We would like to praise Dr. Liguori and colleagues for their elegant study showing that the use of continuous positive airway pressure (CPAP) for one week is able to revert vitamin D deficit in men.1 Recent studies have demonstrated that vitamin D deficiency has been associated with sleep disorders in different populations.2 Even being highly discussed, there are few studies relating its function to sleep.3
Currently due to the modern lifestyle, the population has remained indoors, preventing the sunlight exposure needed for the synthesis of the vitamin D precursor. It has been demonstrated that staying most of the time indoors can alter the cortisol and melatonin levels, causing a disturbance in the sleep-wake cycle.4 It is important to emphasize that in the current study the absence of clinical information related to the groups could be a bias. Vitamin D deficiency is associated with obesity, as well as weight reduction is correlated with increased serum levels of cholecalciferol.5 Thus, the increase in vitamin D levels found in male patients after the treatment with CPAP could possibly be linked to weight loss.
Another point to be considered is ethnicity. It is known that in Afro-descendent individuals with dark skin, solar exposure time must be greater for a suitable endogenous synthesis of vitamin D. They present vitamin D deficiency more frequently than other people, as well as daytime sleepiness index is higher.6 Therefore, OSA individuals with different ethnic groups could require different time of CPAP treatment to achieve the same effect on the concentrations of vitamin D.
As a perspective, we would like to emphasize the importance of evaluating the inter-individual variability. There is evidence showing that the presence of polymorphisms in the vitamin D receptor gene is associated with vitamin D deficit.7 Finally, we congratulate the authors for their work and expect that these results may help developing the research in this line, allowing a better understanding of vitamin D metabolism and its regulatory mechanisms associated with sleep.
Studies are supported by fellowship from the Associação Fundo de Incentivo à Pesquisa (AFIP) and the São Paulo Research Foundation (grant #2014/15259-2 to CH). ST and MLA received CNPq Fellowships. The authors have indicated no financial conflicts of interest.
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