Vitamin D is a fat-soluble vitamin having well known effects on bone health and calcium homeostasis. Vitamin D3 (cholecalciferol) is the chemical form of vitamin D in humans, synthesized from photochemical conversion of 7-dehydrocholesterol in the skin. It is activated by hydroxylation in the liver and kidney to form 1,25-dihydroxyvitamin D3 (calcitriol), which exerts its biochemical and physiological effects through the vitamin D receptor (VDR). In addition to maintenance of skeletal mineral balance, vitamin D has multiple other roles in the body including regulation of cell cycle, cellular differentiation and immune functions, neuroprotection, antioxidant action, xenobiotic metabolism as well as antimicrobial, anti-inflammatory and antitumor effects. Vitamin D is involved in the expression of several genes having oncogenic potential. The anti-cancer effects of vitamin D are mediated via its stimulation of apoptosis and cell differentiation and inhibition of invasion, metastasis and angiogenesis.

In the last two decades, numerous studies have addressed the potential of vitamin D in curtailing development and progression of multiple malignancies and/or improving their prognosis. Most of the data accumulated over this time has linked lack of exposure to sunlight and deficiency of vitamin D to an increased risk of several cancer types but a few studies have yielded conflicting results particularly on the therapeutic role of vitamin D supplementation. Recent findings have provided detailed insights into the mechanism of anti-cancer actions of vitamin D. Disruption of the VDR signalling pathway at multiple levels seems to be at the core of oncologic transformation in various types of cancers. Based on the available mechanistic evidence, future research trials using novel pharmacological interventions targeting the vitamin D pathway need to be instituted in order to derive conclusive clinical recommendations. At present, adequacy of vitamin D status appears to be a pertinent factor in mitigating the risk of cancer development, thus maintenance of vitamin D levels through appropriate sun exposure and dietary intake is advisable.