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STUDY QUESTION Is there any relationship between vitamin D [25 (OH) vitamin D], total plasma homocysteine and methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism in women with recurrent pregnancy losses (RPL)? SUMMARY ANSWER Women with MTHFR 677TT (homozygous mutation, TT) genotype have significantly lower vitamin D levels, higher homocysteine and natural killer (NK) cell cytotoxicities than those of women with MTHFR 677CC (wild type, CC) and 677CT (heterozygous mutation, CT) genotypes. WHAT IS KNOWN ALREADY Vitamin D insufficiency, MTHFR C677T polymorphism and hyperhomocysteinemia have been reported as risk factors for RPL. However, the relationship between these risk factors is not known in this population. STUDY DESIGN, SIZE, DURATION This is a retrospective cross-sectional study, including 837 women with RPL, who were enrolled in Reproductive Medicine and Immunology, Chicago Medical School, between 2012 and 2017. PARTICIPANTS/MATERIALS, SETTING, METHODS Women with two or more RPL prior to 20 weeks of gestation were included. To investigate whether the MTHFR C677T polymorphism affects the levels of homocysteine and vitamin D as well as immune parameters in women with RPL, biochemical data, such as plasma total homocysteine and serum vitamin D levels, and immune parameters, including NK cell cytotoxicity, were analyzed by MTHFR C677T genotype (CC, CT and TT). MAIN RESULTS AND THE ROLE OF CHANCE The serum level of vitamin D in TT was significantly lower when compared with those of CT (P = 0.001) and CC (P = 0.003), while the level of homocysteine in TT was significantly higher than those in CT (P = 0.01) and CC (P = 0.01). NK cytotoxicity in TT was significantly higher than that of CC (P = 0.04) but not CT (P = 0.09). There was a significant negative correlation between the levels of vitamin D and homocysteine in TT (r = −0.357, P &amp;lt; 0.01). In multivariate analysis, vitamin D insufficiency (&amp;lt;30 ng/ml) was an independent risk factor for hyperhomocysteinemia (adjusted odds ratio 1.89, 95% CI 1.41–2.52). LIMITATIONS, REASONS FOR CAUTION The study was retrospective and included only women with RPL but not healthy fertile controls. In addition, folic acid, vitamin B6 and B12 intake, which could modify the level of homocysteine and vitamin D, were not investigated. Thus, a considerable part of women might have folic acid and vitamin D supplementation and prenatal vitamin pills, and there are probable confounders in this study associated with unrestricted vitamin supplementation. Therefore, the findings should be carefully interpreted and applied to RPL women with MTHFR gene polymorphism. WIDER IMPLICATIONS OF THE FINDINGS The findings attained in this analysis regarding the MTHFR polymorphism and its relationship with vitamin D, homocysteine and NK cytotoxicity may aid in uncovering the underlying etiology and mechanism for RPL. The study highlights an interplay between nutrition and immune responses in RPL. STUDY FUNDING/COMPETING INTEREST(S) No external funding was received for this study. None of the authors have any conict of interest to declare. TRIAL REGISTRATION NUMBER N/A.

Effects of MTHFR C677T polymorphism on vitamin D, homocysteine and natural killer cell cytotoxicity in women with recurrent pregnancy losses