Previous cohort studies examining the association of serum antioxidant levels and risk of colorectal cancer have used a single (baseline) measurement only. In the present study, we assessed the association of serum levels of eight antioxidant nutrients in relation to risk of colorectal cancer, using repeated measurements.
Data on a subsample of women in the Women's Health Initiative with repeated measurements of serum retinol, α-carotene, β-carotene, β-cryptoxanthin, lutein+zeaxanthin, lycopene, α-tocopherol and γ-tocopherol during follow-up were included in the analysis. Cox proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (CIs).
Among 5477 women with baseline serum antioxidant values, 88 incident cases of colorectal cancer were identified over a median follow-up time of 12 years. Serum antioxidants measured at baseline generally showed no association with risk of colorectal cancer, although serum β-carotene at baseline showed a non-significant inverse association with colon cancer alone. Furthermore, using the repeated measurements of β-carotene, the average of all measurements was inversely associated with risk of both colorectal and colon cancer: HRs for highest vs lowest tertile 0.54, 95% CI 0.31-0.96, and 0.47, 95% CI 0.25-0.88, respectively. No associations were seen with other antioxidant nutrients in the repeated measure analyses.
In this study, baseline levels of antioxidant nutrients were not associated with risk of colorectal or colon cancer; however, using repeated measures, a relatively high serum level of β-carotene (average of all measurements) was inversely associated with risk of colon and colorectal cancer in postmenopausal women.