Ensuring the quality of vitamin A capsules used in supplementation programs.

acid, or both and after cessation of such supplementation, which would, by necessity, involve dynamics between RBC kinetics and whole-body folate kinetics. Quinlivan further comments that a steady state of RBC folate may have been reached between weeks 16 and 24. In our intervention study, blood drawings were performed every 4 wk, and RBC folate significantly increased between week 16 and week 20 and between week 20 and week 24 (4). This continuous increase was observed in all 3 supplementation groups that received either 400 g folic acid/d, 416 g [6S]-5-methyltetrahydrofolate/d, or 208 g [6S]-5-methyltetrahydrofolate/d (4). To our knowledge, no intervention study has yet been conducted to measure the long-term effect of supplementation with folate, folic acid, or both on saturation kinetics of RBC folate. To date, the longest intervention trials lasted 24 wk, and neither of those trials (4, 5) could determine a plateau in RBC folate concentration. Meanwhile, Quinlivan states that it will take 21 wk for RBC folate to reach steady state. He explains that “plateau RBC folate enrichment may be further delayed by the amount of time it takes for the RBC progenitor cells, presumably in the bone marrow, to reach maximum enrichment.” This statement supports our hypothesis that RBC folate concentrations plateau later than after 24 wk of supplementation. In conclusion, we wish to emphasize that our model is in fact a testable hypothesis that suggests a possible description for the behavior of RBC folate during and after long-term supplementation with folate, folic acid, or both. The appearance of RBC folate in the model is partly confirmed by 2 independent studies, and the kinetic model for RBC folate elimination is currently being tested in a long-term intervention trial.