Physicians’ Health Study
The Physicians’ Health Study randomly assigned 22,071 male physicians aged 40 to 94 living in the United States to receive beta-Carotene (50 mg every 2 days) or placebo for a mean of 12 years.14 The cohort included 11% who were current smokers and 30% who were former smokers.
There was no significant difference in the incidence of overall cancer (RR, 0.98; 95% CI, 0.91-1.06; P = .65), as well as lung, colorectal, prostate, stomach, pancreas, brain cancer, or non-melanoma skin cancers, and melanoma, leukemia, or lymphoma between the beta-Carotene and placebo groups.14,15 There were higher, but nonsignificant, rates of bladder and thyroid cancer in the beta-Carotene group compared with the placebo group.
No significant difference in overall or lung cancer incidence was observed among smokers who received beta-Carotene compared with placebo (RR, 1.05; 95% CI, 0.86-1.28 and RR, 0.90; 95% CI, 0.58-1.40, respectively).
beta-Carotene supplementation was also not associated with differences in rates of cancer-related mortality compared with placebo.
Women’s Health Study
The women’s health study randomly assigned 39,876 female health professionals aged 45 or older living in the United States to receive beta-Carotene (50 mg every 2 days) with or without aspirin (100 mg every 2 days) and/or vitamin E (600 IU every 2 days), or placebo.16 The trial lasted for 12 years, though the beta-Carotene component ended after a median treatment duration of 2.1 years. Current smokers comprised 13% of the cohort at baseline.
There was no significant difference in the incidence of cancer (RR, 1.03; 95% CI, 0.89-1.18), including breast, colorectal, uterus, lung, ovary, thyroid, bladder, brain, pancreas, cervix, or stomach cancers, and melanoma, leukemia, and lymphoma. The risk of invasive cancer was similar among smokers and nonsmokers. The smokers’ cohort was too small for analyses of site-specific cancers.
There was also no difference in risk of overall mortality (RR, 1.07; 95% CI, 0.74-1.56) or cancer-specific mortality (RR, 1.11; 95% CI, 0.67-1.85) between the beta-Carotene alone and placebo groups.
Other randomized, controlled trials demonstrated no significant reduction in risk of non-melanoma skin cancers, head and neck cancer, or aerodigestive tract cancers with beta-Carotene supplementation compared with placebo.17-19
One randomized, placebo-controlled trial of 103 women with cervical intraepithelial neoplasia 2 or 3 found no significant difference in regression rates among those who received beta-Carotene (30 mg daily) or placebo for 2 years.20
Recurrent Colorectal Adenoma
The multicenter, double-blind Antioxidant Polyp Prevention Study randomly assigned 864 patients with a colorectal adenoma removed to receive beta-Carotene (25 mg daily) with or without vitamins C (1000 mg daily) and E (400 mg daily) or placebo.21 At baseline, 19% of patients were current smokers.
beta-Carotene supplementation significantly reduced the risk of adenoma recurrence among patients who were nonsmokers and did not consume alcohol (RR, 0.56; 95% CI, 0.35-0.89). There was no difference in risk of adenoma recurrence among patients who smoked or consumed alcohol.
Among patients who smoked and consumed alcohol, there was a significant increase in adenoma recurrence with beta-Carotene compared with placebo (RR, 2.07; 95% CI, 1.39-3.08; P < .001).