Seasonalaffectivedisorder (SAD) is a polyfactorial and polygenetic disorder that involves biological and psychological sub-mechanisms that differentially involve depression, seasonality, circadian rhythms, retinal sensitivity, iris pigmentation, sleep factors, and the neurotransmitters involved with these systems.
Within the framework of the polyfactorial conceptualization of SAD, we review the possiblecontributions of vitamin D3 with respect to the aforementioned sub-mechanisms.
We hypothesize that rather than functioning primarily as a proximal or direct sub-mechanism in the etiology of SAD, vitaminD likely functions in a more foundational and regulative role in potentiating the sub-mechanisms associated with the depressive and seasonality factors. There are several reasons for this position:
1. vitaminD levels fluctuate in the body seasonally, with a lag, in direct relation to seasonally-available sunlight;
2. lower vitaminD levels have been observed in depressed patients (as well as in patients with other psychiatric disorders) compared to controls;
3. vitaminD levels in the central nervous system affect the production of both serotonin and dopamine; and
4. vitaminD and vitaminD responsive elements are found throughout the midbrain regions and are especially concentrated in the hypothalamus, a region that encompasses the circadian timing systems and much of its neural circuitry.
We also consider the variable of skinpigmentation as this may affect levels of vitaminD in the body.
We hypothesize that people with darker skinpigmentation may experience greater risks for lower vitaminDlevels that, especially following their migration to regions of higher latitude, could contribute to the emergence of SAD and other psychiatric and physical health problems.
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