Iron deficiency is a common problem that is often treated with the administration of oral iron.

Most of the iron is not absorbed and remains in the intestine, where it influences the balance of gut bacteria.

This influence is usually undesirable and should be kept as small as possible. With lower iron doses, the amount of unabsorbed iron is smaller and therefore a smaller change in the intestinal bacteria composition is to be expected.

Iron deficiency affects about one in five premenopausal women. The first line treatment for patients with iron deficiency is an oral therapy with iron-supplements, another option is the intravenous iron supplementation.

The oral supplementation causes side-effects such as nausea, vomiting, abdominal pain and many more in a lot of patients. There is a positive correlation between the appearance of gastrointestinal side-effects and the applicated iron dose.

Only a low fraction of orally taken iron is actually absorbed, even in patients with iron-deficiency. Since only a fraction of the orally administered iron is actually absorbed, the toxicity of high concentrations of iron in the gastrointestinal tract and the altered selection advantages of the bacteria presumably leads to microbiome changes.

The unabsorbed iron is potentially changing the microbiome, which leads to undesirable side effects.

With low-dose iron-supplementation the amount of unabsorbed iron is smaller, therefore a dosage dependent microbiome change would be expected.

The aim of the study is to determine the magnitude of the microbiome response to daily iron substitution with 12mg and 100mg.

Inclusion Criteria:

• Informed Consent as documented by signature
• Female gender
• Premenopausal
• Age > 18 years
• Regular menstrual cycle
• BMI in normal range (18-25kg/m2)
• Serum-ferritin ≤30ng/ml
• No anemia (hemoglobin ≥117g/l)
• No intake of dietary supplements for at least 4 weeks

Exclusion Criteria:

• Pregnancy
• Hypermenorrhea (more than 5 unties/tampons per day)
• Chronic inflammatory disease, psychiatric disorders
• Hypersensitivity to iron supplements
• chronic kidney disease (creatinine >80 µmol/l)
• Liver disease (ALT >35 U/l)
• Hypo- or Hyperthyroidism (TSH not between 0.16-4.25 mU/l)
• Intake of medicines that interact with oral iron supplementation (e.g. PPI)*