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Vitamin D and Cancer Vitamin D supplements have been widely marketed for their claimed anticancer properties. The mechanisms whereby vitamin D reduces risk of cancer incidence and death include regulating multiple signaling pathways involved in proliferation, apoptosis, differentiation, inflammation, invasion, angiogenesis and metastasis, as well as regulating microRNA expression and may affect cancer stem cell biology. Associations have been shown in observational studies between low vitamin D levels and the risk of development of several types of cancer including bladder, breast (4), colorectal (5), liver (6), and renal (7). Meta-analyses of observational studies found significant reduced risk of cancer incidence related to individual vitamin D intake for colorectal (5), lung (8) and renal (7) cancer. While randomized controlled trials have not confirmed that vitamin D supplements reduce the risk of cancer incidence, a meta-analysis of five RCTs of vitamin D supplementation including 75,241 participants of whom 1107 died from cancer, found a 13% (95% CI, 4 to 21%) reduction in cancer deaths (9).