Obese volunteers who have scheduled a gastric bypass (RYGB) in UZ Leuven will be asked to participate in a cross-over study using commercial available iron supplements. The aim of this study is to evaluate the absorption rate of different oral iron supplements in volunteers before and after RYGB (1, 3, 6 and 12 months post-RYGB).

Patients who met the criteria will be invited to come to the clinical research center at 10 occasions (two times before and 1, 3, 6 and 12 months after RYGB). During each visit the following information will be collected:

• General information concerning sociodemographic and medical background
• The medical history
• Anthropometric measures
• Physical activity by questionnaire
• Food frequency by questionnaire
• Information about menstrual blood loss.

Following an overnight fast, subjects will come to the clinical research center. An intravenous catheter will be inserted to collect blood samples. Before the oral administration, blood samples will be collected for the determination of the serum concentration of different iron status markers. The volunteers will receive first an iron(II)sulfate supplement, followed by a Fe-chelate supplement (at least 7 days between both supplements). After the administration of the supplement, blood samples will be taken at various time points to determine the amount of iron absorption and to investigate if there are any changes in the iron metabolism (at 15, 30, 60, 90 minutes and 2; 2.5; 3; 3.5; 4 and 5 hours after oral administration). There will be a one-week washout period between the tests with each iron supplement. The volunteers will be asked to repeat the test before surgery and 1, 3, 6 and 12 months after surgery for each iron supplement.

Inclusion Criteria:

• Obese patients who have planned a RYGB surgery in the University Hospital Leuven
• Low iron status at the preoperative consultation (ferritin < 30 µg/L and/or transferrin saturation <20%)

Exclusion Criteria:

• Patients with a bariatric surgery history
• Smokers (> 15 cigarettes/day)
• Pregnant women
• Lactating women
• Patients with a positive helicobacter pylori screening