Comparative, literature-based, analysis of the effect of vitamin D/sunlight in endometriosis between Greece and Finland

This is a literature-based comparative analysis for the delineation of the role of sunshine in vitamin D (vitD) synthesis and its impact on endometriosis, an immune, inflammatory disease. The findings, between high (Greece) and low (Finland) sun exposed countries show that the prevalence of endometriosis does not change in the general population and that vitD supplementation through sun and/or vitD food fortification does not lower the disease’s risk, thus pointing to a scientific myth that has to be either re-examined or abandoned after thorough further research. *Correspondence to: Vassiliadis S, Association of Greek Immunology Graduates, 33 Voriou Ipirou Street, Maroussi, 15125 Athens, Greece, Tel: +3


Introduction Background concerning vitamin D in healthy subjects
Vitamin D is produced in the skin between March and October, with the highest quantities being produced in the summer months. The intake of vitD both through the skin and from dietary sources is described as the vitD level in the blood, or serum 25 hydroxy vitD (25(OH)D) concentration. Vitamin D is classified as a pro-hormone which exists in circulation in two major forms, 25(OH)D: 25(OH)D2 (ergocalciferol) and 25(OH)D3 (also known as cholecalciferol) [1,2]. These precursors bind to a binding protein (VDBP) to transit in the blood and reach the liver where they are modified into calcidiol (25-OH cholecalciferol) then metabolized a second time by the kidney to generate calcitriol (or 1,25 dihydroxycholecalciferol), the active form of vitD [3]. Vitamin D levels are positively associated with bone mineral density [4] and several studies have reported a link with muscle mass and strength [5]. There exists evidence that vitD insufficiency and/ or deficiency in elderly people, especially women, leads to secondary hyperparathyroidism and, consequently, to increased bone turnover and, finally, bone loss [6].
Greece is considered a country of high sunlight levels according to the Hellenic National Meteorological Service (2,600 to 2,800 hours yearly) [7]. Based on the important contribution of sunlight exposure to the production and maintenance of serum 25(OH)D levels, one might consider that 25(OH)D deficiency may be a problem limited to countries located at higher latitudes. Nevertheless, numerous studies from countries with high sunlight levels, Greece included, have shown that vitD deficiency is a common occurrence and a global health problem [8-10]. A 2016 state-of-the-art review article by Holick, a pioneer in the field of vit D synthesis, concisely outlines all the biological effects of sunlight, ultraviolet radiation, visible light and infrared radiation on vitD production and benefits for health [11]. The vitD status within different European countries shows a high variation. A serum 25(OH) D lower than 25nmol/L has been monitored in 2 to 30% of adults [12].

Discussion / Conclusions
According to the knowledge acquired thus far, vitD deficiency in the Greek population, under an average sunshine exposure of 2,700 hours yearly, constitutes an absolute paradox that matches that of the Finnish population, with an average sun exposure time of 1,300 hours per year, respectively.
In terms of endometriosis, there is no solid evidence that vitD lowers the risk of the disease, as anticipated, while increased levels of serum vitD through food fortification do not appear to ameliorate the patients' status and/or disease's prevalence.
Endometriosis is an immune, inflammatory disease and vitD, as an immunomodulator and anti-inflammatory agent, is expected to have an -at least-corrective, if not, restorative action. The above-described literature-based data, however, point to a null correlation between endometriosis and role of vitD or insinuate unknown yet mechanisms required to trigger the proper biochemical reactions towards a positive result. Therefore, the synergistic actions of other dietary bioactive compounds should be studied in order to elucidate whether vitD can be activated beneficially towards endometriosis or whether this theory has to be abandoned.