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Surveys suggest an association between diets rich in beta-carotene and vitamin A and a lower risk of some types of cancer. 

There is evidence that a higher intake of green and yellow vegetables or other food sources of beta-carotene and/or vitamin A may decrease the risk of lung cancer. However, a number of studies that tested the role of beta-carotene supplements in cancer prevention did not find it to be protective. In a study of 29,000 men, incidence of lung cancer was greater in the group of smokers who took a daily supplement of beta-carotene. 

The Carotene and Retinol Efficacy Trial, a lung cancer chemoprevention trial that provided randomized subjects with supplements of beta-carotene and vitamin A, was stopped after researchers discovered that subjects receiving beta-carotene had a 46% higher risk of dying from lung cancer than those who did not receive beta-carotene. The Institute of Medicine (IOM) states that “beta-carotene supplements are not advisable for the general population,” although they also state that this advice “does not pertain to the possible use of supplemental beta-carotene as a provitamin A source for the prevention of vitamin A deficiency in populations with inadequate vitamin A nutriture”.