Validation of a food frequency questionnaire to assess folate intake of Dutch elderly people.

Folate is required for 1-carbon metabolism and deficiency in folate leads to megaloblastic anemia. Low levels of folate have been associated with increased risk of vascular disease. To investigate whether RDA of folate are met, habitual folate intake needs to be assessed reliably. We developed a FFQ to specifically measure folate intake over the previous 3 months in elderly people in the Netherlands. Major sources of folate intake, i.e. foods contributing to at least 80% of the average folate intake, were identified through an analysis of the second Dutch Food Consumption Survey for the sub-population of men and women aged 50-70. In 2000 and 2001, folate intake was estimated with this questionnaire in 1286 individuals aged 50-75 years. Concentrations of serum and erythrocyte folate served as biomarkers with which relative validity of the questionnaire was assessed. The same FFQ was repeated after 3 years in 803 subjects in order to assess long-term reproducibility. Mean folate intake was estimated to be 196 (SD 69) microg/d. Spearman correlation coefficients between folate intake and serum and erythrocyte concentrations were 0.14 (P < 0.01) and 0.05 (P = 0.06) respectively. Spearman correlations between folate intakes measured at baseline and after 3 years were 0.58 (P < 0.01). 47% of the participants were classified in the same quartiles on the two occasions. Our FFQ showed a weak correlation between folate intake and blood folate concentrations and reproducibility was acceptable. This FFQ is able to rank subjects according to their folate intake.