Iron supplementation is the first line of treatment of iron deficiency in most women, but we do not know when the best time is to supplement in the context of the menstrual cycle. With this study, we aim to measure and compare iron status, hepcidin, inflammatory markers, hormones estrogen and progesterone and changes in iron absorption at various points throughout the menstrual cycle, with a long-term view to determine best time for iron supplementation in relation to the menstrual cycle.

To date, very few studies have assessed iron absorption in relation to a woman's menstrual cycle. Although oral iron supplementation is considered the first line of treatment for iron deficiency in most women, there is no knowledge on the relationship between this intervention in the context of the menstrual cycle, and more specifically there is no knowledge on its potential implications for the most appropriate timing of iron supplementation. A better understanding of iron absorption during the menstrual cycle as well as kinetics of key parameters could allow the design of adapted and/or new interventions (e.g., timing of iron supplementation in relation to different phases of the menstrual cycle) to mitigate iron deficiency in menstruating women and can inform programs aimed at addressing iron deficiency

Inclusion Criteria:

• Female, 18-30 years old
• Weight <70 kg
• Normal body mass index (18.5 - 25kg/m2)
• Regular menstrual cycle (self-reported cycle length between 28 and 35 days in the past 6 months)
• Depleted iron stores (serum ferritin ≤ 30 µg/L)
• Signed informed consent
• Able to read and understand English

Exclusion Criteria:

• Use of hormonal contraceptives within a 3-month recall period
• Anemia (hemoglobin < 117 g/L)
• Any known major 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)
• Women with severe menstrual cramps that would prevent them from attending study meetings during menstruation (judged by the women)
• Consumption of iron-containing supplements within 1 month prior to the start of study
• Known difficulties with blood sampling
• Pregnancy (serum human chorionic gonadotropin (hCG) < 5 mIU/mL)
• Current smoking (>1 cigarette per week over a 1-month recall period)
• Women who are planning to get pregnant within the study period, as this would affect iron metabolism and subsequent study measurements.
• Inability to follow the study protocol