Erythrocyte Folate and Its Response to Folic Acid Supplementation Is Assay Dependent in Women ,

Abstract

Optimizing folate status requires continued monitoring of erythrocyte (RBC) folate and folate intake. The accuracy of RBC folate assays remains a concern. Therefore, we measured RBC folate with 4 different assays, examined the interassay correlations, and compared RBC folate with folate intake as measured by an abbreviated folate-targeted food/supplement screener. The screener had 21 questions (19 diet, 2 supplement) and measured usual and customary intakes of dietary folate equivalents (DFEs). Our design was a 4 × 2 × 2 factorial, 4 assays in pregnant and nonpregnant women before and after each group received a folic acid supplement (1814 nmol/d) for 30–60 d. Folate assays included L. casei, chemiluminescence, GC-MS, and radioassay (RA). Baseline RBC folate levels ranked low to high by assay (mean ± SE) were as follows: 1155 ± 44 nmol/L (L. casei) < 1390 ± 43 nmol/L (chemiluminescence) < 1531 ± 39 nmol/L (GC-MS) < 1727 ± 55 nmol/L (RA) (P < 0.0001). Supplementation raised RBC folate levels (mean ± SE) as follows: 138 ± 63 nmol/L (chemiluminescence) < 267 ± 64 nmol/L (GC-MS) = 285 ± 75 nmol/L (L. casei) < 351 ± 87 nmol/L (RA). Pregnant women had higher RBC folate than nonpregnant women using chemiluminescence and RA. Interassay correlations (r) ranged from 0.4679 to 0.8261 (P < 0.001). Correlations of RBC folate with folate intake ranged from 0.2676 to 0.4622 (P < 0.0004). We conclude that RBC folate levels are assay dependent, as is the definition of optimized status; there continues to be a need for an accurate assay of RBC folate. RBC folate correlated with total folate intake using a folate-targeted food/supplement screener.