The puzzling role of vitamin A in cancer prevention (review).

This review focuses on the question of whether the cancer preventive effect of dietary vitamin A should be attributed to retinol or to carotene. Knowledge about the metabolism of retinol and carotene and results from epidemiological studies point to an effect of carotene. Studies on the mechanisms of the vitamin A effect and results of experimental studies in animals lend, at the same time, much support to an effect exerted by retinol. Integration of data from experimental and epidemiological research leads to the conclusion that, in contrast to retinol, a high intake of carotene most likely influences cancer incidence in humans, either via an effect exerted by carotene itself or after conversion into retinol.