“…Based on findings from genetic and neurobiological studies, which suggested that disturbed serotonergic neurotransmission underlies the development of both, depressive disorders (Kraus et al ., 2017) as well as alexithymia (Gong et al ., 2014; Terock, et al ., 2018; Li et al ., 2020; Terock, et al ., 2021a), it could be speculated that vitamin D impacts on these conditions via an interaction with the central serotonergic metabolism. In fact, there is growing evidence that vitamin D, binding to intracellular vitamin D receptors and subsequently to promotor regions (Uitterlinden et al ., 2004), regulates the expression of various genes of the serotonergic pathway: For example, the expression of tryptophan hydroxylase 2 (TPH-2), the initial and rate-limiting enzyme in the biosynthesis of serotonin, was found to be associated with various neuropsychiatric disorders, including depression (Ottenhof et al ., 2018) and its expression is regulated by activated vitamin D (Kaneko et al ., 2015).…”