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Unformatted text preview: Nutritional Methodology Erythrocyte Folate and Its Response to Folic Acid Supplementation Is Assay Dependent in Women 1,2 Andrew J. Clifford, 3 Elizabeth M. Noceti, Amy Block-Joy, Torin Block,* and Gladys Block Department of Nutrition, University of California, Davis, CA 95616; *Block Dietary Data Systems, 2634 Le Conte Avenue, Berkeley, CA 94709; and University of California, Berkeley School of Public Health, Berkeley, CA 94720 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 3060 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). Supplemen- tation 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. J. Nutr. 135: 137143, 2005. KEY WORDS: RBC folate analytical method low-income women folate-targeted food screener Adequate intake of folate is important to minimize the risk of neural tube defects (NTDs) 4 (1,2), cardiovascular disease (3), and some cancers (4). An adequate folate intake by women of child-bearing age is especially important because it can raise RBC folate levels, and has been shown to reduce the risk of NTDs by 70% (5). The RBC folate level is rated a good biomarker for nutritional status of this important nutri- ent because of its correlation with liver folate, a significant tissue store (6). Cell accumulation of folate may be key to ameliorating NTD susceptibility with increased folate intake (7). Therefore, the strength of the relations between folate intake and biomarkers of folate status such as RBC folate is of considerable interest....
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