A natural pigment synthesized by plants, beta-carotene is used as an antioxidant and an immunostimulant, and to prevent or treat cancer, HIV, heart disease, and leukoplakia. Beta-carotene, along with alpha-carotene and beta-cryptoxanthin, can be converted to retinol and is classified as a provitamin A carotenoid. Supplementation with beta-carotene does not increase overall vitamin A levels or lead to vitamin A toxicity.
Animal studies show that beta-carotene promotes the development of pulmonary adenocarcinoma via increased cAMP signaling (29).
Available data concerning beta-carotene supplementation for HIV-positive patients and its effects on CD4 counts (10) as well as cardiovascular disease are conflicting. A meta-analysis of eight randomized trials demonstrated a small but significant increase in all-cause mortality and cardiovascular death for the beta-carotene arm over placebo (11), whereas other studies have reported no benefits of beta-carotene supplementation on cardiovascular disease (12) or its risk factors (13).
Consistent associations between serum beta-carotene levels and risk of developing type 2 diabetes are also lacking (14)(15). One study reported that serum beta-carotene was inversely associated with the incidence of cataract formation (16).
Long-term beta-carotene supplementation may increase cognitive function (17).
Epidemiological associations between beta-carotene and cancer risk are conflicting. Whereas high dietary beta-carotene intake was associated with reduced risk of cervical cancer (1), high serum levels were also associated with increased risk for aggressive prostate cancer (2) but decreased the risk of aggressive urothelial cell carcinoma (32). In addition, studies of beta-carotene supplementation and chemoprevention for cancer are inconsistent. Studies show that consumption of beta carotene, vitamins A, C, fruits and vegetables does not influence the risk of renal cell carcinoma (31). A review of 14 clinical trials shows that supplementation with antioxidants, beta-carotene and vitamins A, C, and E, does not prevent gastrointestinal cancer, and that beta-carotene may actually increase overall mortality (3) (4). Data from large, multicenter trials suggest that beta-carotene supplementation may not lower the risk of prostate cancer (5) (6), and in male smokers over the age 40, it may increase lung cancer incidence (7) (30).
Beta-carotene supplementation especially when combined with cigarette smoking may also reduce the efficacy of cancer therapies, resulting in increased recurrence and mortality (8).
One large-scale cohort study in the Netherlands suggests alcohol consumption has a negative effect on the chemopreventive property of beta-carotene (9).
The U.S. Preventive Services Task Force (USPSTF) recommends against beta-carotene or vitamin E supplements for the prevention of cardiovascular disease or cancer (33).