It is very difficult to quantify menstrual blood loss, the reference method is a tedious one. This is a problem, as it is not conducive to objectively measuring menstrual blood loss and understanding the contribution of menstrual iron loss to iron deficiency anemia. With this study, the investigators aim to investigate iron loss during the menstrual cycle and aim to validate a much simpler technique.

Anemia reduction efforts have largely focused on increasing iron intakes such as improving diet quality, food fortification with iron, iron supplementation, biofortification. There is little information on the contribution of menstrual iron loss to iron deficiency anemia.

Indeed, the accurate measurement of menstrual blood loss volume and iron loss is difficult. Self-perception of heavy menstrual blood loss poorly predicts actual blood loss, and the objective measurement of menstrual blood loss remains a tedious method. Various methods have been used to objectively measure menstrual blood loss volume including radioisotopes, but these methods are invasive. The investigators aim to validate a much simpler technique, namely the stable iron isotope dilution methodology. This is a promising new method for quantifying long-term body iron balance, absorption, and loss which has not been applied previously to measure menstrual iron losses. Validating this new method against the alkaline hematin reference method would be an important step to encourage menstrual blood loss measurements and shed light on the contribution of menstrual blood loss to iron deficiency and iron deficiency anemia.

Inclusion Criteria:

• Female, 18-30 years old
• Already labelled with stable iron isotopes at least 12 months prior to study start
• 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)
• 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
• 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
• Inability to follow the study protocol

Female 18-30 year olds, who have already been labelled with stable iron isotopes.