To the Editor In a recent publication by Yao et al,1 serum level of vitamin D was associated with lower risk of breast cancer morbidity and mortality, most prominently among premenopausal women, based on their findings from a large, well-characterized prospective cohort providing compelling observational evidence. Vitamin D has been shown to suppress cell proliferation and induce apoptosis in a variety of cancer cell models including breast cancer.2 It has been found that vitamin D promotes the inhibition of cancer cell proliferation by the suppression of survivin.2 Survivin is the smallest member of the inhibitor of apoptosis protein family that is overexpressed in virtually all cancer types, but hardly detectable in normal differentiated tissues.2,3 Accumulating data have suggested that survivin expression in cancer is associated with tumor progression, poor prognosis, drug resistance, and shorter patient survival.2,3 Concordantly, positive expression of survivin has been associated with a significantly higher risk of recurrence and decreased overall survival rates in patients with breast cancer.3 Although it is described as a biomarker predictive of aggressive cancer, survivin has been corroborated to be the key player of a mechanism by which myocytes at risk of apoptosis retain their viability.4 Survivin myocardial expression after acute myocardial infarction (AMI) has been related to the survival of at-risk myocardium and to a more favorable remodeling after AMI restraining tissue damage and improving functional outcome.4 Noncancer causes of death have been reported highest in patients with a number of cancers including breast cancer.5 Women with breast cancer seem to be at increased risk for cardiovascular diseases.5 In this category of patients, risk of noncancer death has been detected to surpass that of cancer deaths, particularly for young patients in the first year after diagnosis, more than 40% of deaths are from heart disease.5 All these contentions led us to hypothesize that vitamin D supplementation may not be advisable in young women with breast cancer. We speculate that vitamin D might adversely affect outcomes during the acute phase of cardiovascular conditions, such as AMI suppressing survivin expression, taking into account the enhanced risk status of women with breast cancer. We suggest that vitamin D should be considered for use in women with breast cancer after excluding potential risk factors for AMI and performing a careful surveillance by cardiovascular disease screening. Research studies are needed to outline primary end points in breast cancer and also to define threshold vitamin D levels above which supplementations might not be safe for the heart.