title: 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 x 2 x 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.
authors.affiliation: Department of Nutrition, University of California, Davis, CA 95616, USA. ajclifford@ucdavis.edu
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Clifford
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Noceti
authors.forename: Amy
authors.initials: A
authors.lastname: Block-Joy
authors.forename: Torin
authors.initials: T
authors.lastname: Block
authors.forename: Gladys
authors.initials: G
authors.lastname: Block
mesh_terms: D019587:Dietary Supplements; D004906:Erythrocyte Count; D004912:Erythrocytes / Q000187:drug effects / Q000378:metabolism*; D005247:Feeding Behavior; D005260:Female; D005492:Folic Acid / Q000008:administration & dosage / Q000097:blood* / Q000494:pharmacology*; D006801:Humans; D011247:Pregnancy; D012727:Sex Characteristics; D014815:Vitamins
chemical_list: D014815:Vitamins; D005492:Folic Acid