OBJECTIVE: To review the evidence on vitamin D (VTD) insufficiency and deficiency from a Canadian perspective and to highlight some of the known and evolving implications of insufficiency or deficiency for health.

QUALITY OF EVIDENCE: PubMed was searched for articles on VTD insufficiency or deficiency and the role they play in various diseases and conditions. Level I and II evidence indicates that lack of VTD has a major role in short- and long-latency diseases.

MAIN MESSAGE: The long winters in Canada and lack of exposure to the sun contribute to lower levels of VTD among Canadians in late winter and spring. Currently recommended levels of fortification and supplementation are likely not high enough to restore adequate levels of VTD in the body. Repletion and maintenance therapy might be needed.

CONCLUSION: Many Canadians are at risk of VTD insufficiency or deficiency. Assessment of VTD status is important because optimal levels of VTD have been determined for various conditions. Low levels of VTD have negative implications for bone health and the health of other cell types.