In a study of non-pregnant, pregnant and lactating women who were on a diet containing 400 mcg folate and given 750 mcg of folic acid as a supplement, 84% of all the participants had un-metabolized folic acid in their blood. In other words, the vast majority of the folic acid given over and above the diet natural folate, was not converted to the active form. 24-hour urine excretion is an effective way to assess the immediate bioavailability and utilization of an ingested folic acid or folate dose (compared with serum or red blood cell folate levels) and this study showed some surprising results.

100% of un-metabolized folic acid was excreted by non-pregnant women at the end of the study (10-12 weeks) compared to 77% un-metabolized folic acid excreted by pregnant women (in their 3rd trimester) an 89% of lactating women. This indicates that pregnant women convert more folic acid to the active form than lactating women who convert more than non-pregnant women.