About 10% of Caucasians have the MTHFR 677 TT genotype, which is responsible for a less-active thermolabile variant of 5, 10-methylenetetrahydrofolate reductase [7] and associated with lower folate levels [28]. In this study, patients with a BMI above 35 kg/m2 and with the risk-associated MTHFR 677 TT genotype had significantly lower concentrations of serum folate than C allele carriers. Our results are in line with the study by Bueno et al. [29], where folate serum levels were lower in 781 adults from a Spanish population with the MTHFR 677 TT genotype compared to the wild-type genotype. However, it cannot be excluded that the risk of low-folate status associated with the MTHFR 677 TT genotype may depend on a combination with other polymorphisms, as previously reported [29,30]. Folate consumption also influences folate status. Previously, serum folate concentrations were measured in young Japanese women. The intake of dietary folates was calculated based on dietary records. Even if taking in the same amounts of folates, patients with the TT genotype had lower folate concentrations. The authors suggested that people with the TT genotype consume more folates in their diet [31]. On this basis, it can be concluded that the MTHFR C677T variant contributes to disturbances in the folate cycle, as folate cannot be sufficiently converted into a biologically active form. Additionally, although the MTHFR 677 TT genotype was not directly related to BMI among the studied Polish adults, it does have a detrimental effect on folate concentrations in individuals with high BMI values. An important strength of our study is that it is the first to consider the influence of radical weight reduction on folate status according to the MTHFR genotype. Our data showed that the folate-lowering effect of the TT genotype was pronounced among participants with excess body weight and low baseline folate. Radical weight loss significantly modified this interaction. The mechanisms linking fat accumulation and folate deficiency remain unknown. It has previously been reported that the folate serum concentration was lower in overweight/obese people and that it was independent of the total folate intake [32,33]. Similarly, in an earlier Polish study, participants with high BMI values (n = 213) had 8.5% lower serum folate concentrations than the controls [25]. An analysis carried out by Kim et al. is in line with the findings mentioned above. The authors found a significant negative correlation between serum folate concentrations and BMI values in a group of 1462 pregnant Korean women [33]. The folate deficiency and altered folate metabolism in individuals with high body fat can be explained by the fact that adiposity is associated with systemic oxidative stress. Fat mass expansion impairs the synthesis of adipocyte-derived mediators and causes inflammation, which is commonly associated with increased ROS generation [34,35], resulting in an increased need for folates. The beneficial effect of folate is partially attributed to its maintenance of the cellular redox status by decreasing oxidative stress [36]. In our study, positive relationships were noted between levels of folate and vitamin B12 in patients with MTHFR 677 CC and CT genotypes. The metabolism of folate is associated with the action of vitamins B6 and B12. Insufficient concentrations of these or an ineffective MTHFR enzyme can result in a decrease in methylation and in the higher production of toxic metabolites. For instance, vitamin B12 and folate are involved in the metabolic pathways of asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA), which are toxic amino acids that inhibit nitric oxide (NO) production. Thus, it can be hypothesized that folate and vitamin B12 deficiency results in high ADMA or SDMA levels and in the development of ADMA-related pathologies in individuals with obesity [22]. This can, at least in part, be explained by altered patterns of epigenetic modifications, together with the expansion of fat mass related to a greater concentration of serum lipid peroxides [37].Variants of MTHFR can increase the genetic risk of obesity [38]. The functional integrity of the enzymes involved in folate/methionine cycles is necessary for the synthesis of the methyl-donor groups that are responsible for proper DNA methylation. DNA methylation is involved in regulating gene expression, including the leptin gene, which increases the risk of developing obesity [39]. However, the results of this study showed no statistical differences between obese and non-obese individuals in terms of the prevalence of the MTHFR C677T polymorphism, which is consistent with results obtained in a young population in Mexico [40], in 421 patients of Polish origin [41] and in a population of Brazilian patients [19]. The advantage of this study is that it involved patients with a wide range of BMIs, which allowed a comparison to be made of the distribution of genotypes of MTHFR C677T across four large groups categorized according to BMI (BMI 18,42,43]. Further studies on a larger number of patients are still necessary to clarify the role of the MTHFR C677T genotype in the development of obesity.

This study had some limitations. We have not considered other possible genetic and metabolic factors which are known to affect the balance between the folate cycle and the methionine cycle. Homocysteine concentrations were not measured, data on which could provide additional insight into 5-MTHF synthesis or bioavailability.

In conclusion, the MTHFR 677 TT genotype was not found to have a significant influence on BMI or the obesity risk in codominant, dominant or recessive inheritance models. Patients with obesity and with the MTHFR TT genotype had significantly lower folate concentrations. A disturbed balance between folate and vitamin B12 concentrations after significant weight loss in individuals with the MTHFR T677T genotype is suggested by the data. Further large-scale studies are still needed to confirm our results.