Cancer-related behavior of vitamin supplement users

Epidemiological studies suggest that certain vitamin supplements may reduce the risk of some cancers. However, observational studies can be compromised by confounding, because supplement use is related to other factors that affect cancer risk. The purpose of this paper is to identify cancer-related behaviors that could confound studies of the associations between vitamin supplement use and cancer risk. Data are from a random digit dial survey to monitor cancer risk behavior in adults in Washington State (n = 1449). Unconditional logistic regression was used to examine whether regular supplement users were more likely to practice other cancer-related behaviors than nonusers, after adjustment for age, education, and smoking. Among women, supplement users were more likely to have had a sigmoidoscopy [odds ratio (OR), 2.3; 95% confidence interval (CI), 1.2-4.5], hemoccult (OR, 2.3; CI, 1.5-3.5), or mammogram (OR, 1.5; CI, 1.0-2.1) in the past 2 years. Among men, supplement users were twice as likely to have had a prostate-specific antigen test (OR, 2.2; CI, 1.3-3.7) and to regularly take aspirin (OR, 1.7; CI, 1.1-2.6). Supplement users were statistically significantly more likely to exercise regularly, eat four or more servings of fruits and vegetables per day, follow a low-fat diet pattern, and believe in a connection between diet and cancer. The association was especially strong for fruits and vegetables (women, OR, 1.9; and CI, 1.3-2.6; men, OR, 2.4; CI, 1.6-3.8). Those investigating the benefits and risks of vitamin and mineral supplements need to be aware of the lifestyle characteristics of supplement users to assess the potential for bias in their studies.