Quantifying folate bioavailability: a critical appraisal of methods.

Abstract

Dietary reference intakes for folate rely on a good estimate of folate bioavailability from the general diet. In this review, current methods for quantifying the bioavailability of dietary folate and specific folate vitamers in humans are reviewed. Emphasis is on isotopic labeling techniques that have been developed during the past 15 years. Most reported studies applied single-dose designs, in which blood or urine concentrations of folate are measured for several hours after oral folate administration. To obtain a measurable biochemical response, however, relatively high doses of folic acid are administered and individuals are often saturated with large doses of folic acid prior to study. The effect of this on folate absorption and metabolism is poorly understood. Therefore, study designs in which multiple oral doses are administered are preferred. Several such studies, both with unlabeled and isotopically labeled folic acid, are discussed. Although many studies have been performed on the bioavailability of specific folate compounds and of folate from single foods, reliable data in which the bioavailability of folate from total diets have been measured are currently lacking. A multiple oral dose design is the best approach for measuring folate bioavailability because there are several serious drawbacks to designs based on the use of a single oral dose. Studies on folate bioavailability from total diets are urgently required in order to evaluate current recommendations for folate intake.