The most important of the provitamins A, β-carotene can be classified as an antioxidant due to its inhibition of radical initiated peroxidation in vitro. However, in vivo it appears to act either as an antioxidant or a prooxidant depending on cellular environment. It reduces the incidence of many cancers.
R. Peto.; R. Doll.; J. D. Buckley.; M. B. Sporn. Can dietary beta-carotene materially reduce human cancer rates? Nature. 1981, 290201 - 208.