Beta-Carotene and Cancer

Beta-carotene is a plant pigment and provitamin converted inside cells into active vitamin A forms such as retinoic acid or retinol. Vitamin A, associated with carrots, is important for immune function, intracellular signaling systems, reproduction, vision and the structure of many organs. Studies suggest adequate beta-carotene should come from fresh vegetables and fruit, but excessive supplementation may increase cancer risk.

The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study

The study enrolled smokers aged 50-69, totaling 29133 Finnish men. It assessed whether beta-carotene could reduce cancer risk in this smoking-related high-risk group. These nearly 30 thousand participants were divided into two groups receiving beta-carotene or placebo, a product without an active molecule, and followed for a median of 6.1 years.

• At the 18-month assessment, lung, prostate and stomach cancer incidence was significantly higher with beta-carotene supplements. Lung cancer risk was even higher among those smoking 1 pack or more daily.
• With beta-carotene supplements compared with placebo, colorectal cancer, colon polyp recurrence, pancreatic cancer, bladder cancer or liver cancer rates did not differ.

Beta-Carotene and Retinol Efficacy Trial (CARET)

Beginning in 1985, the study enrolled people aged 46-74 who smoked or had asbestos exposure, totaling 18314 people in the United States. Asbestos is a known risk factor for pleural cancer, malignant mesothelioma , and lung cancer. Patients were divided into two groups receiving beta-carotene plus retinol or placebo.

• Lung cancer incidence increased in the beta-carotene plus retinol group compared with placebo. 
• Among asbestos-exposed people and heavy smokers, no increased lung cancer risk was seen compared with placebo. 
• Prostate cancer risk did not increase with beta-carotene plus retinol compared with placebo. 
• Lung cancer deaths were significantly higher in the beta-carotene plus retinol group than placebo. 
• CARET was stopped early in 1996 because of the marked mortality difference.

Physicians’ Health Study

In the United States Physicians’ Health Study, men aged 40-94, totaling 22071 physicians, were randomized into two groups. For 12 years, one received beta-carotene (every 2 days, 50 mg), the other placebo. % 11 were current smokers and % 30 former smokers.

• Overall, beta-carotene supplementation did not significantly increase cancer incidence.
• Bladder and thyroid cancer risks increased somewhat with beta-carotene, but not significantly.

Against Colorectal Polyp (Adenoma) Recurrence

The Antioxidant Polyp Prevention Study assessed 864 people with colorectal polyps. Half received beta-carotene (25 mg daily) and vitamin E (400mg daily); controls received placebo. At baseline, %19 smoked.

• In people who neither smoked nor drank alcohol and received beta-carotene, colorectal polyp recurrence risk fell significantly.
• In people who smoked or consumed alcohol, beta-carotene increased colon and rectal polyp recurrence compared with placebo.

Other Studies

Cancer treatmentfollowed by beta-carotene supplementation has also been studied to determine whether it reduces recurrence risk.

In one study, stage 1 and 2 squamous cell head and neck cancer patients, 224 in total, were divided into two groups. One received beta-carotene for 3 years (75 mg daily). Adherence was 68.7, and follow-up lasted 59 months. Disease-free or 10-year survival did not differ significantly between supplemented and nonsupplemented patients. However, mortality risk was lower in those not treated with beta-carotene.

A Physicians’ Health Study subgroup analysis assessed beta-carotene in aggressive prostate cancer patients receiving radiotherapy. It likewise found no benefit for recurrence or survival.

Conclusion

• None of the discussed studies found a protective effect against cancer from beta-carotene supplements compared with placebo.
• Some studies show increased lung cancer risk, particularly among smokers.
• Although some studies suggest benefits for colorectal polyp outcomes, very few have evaluated this effect.

In conclusion, These studies should not be interpreted as showing that vitamin A and its derivatives are harmful. Adequate vitamin A is essential for normal bodily function, and deficiency is uncommon with normal nutrition. However, vitamin A illustrates how unnecessary excess supplementation, even if natural, can cause serious harm. Overall, data suggest beta-carotene for cancer prevention should not be recommended, particularly to smokers, because of potential harms.