Serum Iodine and Breast Cancer Risk: A Prospective Nested Case–Control Study Stratified for Selenium Levels

Background: Iodine has been suggested to protect against breast cancer, but there are no epidemiologic studies on individual risk. An interesting finding is that in areas where the exposure to both selenium and iodine are high (e.g., Japan), the risk of breast cancer is lower than in areas where selenium is high and iodine low (e.g., United States), or in areas where both are low (e.g., Northern Europe). The aim of this study was to investigate the association between prediagnostic serum iodine levels and subsequent breast cancer risk, and to investigate if this potential association was modified by selenium levels.

Methods: The Malmö Diet and Cancer Study provided prediagnostic serum samples and the current analysis included 1,159 breast cancer cases and 1,136 controls. Levels of baseline serum iodine and selenium were analyzed. A logistic regression analysis yielded ORs with 95% confidence intervals adjusted for potential confounders.

Results: There was no evidence of an overall association between iodine levels and risk of breast cancer. Among women with high selenium levels (above the median), high iodine levels were associated with a lower risk of breast cancer; the OR for above versus below the median was 0.75 (0.57–0.99). The corresponding OR for women with low selenium was 1.15 (0.87–1.50), and the P_interaction was 0.06.

Conclusions: The combination of high serum iodine levels and high selenium levels was associated with a lower risk of breast cancer.

Impact: A high iodine and selenium exposure may decrease the risk of breast cancer.