Comparison of faecal monitoring and area under the curve techniques to determine iron absorption in humans using stable isotope labelling

The objective of the study was to compare faecal monitoring and area under the curve (AUC) techniques for measuring iron absorption from a stable isotope label. Nine healthy female subjects (age: 33±8 yr) were recruited to take part in the study. After an overnight fast, each subject received a 5 mg dose of highly enriched Fe-57 and all faecal samples were collected for 10 days post-dose to allow for estimation of iron absorption by the faecal monitoring method (geometric mean: 25%). Serial blood samples (12 in total) were also collected from each volunteer for 6 h post-dose to estimate iron absorption by the well-validated area under the plasma iron concentration curve (AUC) method (geometric mean: 18%). The faecal monitoring method yields higher iron absorption results compared to AUC. This discrepancy may be due to the steps involved in faecal processing where systematic errors are likely to take place and also to the lack of complete faecal recovery associated with issues of cleanliness and hygiene when passing stools. Both of these lead to losses of the unabsorbed isotope and to an overestimate of iron absorption.