Vegetables, fruits, and carotenoids and the risk of cancer. Low intake of vegetables, fruits, and carotenoids is consistently associated with increased risk of lung cancer in both prospective and retrospective studies. In addition, low levels of beta-carotene in serum or plasma are consistently associated with the subsequent development of lung cancer. The simplest explanation is that beta-carotene is protective. Since retinol (preformed vitamin A) is not related in a similar manner to lung cancer risk, beta-carotene appears to function through a mechanism that does not require conversion into vitamin A. However, the importance of other carotenoids and other constituents of vegetables and fruit has not been adequately explored. Both prospective and retrospective studies suggest that vegetable and fruit intake may reduce the risk of cancers of the mouth, pharynx, larynx, esophagus, stomach, colon, rectum, bladder, and cervix. But because of fewer studies and less consistency among studies, the epidemiologic evidence is at present less persuasive than for lung cancer. 