Table 6.

Inconsistencies between serum MeFox and 5-methyltetrahydrofolate concentrations in the US population ≥1 year, NHANES 2011–2

Variable	Finding
Correlation between MeFox and 5-methylTHF	Unexpectedly low (r = 0.25) considering that MeFox is an oxidation product of 5-methylTHF
Age	MeFox concentrations seemed to increase linearly with age while 5-methylTHF concentrations showed a U-shaped age pattern
Sex	MeFox concentrations did not differ by sex while 5-methylTHF concentrations were higher in females
Fasting status	MeFox concentrations were much lower in fasted (≥8 hours) compared to nonfasted (<3 hours) persons (39%) while 5-methylTHF concentrations were only a little bit lower (7%); the central 95% reference intervals for serum total folate excluding and including MeFox were almost the same in fasted persons, but not in nonfasted persons
Kidney function	MeFox concentrations were much higher (111%) in persons with poor (eGFR <60 mL/(min×1.73 m2) compared to good (eGFR ≥90 mL/(min×1.73 m2) kidney function while 5-methylTHF concentrations were only a little bit higher (22%)
BMI	MeFox concentrations were higher (21%) with higher BMI (obese vs. underweight) while 5-methylTHF concentrations were lower (32%)
BSA	MeFox concentrations were higher (11%) with higher BSA (≥2 vs. <1.5 cm×kg) while 5-methylTHF concentrations were lower (34%)
Smoking status	MeFox concentrations did not differ by smoking status while 5-methylTHF concentrations were lower (26%) in smokers

MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methylTHF; NHANES, National Health and Nutrition Examination Survey; 5-methylTHF, 5-methyltetrahydrofolate; eGFR, estimated glomerular filtration rate; BSA, body surface area.