Dietary selenium intake and thyroid cancer risk in postmenopausal women.

OBJECTIVE: It has been suggested that higher selenium intake and consumption of supplements protect against several cancers. To our knowledge, epidemiologic evidence is rare and inconsistent on the association of selenium level and the risk for thyroid cancer. Therefore, the aim of this study was to examine the association between selenium intake and thyroid cancer risk in postmenopausal women using the Women's Health Initiative (WHI) database. METHODS: The WHI recruited 161 808 postmenopausal women 50 to 79 y of age between September 1, 1993 and December 31, 1998. The present study included 147 348 women 63.15 y of age (SD = 7.21) at baseline. The main exposure was baseline total selenium intake including dietary selenium measured by food frequency questionnaire (FFQ) and supplemental selenium. The outcome was thyroid cancer, which was adjudicated by trained physicians. Cox proportional hazard models were used to analyze the association. RESULTS: During a mean follow-up of 16.4 y until September 30, 2020, 442 thyroid cancer cases were identified. There was no significant association between total selenium intake and thyroid cancer risk after adjusting for multiple covariates (highest versus lowest quartile: hazard ratio [HR], 0.88; 95% confidence interval [CI], 0.60-1.29). Association between total selenium intake and the risk for papillary thyroid cancer was also not significant (highest versus lowest quartile: HR, 1.02; 95% CI, 0.66-1.52). CONCLUSIONS: The present data did not support that either total or dietary selenium intake was associated with the risk for thyroid cancer or the papillary subtype in postmenopausal women ages 50 to 79 y in the United States.

Association between selenium intake and breast cancer risk: results from the Women's Health Initiative.

PURPOSE: It has been hypothesized that selenium (Se) can prevent cancer, and that Se deficiency may be associated with an increased risk of breast cancer. However, findings from epidemiological studies have been inconsistent. The objective of this study was to assess the association between Se intake and risk of breast cancer in the Women's Health Initiative (WHI). METHODS: This study included 145,033 postmenopausal women 50-79 years who completed baseline questionnaires between October 1993 and December 1998, which addressed dietary and supplemental Se intake and breast cancer risk factors. The association between baseline Se intake and incident breast cancer was examined in Cox proportional hazards analysis. RESULTS: During a mean follow-up of 15.5 years, 9487 cases of invasive breast cancer were identified. Total Se (highest versus lowest quartile: HR 1.00, 95% CI 0.92-1.09, Ptrend = 0.66), dietary Se (highest versus lowest quartile: HR 0.99, 95% CI 0.89-1.08, Ptrend = 0.61), and supplemental Se (yes versus no: HR 0.99, 95% CI 0.95-1.03) were not associated with breast cancer incidence. CONCLUSIONS: This study indicates that Se intake is not associated with incident breast cancer among postmenopausal women in the United States. Further studies are needed to confirm our findings by using biomarkers such as toenail Se to reduce the potential for misclassification of Se status.