Wednesday, October 12, 2016

Major Depression is More Common in More Northerly Latitudes in Canada

Major depression is thought to be more common in the winter than the summer months in Canada (e.g. follow this link), and changing circadian patterns are believed to have a role in this. It has sometimes been suspected that there would be a north-south latitude gradient as well. However, the small number of studies that have looked at this have failed to find an association, see Partonen et al. and Grimaldi et al.  Using data from many large scale Canadian surveys, were able to examine this association with a much larger sample size than was previously possible. These surveys included a measure of past year major depressive episodes called the Composite International Diagnostic Interview, Short Form and by linking to postal code files we were able to determine the approximate latitude of each respondent. A small gradient was found, with more northerly latitudes having a higher prevalence. A link to be the abstract is available here. It is possible that the etiology of such depression is related to factors such as sunlight exposure or shifting circadian patterns with more northerly latitudes. Of course, the difference could also be due to social determinants. Additional studies will be needed to determine this, but with adjustment for many potential determinants (age, sex, marital status, income, education) the association persisted. These results may contain clues to understanding the causes of depression more clearly, but they certainly also have implications for planning health services in more northerly places.