Iron deficiency (ID) is a major public health problem worldwide and oral iron supplementation can be an effective strategy to treat and prevent ID. To maximize iron bioavailability form oral iron supplements the simultaneous intake of the iron absorption enhancer ascorbic acid (AA) is recommended, and the simultaneous intake of coffee or tea containing the iron absorption inhibitors polyphenols should be avoided. Also, oral iron supplements are recommended to be taken on an empty stomach in the morning and without a meal to avoid any interaction with phytic acid, another iron absorption inhibitor present in many foods. However, the effects of these iron absorption enhancers and inhibitors have only been shown on iron absorption from dietary iron (up to 10mg). Also, the effect of the diurnal hepcidin increase on absorption from an iron supplement given in the afternoon without a preceding morning dose is unclear. Whether AA also increases iron bioavailability from a supplemental iron dose and whether a cup of coffee, a breakfast or iron administration in the afternoon decreases iron bioavailability from a supplemental dose is uncertain.

Inclusion Criteria:

• Female, 18 to 45 years old,
• SF levels <30 μg/L,
• Body weight < 70 kg
• Normal Body Mass Index (18.5-25 kg/m2),
• Signed informed consent.

Exclusion Criteria:

• Anemia (Hb < 12 g/dL)
• Elevated CRP > 5 mg/L,
• Any metabolic, gastrointestinal, kidney or chronic disease such as diabetes, renal failure, hepatic dysfunction, hepatitis, hypertension, cancer or cardiovascular diseases (according to the participants own statement) affecting iron metabolism,
• Continuous/long-term use of medication during the whole study, which may interfere with iron absorption, gut physiology and iron metabolism,
• Consumption of mineral and vitamin supplements since screening and over the study period until last blood sample collection,
• Difficulties with blood sampling,
• Blood transfusion, blood donation or significant blood loss (accident, surgery) over the past 6 months,
• Known hypersensitivity or allergy to iron capsules in the given amount (ferrous fumarate, brilliant blue FCF (E133), titandioxide (E171) and sodium lauryl sulfate)
• 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.