New results from stable isotope studies show that folic acid should not be used as a reference folate for estimating relative absorptions of natural food folates

Summary  An adequate supply of folate minimises the risk of neural tube defects, vascular disease (CHD, stroke) and possible cancers. Whilst the USA, Canada and Chile have a policy of mandatory fortification of white flour with folic acid, most countries of the EU do not permit the fortification of foodstuffs with folic acid. The UK allows folic acid fortification and in practice this has been mainly limited to some breakfast cereals and breads. Governments can only assess whether an optimal folate intake can be achieved through food sources alone if the proportion of food folate actually absorbed can be accurately established. Our recent use of stable‐isotope‐labelled folates has now questioned the reliability of experimental methods used for the past four decades to estimate the ‘relative absorption’ of food folates. Historically, the plasma 5‐methyltetrahydrofolic acid response to oral test doses of food folates are compared to the plasma response induced by a similar sized ‘reference’ dose of folic acid. We have found that a significant part of the plasma response does not actually originate from the test dose, and the accepted understanding of the initial site of biotransformation of folic acid upon which the experimental approach depends may be incorrect.