A study that suggests vitamin A could reduce the risk of melanoma should be treated with caution, according to Australian cancer experts who say the results are inconclusive and involve potentially toxic levels of vitamin supplements.
The study, published online this week in the Journal of Investigative Dermatology, suggests that retinol - a form of vitamin A present in foods of animal origin such as liver, eggs, and milk - could act as a chemoprotective agent against melanoma when taken as a supplement.
US researchers examined 69,635 people - divided almost evenly between men and women - over almost six years to see whether dietary and supplemental vitamin A affected the risk of developing melanoma.
They found that high doses - 41,200 mg per day - of retinol led to “a significant reduction” of melanoma risk, particularly among women. The protective effect was achieved only with supplemental retinol, the researchers reported. When consumed as part of a diet, Vitamin A had no effect.
Graham Mann, a Professor in Medicine at the University of Sydney, said the main limitation of the study was that the researchers did not assess sun exposure, the major risk factor for melanoma. “Women who take vitamin supplements may well be better at sun protection, and this could account for the reduced number of melanomas, as the authors acknowledge,” said Professor Mann, an expert in the genetic and environmental causes of skin cancer.
“Another contributing factor is that even though the study is large, the number of melanoma cases is small. Three or four more cases of melanoma in the 3,288 female retinol users would have wiped out the difference, so the result may well be entirely a chance finding.”
Australia has the highest rate of melanoma in the world, said Rod Sinclair, a Professor of Dermatology at the University of Melbourne. And although the study was interesting, it only revealed benefits for women who took very high doses of retinol - which “have significant side-effects”.
Andrew Miller, a Clinical Senior Lecturer in Dermatology at the Australian National University, also warned about the dangers of taking high levels of retinol.
“When the potential for foetal damage and systemic toxicity from taking higher than recommended daily intakes is considered, I would not recommend routine supplementation,” Dr Miller said.
The use of a cancer registry to determine the number of melanomas may also have yielded misleading results, he said, “and since there was no attempt made to obtain an independent histologic confirmation of the diagnosis, any variation in diagnosis validity could significantly skew the results of the study”.
“The best advice that I can give for mitigation of melanoma risk is careful sun protection and routine self-examination; particularly in high risk families.”
Professor Sinclair agreed: “‘Slip, Slop, Slap’ is still the best advice going around.”