Toenail trace element levels and breast cancer: a prospective study.

The associations between toenail levels of five trace elements and breast cancer risk were studied among a cohort of 62,641 US women who provided toenail clippings and were free from diagnosed breast cancer in 1982. Among 433 cases of breast cancer identified during 4 years of follow-up and their matched controls, the odds ratios comparing the highest with the lowest quintiles and adjusted for established breast cancer risk factors were as follows: for arsenic, 1.12 (95% confidence interval (CI) 0.66-1.91); for copper, 0.91 (95% CI 0.59-1.42); for chromium, 0.96 (95% CI 0.61-1.52); for iron, 0.89 (95% CI 0.56-1.40); and for zinc, 1.09 (95% CI 0.70-1.70). Among postmenopausal women, a marginally significant positive association was observed between toenail chromium levels and breast cancer risk (odds ratio = 1.71, 95% CI 0.87-3.35) (p for trend = 0.07). However, the association between chromium and breast cancer risk was inverse among premenopausal women. Although data on the validity of toenail levels of certain of these elements are limited, these results do not provide evidence for an important effect of arsenic, copper, chromium, iron, or zinc on breast cancer risk.