Relationship of several serum folate forms with kidney function and albuminuria: Cross-sectional data from the NHANES 2011-2018.

We aim to examine the relation of several folate forms (5-methyltetrahydrofolate [5-mTHF], unmetabolized folic acid [UMFA], and MeFox) with kidney function and albuminuria, which remained uncertain. The cross-sectional study was conducted in 18,757 participants from National Health and Nutrition Examination Survey 2011-2018. The kidney outcomes were reduced estimated glomerular filtration rate (eGFR) (<60 mL/min/1.73 m2), microalbuminuria (albumin-to-creatinine ratio of 30-299 mg/g), and macroalbuminuria (albumin-to-creatinine ratio ≥ 300 mg/g).Overall, there were significant inverse associations between serum 5-mTHF and kidney outcomes with significant lower prevalence of reduced eGFR (OR, 0.71; 95%CI: 0.57-0.87) and macroalbuminuria (OR, 0.65; 95%CI: 0.46-0.91) in participants in quartile 3-4 (vs. quartile 1-2; ≥34.0 vs. <34.0nmol/L; both P for trend across quartiles <0.05). In contrast, there were significant positive relationship between serum UMFA and kidney outcomes with significant higher prevalence of reduced eGFR in participants in quartile 2-4 (vs. quartile 1; ≥0.5 vs. <0.5nmol/L; OR, 2.12; 95%CI: 1.45-3.12; P for trend <0.001) and higher prevalence of macroalbuminuria in participants in quartile 4 (vs. quartile 1-3; ≥ 1.0 vs. <1.0 nmol/L; OR, 1.46; 95%CI: 1.06-2.01; P for trend <0.001). However, there was no significant associations of 5-mTHF and UMFA with microalbuminuria. In addition, there were significant positive relationships of serum MeFox with reduced eGFR, microalbuminuria and macroalbuminuria (all P for trend <0.01). In conclusion, higher 5-mTHF level, along with lower UMFA and MeFox level, were associated with lower prevalence of kidney outcomes, which may help counsel future clinical trials and nutritional guidelines regarding the folate supplement.