The primary objective of the study is to compare the incidence of hypophosphatemia in RYGB patients treated with intravenous (IV) single dose of iron isomaltoside (Monofer®) or ferric carboxymaltose (Ferinject®).

Inclusion Criteria:

• Patients over 18 years
• Signed informed consent
• Patients with previous RYGB surgery performed > 12 months ago
• Failed response to oral iron supplementation
• Established diagnosis of iron deficiency by ferritin < 50 ug/l or serum ferritin ≤100ug/l and low transferrin saturation (TSAT) ≤ 30%
• Normal phosphate blood level (0.8 - 1.45 mmol/l) before infusion
• Normal magnesium blood level (0.65-1.05 mmol/l)
• Outpatient

Exclusion Criteria:

• - Patients with known hypersensitivity to iron preparation and/or anaphylaxis from any cause
• Patients for whom a treatment with one of the IV iron is contra-indicated (based on product summary of product characteristics)
• Women who are pregnant or breastfeeding
• Intention to become pregnant during the course of the study
• Renal failure, chronic kidney disease stage 3b or worse (eGFR ≤ 45 ml/min/1.73m2)
• Patients who received IV iron infusion during the last 3 months before screening
• Treatment with erythropoietin or erythropoietin-stimulation agents, red blood cell transfusion within the last 3 months prior to screening
• Alcohol or drug abuse within the past 6 months
• Planned surgical procedure within the clinical trial period
• Surgery under general anaesthesia within the last 3 months prior to screening
• Hyperparathyroidism
• Kidney transplantation
• Inability to follow study procedures or give informed consent
• Use and inability to stop from V0 to study end, phosphate supplementation (except daily multivitamin preparation recommended after bariatric surgery; the exact amount of phosphate and iron supplementation in the daily multivitamin will be recorded)
• Osteoporosis drug therapy (bisphosphonates, denosumab or selective oestrogen receptor modulators).
• Patient who refuses to be informed of incidental discoveries that may contribute to the prevention, diagnosis, and treatment of existing or probable future illnesses.