Introduction: There are indications that the absorption of oral iron supplementation is reduced after a Roux- en -Y gastric bypass. Nevertheless, oral preparations are used as standard therapy for iron deficiency, even in patients who underwent a Roux- en -Y gastric bypass. Our goal is to evaluate if iron absorption is disturbed after a RYGB, which leads to a insufficient treatment of oral iron suppletion.

Methods: an iron absorption test will be performed pre- and postoperatively in 24 patients. Two groups will be created.

Preoperatively group 1 receives a daily dose of ferrous fumarate (600mg) and group 2 receives a daily dose Losferron (1390mg).

Before intake of the medicines, a fasting blood sample is taken (baseline), serum iron including ferritin, transferrin and transferrin saturation will be measured. After intake of losferron/ferrous fumarate blood samples will be taken 1, 2, 3, 4, 5 and 6 hours after intake, using a drip. An increase of 80 microgram/l is representative for a sufficient iron absorption. All patients undergo a Roux- en -Y gastric bypass.

Postoperatively; one month postoperatively the same absorption test will be repeated in the same patients.

Inclusion Criteria:

• patients who are eligible for a primary Roux- en -Y gastric bypass and who have no pre-existing iron deficiency (serum ferritin between 20-200 micrograms/L)

Exclusion Criteria:

• blood transfusion one month before ans in the study period. The use of iron containing nutritional supplements, except our standardized multivitamin supplements. Decreased function of the kidney with a GFR of < 30ml/min and a serum creatinin below 50 micromol/L. Hb< 7.4 mmol/L in females en Hb< 8.4 mmol/L in males. Accumulation of iron. Hypersensitivity for ons of the medicinal products. Psychiatric illness. Pregnancy