronic intake of more than 30,000 IU of vitamin A. The clinical picture is complex and nonspecific and may include diplopia, headache, insomnia, microcytic anemia, neutropenia, coagulation abnormalities, hepatotoxicity, bone and skin changes, and other nonspecific adverse effects. (14)(16)(17)
Warfarin: Large doses of vitamin A may increase the anticoagulant effects of warfarin. Alcohol: Ethanol can compete with retinol for alcohol dehydrogenase, leading to reduced levels of retinol oxidation to retinaldehyde and retinoic acid. (14)(18)
Van Zandwijk N, et al. EUROSCAN, a randomized trial of vitamin A and N-acetylcysteine in patients with head and neck cancer or lung cancer. J Natl Cancer Inst 2000;92:977-86. A prospective, open-label, randomized evaluation of vitamin A (300,000 International units (IU) daily for 1 year followed by 150,000 IU daily for 1 year), N-acetylcysteine (NAC) (600 mg once daily for 2 years), both agents, or placebo in patients with non-small cell lung cancer (NSCLC), laryngeal cancer, or cancer of the oral cavity. Primary end points were event-free survival, overall survival, and occurrence of second primary tumor. A total of 2,573 patients were randomized to vitamin A (n=647), NAC (n=642), both agents (n=643), or placebo (n=641). Demographics appear similar between treatment arms, but no statistical tests were reported. Five-year survival, event-free survival, and development of secondary tumors were not significantly different between treatment arms. Adverse events were reported by approximately 25% of patients receiving vitamin A alone or in combination with NAC. Reactions included gastric events, skin rash, transient elevations in liver function tests, and bone pain. Nearly 18% of patients receiving NAC alone reported adverse events related to gastric events and skin rash. The authors conclude that vitamin A alone or in combination with NAC or NAC alone is no better than placebo in improving survival or decreasing second tumors for patients with primary NSCLC or head and neck cancers.
