Inclusion Criteria:  Female, 18 to 45 years old, SF levels <25 μg/L, Hb levels ≥ 11 g/dL Normal Body Mass Index (18.5-25 kg/m2), In possession of a mobile phone on which the study app can be loaded, Signed informed consent, Not mandatory (only for subgroup analysis): having participated in a former stable iron isotope study in our laboratory between 1-5 y ago  Exclusion Criteria:  Elevated CRP > 5 mg/L, Any metabolic, gastrointestinal, kidney or chronic disease (self-reported) affecting iron metabolism, Continuous/long-term use of medication, which may interfere with iron absorption, gut physiology and iron metabolism, Consumption of additional iron supplements over the study period, Consumption of iron supplements since screening, Difficulties with blood sampling, Known hypersensitivity or allergy to iron or placebo capsules in the given amount (ferrous sulfate, mannitol, silica) Pregnancy, breastfeeding Women who intend to become pregnant during the course of the study, Known or suspected non-compliance, drug or alcohol (more than 2 drinks/day) abuse, Smokers (> 1 cigarette per week), Participant is likely to be absent on one the study appointments, Inability to follow the procedures of the study, e.g. due to language problems, self-reported psychological disorders, etc. of the participant.