Background: The use of bisphosphonates in the results of breast cancer is increasing for patients with metastatic and early-stage disease. Vitamin D deficiency may be a risk factor for breast cancer development and is very common in the northeastern US. Vitamin D deficiency appears to increase the risk of hypocalcemia following IV bisphosphonate . The purpose of this study is to evaluate the prevalence of vitamin D deficiency following the initial diagnosis of breast cancer.

Methods: We analyzed fasting morning blood from premenopausal women, from the northeastern US, diagnosed with stage I/II breast cancer. Bloods were obtained following curative surgery, and prior to initiating . Serum was archived, stored at -70^o and analyzed in batches in a research laboratory for 25-hydroxyvitamin D (25-OHD; Diasorin RIA), albumin-corrected calcium, phosphate, parathyroid hormone (PTH), markers of bone turnover, FSH, LH, estradiol and sex hormone binding globulin (SHBG). Information on demographic, clinical and tumor characteristics were collected and bone mineral density (BMD) was measured (Hologic 4500). Data were analyzed using SAS version 9.

Results: We analyzed sera from 36 premenopausal women, none of whom were taking calcium or vitamin D supplements. The mean age was 42 (range 27-54). The mean serum 25-OHD was 22.6 ng/dl, (SD 7.4) and similar among all racial/ethnic groups. Only 4 patients met contemporary criteria for optimal levels (>30 mg/ml); 18 patients had levels between 20-30 mg/ml, 14patients had "insufficient" levels (<20 mg/ml), and 3 patients had "deficient" levels (<12). Serum 25-OHD was inversely associated with serum PTH and free estradiol, and directly associated with SHBG. There were no associations between serum 25-OHD and serum calcium, phosphate, bone turnover markers, or BMD.

Conclusions: Suboptimal vitamin D levels are surprisingly common among premenopausal women with early stage breast cancer. The associations between serum 25-OHD, SHBG and free estradiol, suggest that serum levels of vitamin D binding protein may be influencing 25-OHD measurements. Serum 25-OHD should be measured and low levels supplemented before initiating adjuvant bisphosphonate , as vitamin D deficiency may increase the risk of hypocalcemia.