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Eating a diet high in beta-carotene or taking dietary supplements may lower the risk of getting smoking related cancer in non-smokers. But in people who smoke, a high dietary intake of beta-carotene may backfire and increase the risk of getting a tobacco related cancer as the study below shows.

Cancer rate (number of cancers per 10,000 women over a 10 year period) for smokers and non-smokers based on whether they had a low dietary intake of beta-carotene or a high dietary intake

 
Risk of Smoking Related Cancers
  Diet rich in Beta-carotene Supplements of beta-carotene
Non-Smoker risk decreases by 28% risk decreases by 56%
Smoker risk increases  by 43%% risk increases  by 214%

Dual Association of {beta}-Carotene With Risk of Tobacco-Related Cancers in a Cohort of French Women
Mathilde Touvier. J Natl Cancer Inst 2005; 97: 1338-1344.

Intervention studies have demonstrated that, in smokers, {beta}-carotene supplements had a deleterious effect on risk of lung cancer and may have a deleterious effect on digestive cancers as well. We investigated a potential interaction between {beta}-carotene intake and smoking on the risk of tobacco-related cancers in women.

Methods: A total of 59 910 women from the French Etude Epidémiologique de Femmes de la Mutuelle Générale de l'Education Nationale (E3N) prospective investigation were studied from 1994. After a median follow-up of 7.4 years, 700 women had developed cancers known to be associated with smoking. Diet, supplement use, and smoking status at baseline were assessed by self-report. {beta}-carotene intake was classified into four groups: first (low intake), second, and third tertiles of dietary intake, and use of supplements (high intake).

Results: Among never smokers, multivariable hazard ratios of all smoking-related cancers were 0.72 (95% CI = 0.57 to 0.92), 0.80 (95% CI = 0.64 to 1.01), and 0.44 (95% CI = 0.18 to 1.07) for the second and third tertiles of dietary intake, and high {beta}-carotene intake, respectively, compared with low intake (Ptrend = .03). Among ever smokers, multivariable hazard ratios were 1.43 (95% CI = 1.05 to 1.96), 1.20 (95% CI = 0.86 to 1.67), and 2.14 (95% CI = 1.16 to 3.97) for the second and third tertiles of dietary intake, and high {beta}-carotene intake, respectively, compared with low intake (Ptrend = .09).

Tests for interaction between {beta}-carotene intake and smoking were statistically significant (Ptrend =.017). In this population, the absolute rates over 10 years in those with low and high {beta}-carotene intake were 181.8 and 81.7 cases per 10 000 women in never smokers and 174.0 and 368.3 cases per 10 000 women in ever smokers. Conclusions: {beta}-carotene intake was inversely associated with risk of tobacco-related cancers among nonsmokers with a statistically significant dose-dependent relationship, whereas high {beta}-carotene intake was directly associated with risk among smokers.