Iron deficiency (ID) with or without anemia is a major public health problem worldwide, especially in women of reproductive age. Iron supplementation can be an effective strategy to prevent and treat ID and iron deficiency anemia (IDA). Recent studies suggests that giving oral iron every other day would be an optimized dosing regimen with maximized absorption and a lower incidence of gastrointestinal side effects compared to consecutive day dosing. Long-term trials in which participants and investigators are blinded to the dosing interval with iron status and gastrointestinal side effects as study outcomes are needed.

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.

women

double-blinded randomized controlled study: participants and investigators are blinded