The precise impact of calcium absorption in relation to RYGB, SG, and OAGB remains under-researched in terms of statistical power and the diversity of BMS procedures considered. Therefore, this presents a critical area for future investigation to improve patient outcomes in BMS.

Calcium, predominantly absorbed in the duodenum and proximal jejunum, relies heavily on vitamin D and an acidic environment to facilitate absorption. With the increasing prevalence of bariatric metabolic surgery (BMS) procedures and their malabsorptive effects, the likelihood of fat-soluble vitamin malabsorption becomes heightened. This stems from bypassing the stomach, key absorption sites in the intestine, and the inefficient mixing of bile salts.

BMS is often associated with several bone metabolism disorders, including the acceleration of bone remodeling and turnover, bone loss, and decreased bone mineral density (BMD). Postoperative calcium supplementation can mitigate this bone loss over time. For instance, a study demonstrated the beneficial effect of calcium citrate following Roux-en-Y gastric bypass (RYGB). However, the study's statistical power was insufficient; thus, the BMS field still awaits further conclusive and robust research to establish definitive guidelines, which was highlighted in another study.

Moreover, substantial changes in gut hormones, such as peptide YY (PYY), glucagon-like peptide-1, and ghrelin, have been observed following RYGB, sleeve gastrectomy (SG), and One Anastomosis Gastric Bypass (OAGB). While these hormonal changes are typically associated with BMS's numerous positive metabolic benefits, they may also contribute to bone loss.

Consequently, the precise impact of calcium absorption in relation to RYGB, SG, and OAGB remains under-researched in terms of statistical power and the diversity of BMS procedures considered. Therefore, this presents a critical area for future investigation to improve patient outcomes in BMS.

Inclusion Criteria:

• Patients aged 18-75 years old
• After BMS surgery who had an RYGB, SG, or OAGB operation at least 12 months before the study.
• Patients will be selected at random from the hospital's electronic patient system.

Exclusion Criteria:

• Patients on antacids during the study
• Patients onH2 receptor antagonists during the study
• Patients on proton pump inhibitors during the study
• Patients with a previous oophorectomy,
• Liver disease,
• Renal disease,
• Hypercalcemia,
• Hyperthyroidism,
• Hypothyroidism who require levothyroxine supplementation (Levothyroxine forms complexes with calcium)
• Parathyroid disorders
• Use of diuretics,
• Use of calcitonin,
• Use of corticosteroids,
• Use of anabolic steroids,
• Use of anticonvulsants within three months of the study
• Heavy smokers (>10 cigarettes/day)
• Abusing alcohol (>70 ml/day)

The analysis will be performed on a blinded dataset after completing the medical/scientific review. All protocol violations will be identified and resolved, and the dataset will be declared complete. All data will be collected in a data management system (Castor EDC, Amsterdam, The Netherlands; https://www.castoredc.com), handled according to Good Clinical Practice guidelines, Data Protection Directive certificate, and complied with Title 21 CFR Part 11. Furthermore, the data centers where all the research data will be stored are certified according to ISO27001, ISO9001, and Dutch NEN7510.

after study completed, for 15 years

contact study PI

All the patients will be used in a cross-over study design. The same patient will be using study drugs A or B and switching in the study to the other drug.

7 days period will be applied for any wash-out or possible carry-over effect.