Prediagnostic serum iodine and selenium in relation to breast cancer survival

Trace elements like iodine have been suggested to be associated with breast cancer due to their antioxidant properties and ecological differences in incidence and mortality. Yet, there are no epidemiological studies on breast cancer prognosis in relation to iodine levels. The primary aim of this study was to investigate the association between prediagnostic serum iodine levels and breast cancer survival. A secondary aim was to examine if the potential association was modified by serum selenium levels.

This study included women diagnosed with primary invasive breast cancer (1991-2013) from the Malmö Diet and Cancer Study (1991-1996). The patients were divided into quartiles depending on their serum iodine levels at baseline. Crude and multivariable-adjusted Cox regression analyses were used to obtain hazard ratios with 95% confidence intervals, for breast cancer-specific and overall mortality, with follow up until 31 December 2018. Analyses were additionally stratified by selenium levels (below versus above the median).

Out of 17 035 female participants of the Malmö Diet and Cancer Study, 918 patients with breast cancer were included in this study. The mean(standard deviation) follow-up time was 11.4(6.0) years. The adjusted hazard ratios for breast cancer-specific and overall mortality for the highest iodine quartile compared with the lowest were 1.26 (95% confidence interval 0.79 to 1.99) and 1.27 (0.92 to 1.75). Among women with high selenium, high iodine levels were associated with a high breast cancer-specific and overall mortality; hazard ratios of 1.90 (1.09 to 3.32) and 1.81 (1.26 to 2.61), respectively. High selenium levels were associated with a lower breast cancer-specific and overall mortality; the adjusted HRs were 0.69 (0.50 to 0.95) and 0.73 (0.58 to 0.91).

High serum iodine was weakly associated with mortality overall and significantly associated among women with high selenium, whereas serum selenium alone was inversely associated with mortality.