Dietary intake and biological measurement of folate: a qualitative review of validation studies.

Folate is a nutrient of major health significance, but its dietary intake assessment is particularly complex to quantify through traditional approaches. Attempts have been made to validate dietary instruments for assessing folate intake against circulating concentration biomarkers. However, this requires careful attention on various methodological issues. We conducted a qualitative review of 17 recently published validation studies to identify these issues. The majority of the tested instruments were self-administered food frequency questionnaires while the biomarker most frequently used was serum/plasma folate. Seasonality was not considered in most studies. Little attention was given to using updated food composition databases based on reliable chemical methods and including fortified foods and dietary supplements. Time sequence of the test instrument and the reference biomarker used was often ambiguous, and reference periods did not always match. Correlation coefficient was the metric most commonly used, and correlations between dietary folate intake and blood folate concentration varied from weak to moderate (r = 0.05-0.54). The correlations were stronger when dietary supplement use was considered, and when serum/plasma rather than red blood cell folate was used. This review summarises issues that need to be considered in future studies intending to validate instruments for dietary folate assessment against concentration biomarkers.