Folate status and concentrations of serum folate forms in the US population: National Health and Nutrition Examination Survey 2011-2

Abstract

Serum and erythrocyte (RBC) total folate are indicators of folate status. No nationally representative population data exist for folate forms. We measured the serum folate forms (5-methyltetrahydrofolate (5-methylTHF), unmetabolised folic acid (UMFA), non-methyl folate (sum of tetrahydrofolate (THF), 5-formyltetrahydrofolate (5-formylTHF), 5,10-methenyltetrahydrofolate (5,10-methenylTHF)) and MeFox (5-methylTHF oxidation product)) by HPLC-MS/MS and RBC total folate by microbiologic assay in US population ≥ 1 year (n approximately 7500) participating in the National Health and Nutrition Examination Survey 2011-2. Data analysis for serum total folate was conducted including and excluding MeFox. Concentrations (geometric mean; detection rate) of 5-methylTHF (37·5 nmol/l; 100 %), UMFA (1·21 nmol/l; 99·9 %), MeFox (1·53 nmol/l; 98·8 %), and THF (1·01 nmol/l; 85·2 %) were mostly detectable. 5-FormylTHF (3·6 %) and 5,10-methenylTHF (4·4 %) were rarely detected. The biggest contributor to serum total folate was 5-methylTHF (86·7 %); UMFA (4·0 %), non-methyl folate (4·7 %) and MeFox (4·5 %) contributed smaller amounts. Age was positively related to MeFox, but showed a U-shaped pattern for other folates. We generally noted sex and race/ethnic biomarker differences and weak (Spearman's r< 0·4) but significant (P< 0·05) correlations with physiological and lifestyle variables. Fasting, kidney function, smoking and alcohol intake showed negative associations. BMI and body surface area showed positive associations with MeFox but negative associations with other folates. All biomarkers showed significantly higher concentrations with recent folic acid-containing dietary supplement use. These first-time population data for serum folate forms generally show similar associations with demographic, physiological and lifestyle variables as serum total folate. Patterns observed for MeFox may suggest altered folate metabolism dependent on biological characteristics.

Keywords: 5-Methyltetrahydrofolate; Folate oxidation products; Folate vitamers; Liquid chromatography–MS/MS; National Health and Nutrition Examination Survey; Non-methyl folate; Unmetabolised folic acid.

Figure 1

Mean absolute (panel a) and relative (panel b) contribution of folate forms to serum total folate by weighted decile of serum total folate in the US population ≥1 year, NHANES 2011–2. Serum folate forms and serum total folate (sum of folate forms including MeFox) were measured by HPLC-MS/MS. Non-methyl folate represents the sum of three minor forms tetrahydrofolate, 5-formyltetrahydrofolate and 5,10-methenyltetrahydrofolate. 5-MethylTHF, 5-methyltetrahydrofolate; MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methylTHF; UMFA, unmetabolized folic acid.

Figure 2

Spearman correlation between various folate biomarkers by age group in the US population ≥1 y, NHANES 2011–2. Only statistically significant correlations are shown (see supplementary Table S4 for complete information). Serum folate forms and serum total folate (sum of folate forms including MeFox) were measured by HPLC-MS/MS. Non-methyl folate represents sum of three minor forms: tetrahydrofolate, 5-formyltetrahydrofolate and 5,10-methenyltetrahydrofolate. RBC total folate was measured by microbiologic assay. 5-MethylTHF, 5-methyltetrahydrofolate; FOL, folate; MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methylTHF; RBC, red blood cell; UMFA, unmetabolized folic acid.