Garland and Garland first hypothesized that better vitamin D status lowered risk of colorectal cancer in 1980. Subsequently,
the relation between vitamin D status and colorectal cancer risk has been investigated in epidemiologic studies. Various approaches have
been used to estimate vitamin D status, including direct measures of circulating 25(OH)vitamin D levels, surrogates or determinants of
vitamin D (including region of residence, intake, and sun exposure estimates, or a combination of these). These measures of vitamin D
status have been studied in relation to colorectal adenoma, precursors of cancer, and colorectal cancer incidence and mortality.
In general, all lines of inquiry from observational studies indicate that better vitamin D status is associated with lower colorectal cancer
risk. While most of the studies have examined vitamin D status in relation to risk of incident colorectal cancer, some evidence suggests
that vitamin D may be additionally important for colorectal cancer progression and mortality. Although the influence of confounding
factors cannot be entirely excluded, the consistency of the association using various approaches to measure vitamin D, for diverse
endpoints and in diverse populations shows high consistency and is strongly suggestive of a causal association. Thus, improving vitamin
D status could be potentially beneficial against colorectal cancer incidence and mortality.