Estimation of trends in serum and RBC folate in the U.S. population from pre- to postfortification using assay-adjusted data from the NHANES 1988-2010

Abstract

The NHANES has monitored folate status of the U.S. population from prefortification (1988-1994) to postfortification (1999-2010) bymeasuring serumand RBC folate concentrations. The Bio-Rad radioassay (BR)was used from1988 to 2006, and the microbiologic assay (MBA) was used from 2007 to 2010. The MBA produces higher concentrations than the BR and is considered to be more accurate. Thus, to bridge assay differences and to examine folate trends over time, we adjusted the BR results to be comparable to theMBA results. Postfortification, assay-adjusted serumand RBC folate concentrationswere 2.5 times and 1.5 times prefortification concentrations, respectively, and showed a significant linear trend (P < 0.001) to slightly lower concentrations during 1999-2010. The postfortification prevalence of low serum (<10 nmol/L) or RBC (<340 nmol/L) folate concentrations was ≤1%, regardless of demographic subgroup, compared with 24% for serum folate and 3.5% for RBC folate prefortification, with substantial variation among demographic subgroups. The central 95% reference intervals for serum and RBC folate varied by demographic subgroup during both pre- and postfortification periods. Age and dietary supplement use had the greatest effects on prevalence estimates of low folate concentrations during the prefortification period. In summary, the MBA-equivalent blood folate concentrations in the U.S. population showed first a sharp increase from pre- to postfortification, then showed a slight decrease (17% for serum and 12%for RBC folate) during the 12-y postfortification period. TheMBA-equivalent pre- and postfortification reference concentrationswill inform countries that plan folic acid fortification or that need to evaluate its impact. © 2012 American Society for Nutrition.