Vitamin D is a family of prohormones involved in calcium and phosphorus metabolism; it is also directly involved in bone health. The most common isoforms are ergocalciferol (D2), which is found in plants, and cholecalciferol (D3), which is produced by human skin with UV sunlight exposure.1,2 Blood levels reflect exposure to sunlight, skin pigmentation, and dietary intake, among other factors.
The recommended daily dietary allowance for vitamin D is 15 μg (600 International Units [IUs]) for people aged 1 to 70 years and 20 μg (800 IU) daily after age 70 years.3,4 Dietary sources include fish, eggs, fortified cow’s milk, and fruit juice; vitamin D dietary supplements can also be purchased over the counter.
Epidemiological research suggests that cancer mortality might be higher in northern latitudes where people tend to experience less sun exposure, and hence, lower levels of vitamin-production in the skin, sparking interest in a possible connection between vitamin D and cancer risk and outcomes. Preclinical studies suggest possible biological mechanisms for a relationship between vitamin D and tumor growth, including cancer cell inhibition, anti-inflammatory and antiangiogenic effects in vitro, and promotion of apoptosis — bolstering interest in a possible connection.
Exaggerated claims about the evidence base and known anticancer benefits of vitamin D are, however, widespread online. A number of clinical studies were recently undertaken to scientifically assess the potential roles of vitamin D intake and circulating levels of 25(OH)D (the biologically active prehormone formed by the liver’s hydroxylation of cholecalciferol) with cancer risk and cancer outcomes.