IV Iron-induced Hypophosphatemia After RYGB

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.