Vitamin D vs Depression: Associative or Causative?

So, did you all enjoy Black Friday?  My wife went out last night to run with the bulls, or in this case, her girlfriends, to go shop all the post-Thanksgiving sales.  If you notice an improvement in the local economy, y'all can thank them for doing their part. 

As for me, I've been putting in some late nights/early mornings trying to fix up my website, take care of legal paperwork required to start my practice, and read, read, read.  Thus, I've been spending more time indoors than out.  Less sun exposure leads to lower vitamin D.  And now, lower vitamin D may be associated with risk of depression!

I mention this because in a study published earlier this month, the authors performed a cross-sectional analysis of 7,970 participants of the National Health and Nutrition Examination Survey (NHANES III) with age range 15-39 years old.  Using standardized depression scales, they concluded that those with 25OH vitamin D <50 nmol/L (20ng/mL)  were more likely to be depressed than those with 25OH vitamin D >75nmol/L (30ng/mL). 

Interesting, you say.  But this isn't really a new finding.  A prospective study from May 2008 came to similar conclusions after evaluating a cohort of 1,282 participants 65-95 years old and concluding that a reduction of 14% in vitamin D was associated with an increase risk of depression.  And an earlier cross sectional study of 80 participants from December 2006 came to similar conclusion.

However, just as with the chicken and the egg, it's not always clear whether the relationship is associative in nature or causal.  For instance, vitamin D could be low because patients are too depressed to go outside.  But a smaller study published in July concluded that those euthymic women with lower baseline vitamin D were more likely to become depressed in 3-6 years.  So we have 3 studies demonstrating an association and 1 study raising the question of cause & effect.  To hedge my bets since I'm predominantly indoors these days, I supplement my solar-induced manufacture of vitamin D with appropriate vitamin D directed by regular laboratory analysis of my 25OH vitamin D.