A Review of Evidence that Ultraviolet-B Irradiance and Vitamin D Reduce Risk for Cancer

The solar ultraviolet-B (UVB)–vitamin D–cancer hypothesis was proposed in 1980 and has been actively investigated since then. The strongest evidence is from ecologic studies that use indices of solar UVB doses with geographic data on cancer incidence and/or mortality rates. Good evidence also comes from observational studies for breast and colorectal cancer incidence as well as survival rates for several cancers. Laboratory studies have well described the mechanisms whereby vitamin D reduces risk for cancer and increases survival rates after diagnosis. The hypothesis largely satisfies Hill’s criteria for causality for breast and colorectal cancer and moderately satisfies it for several other cancers. However, only two randomized controlled trials (RCTs) found a reduction in cancer incidence for higher vitamin D3 intake. Thus, general acceptance of the UVB–vitamin D–cancer hypothesis will probably be delayed until a large-scale RCT supports it.