Chronic high doses of β-carotene supplementation increases the probability of lung cancer in smokers.The effect is specific to supplementation dose as no lung damage has been detected in those who are exposed to cigarette smoke and who ingest a physiologic dose of β-carotene (6 mg), in contrast to high pharmacologic dose (30 mg). Therefore, the oncology from β-carotene is based on both cigarette smoke and high daily doses of β-carotene.
Increases in lung cancer may be due to the tendency of β-carotene to oxidize, and may hasten oxidation more than other food colors such as annatto. A β-carotene breakdown product suspected of causing cancer at high dose is trans-β-apo-8'-carotenal (common apocarotenal), which has been found in one study to be mutagenic and genotoxic in cell cultures which do not respond to β-carotene itself.
Additionally, supplemental β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos.
Medical authorities generally recommend obtaining beta-carotene from food rather than dietary supplements.Research is insufficient to determine whether a minimum level of beta-carotene consumption is necessary for human health and to identify what problems might arise from insufficient beta-carotene intake,although strict vegetarians rely on pro-vitamin A carotenoids to meet their vitamin A requirements. Use of beta-carotene to treat or prevent some diseases has been studied.
A 2010 systemic meta review concluded that supplementation with β-carotene does not appear to decrease the risk of cancer overall, nor specific cancers including: pancreatic, colorectal, prostate, breast, melanoma, or skin cancer generally.High levels of β-carotene may increase the risk of lung cancer in current and former smokers.This is likely because beta-carotene is unstable in cigarette smoke-exposed lungs where it forms oxidized metabolites that can induce carcinogen-bioactivating enzymes.Results are not clear for thyroid cancer.In a single, small clinical study published in 1989, natural beta-carotene appeared to reduce premalignant gastric lesions.