Purpose: Prior prospective studies, including our own, have evaluated total plasma 25-hydroxyvitamin D [25(OH)D] and breast cancer risk with inconsistent results. However, recent studies suggest that some vitamin D functions may be more relevant to the unbound (free) fraction of 25(OH)D. Vitamin D binding protein (DBP) influences the free 25(OH)D levels and thus possibly the biological activities of vitamin D.
Methods: We conducted a case-control study nested within the Nurses' Health Study II to evaluate the association of plasma free 25(OH)D and DBP with breast cancer risk in predominantly premenopausal women. Plasma samples were assayed for 25(OH)D and DBP in 584 case-control pairs. Free 25(OH)D levels were calculated based on plasma levels of total 25(OH)D, DBP, and a constant value representing average albumin levels. Conditional logistic regression was used to estimate relative risks (RRs) and 95% confidence intervals (CIs).
Results: We found no association between plasma calculated free 25(OH)D and risk of breast cancer overall (highest vs. lowest quartile RR 1.21, 95% CI 0.83-1.77, trend test p value = 0.50). No association was observed for plasma DBP as well (highest vs. lowest quartile RR 0.95, 95% CI 0.67-1.36, trend test p value = 0.96). Results were similar by tumor hormone receptor status. Neither the total nor the calculated free 25(OH)D and breast cancer association substantially varied by plasma DBP levels.
Conclusions: Our study does not support an important role of either calculated circulating free 25(OH)D or circulating DBP levels in breast cancer risk among predominantly premenopausal women.