A study in the Journal of Investigative Dermatology has found that those women who took vitamin A supplements for five years had a 40% reduced risk of melanoma compared to those who did not take the supplements. 
Such studies are useful in that they show that melanoma is not necessarily caused by sunlight. It is important to understand that poor nutrition is a factor in almost all cancers, not just melanoma. And here is something else to remember: high intakes of supplemental vitamin A are associated with birth defects, increased risk of lung cancer, liver abnormalities, increased risk of osteoporosis, and central nervous system disorders. No such adverse side effects are associated with natural dietary beta-carotene consumption (beta carotene is a precursor of vitamin A production in the body); in fact beta carotene from carrots and other vegetables has significant protective effects against cancer, but supplemental doses of beta-carotene may actually increase cancer.
The good news about this study is that it defined an antioxidant that helped prevent melanoma; the bad news is that if people read a report on the study and then start supplementing vitamin A in high quantities, the may commit vitamin A suicide. It is best to eschew the supplementation and eat large quantities of green and yellow vegetables and colorful fruits to help prevent melanoma and other cancers. There are no studies that show anything but positive results for that style of nutrition. For more on the link between nutrition and melanoma, see my earlier blog. skin-cancer-and-nutrition
Also, don’t forget that safe and regular sunlight exposure also decreases the risk of melanoma, contrary to popular belief. See my book for more details.
 Maryam Asgari, M.D., M.P.H., dermatologist and investigator, Kaiser Permanente Northern California, Oakland; Doris Day, M.D., dermatologist, Lenox Hill Hospital, New York City; Robert Graham, M.D., internist, Lenox Hill Hospital, New York City; March 1, 2012, Journal of Investigative Dermatology, online.
 National Institutes of Health, Dietary Supplement Fact Sheet: Vitamin A and carotenoids—Health Professional Fact Sheet. http://ods.od.nih.gov/factsheets/vitamina?print=1. Accessed March 1, 2012.
 Fontham ETH. Protective dietary factors and lung cancer. Int J Epidemiol 1990;19:S32-S42
 Redlich CA, Blaner WS, Van Bennekum AM, Chung JS, Clever SL, Holm CT, Cullen MR. Effect of supplementation with beta-carotene and vitamin A on lung nutrient levels. Cancer Epidemiol Biomarkers Prev 1998;7:211-14.