Serum #iodine and #breast cancer risk: A prospective nested case-control study stratified for selenium levels

Iodine has been suggested to protect against breast cancer, but there are no epidemiological 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. USA), or in areas where both are low (e.g. Northern Europe).

The aim of this study was to investigate the association between pre-diagnostic serum iodine levels and subsequent breast cancer risk, and to investigate if this potential association was modified by selenium levels. Methods: The MalmΓΆ Diet and Cancer Study provided pre-diagnostic serum samples and the current analysis included 1159 breast cancer cases and 1136 controls. Levels of baseline serum iodine and selenium were analyzed.

A logistic regression analysis yielded odds ratios (OR) 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 vs. 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-value for 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 serum exposure may decrease the risk of breast cancer.