We did two prospective, open-label, randomised controlled trials assessing iron absorption using(⁵⁴Fe)-labelled, (⁵⁷Fe)-labelled, or (⁵⁸Fe)-labelled ferrous sulfate in iron-depleted (serum ferritin ≤25 μg/L) womenaged 18–40 years recruited from ETH Zurich and the University of Zurich, Switzerland. In study 1, women wererandomly assigned (1:1) to two groups.