Possible contributions of skin pigmentation and vitamin D in a polyfactorial model of seasonal affective disorder
Stewart AE, Roecklein KA, Tanner S, Kimlin MG (2014) Med Hypotheses. 83(5) 517-25 Elsevier Ltd
Abstract:
Seasonal affective disorder (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 possible contributions 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, vitamin D 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. vitamin D levels fluctuate in the body seasonally, with a lag, in direct relation to seasonally-available sunlight;
2. lower vitamin D levels have been observed in depressed patients (as well as in patients with other psychiatric disorders) compared to controls;
3. vitamin D levels in the central nervous system affect the production of both serotonin and dopamine; and
4. vitamin D and vitamin D 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 skin pigmentation as this may affect levels of vitamin D in the body.
We hypothesize that people with darker skin pigmentation may experience greater risks for lower vitamin D levels that, especially following their migration to regions of higher latitude, could contribute to the emergence of SAD and other psychiatric and physical health problems.
FAB RESEARCH COMMENT:
This review draws together research into the biology of depression, and the biological actions of Vitamin D - focusing on the ways in which this powerful hormone can affect brain function, as well as physical health.
The authors present a strong case for more attention to be paid to Vitamin D deficiency or insufficiency as potential causal factor in Seasonal Affective Disorder in particular, but also depression more generally.
In addition to the evidence of a strong association between depression and low Vitamin D levels, they point out the importance of Vitamin D not only for making serotonin and dopamine (two of the neurotransmitters implicated in depression), but also in brain regions critical for both regulating emotions, and maintaining the 'body clocks' which co-ordinate multiple brain and body rhythms, keeping these aligned with day length and seasonal changes.
Vitamin D deficiency or insufficiency is widespread in the UK and many other developed countries - particularly in the winter months, as from October until March, sunlight is not strong enough to allow any Vitamin D to be made via skin exposure. However, the authors flag the particular vulnerability of individuals with darker skin, who require more sunlight exposure to make the same amount of Vitamin D.
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