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Obesity is a growing pandemic, and consequently the population that has undergone a bariatric procedure is steadily rising. Through various mechanisms, bariatric surgery alters the gastrointestinal anatomy, thereby obtaining weight loss while being associated with various nutrient deficiencies including vitamin D. Over time, vitamin D deficiency may lead to elevated levels of parathyroid hormone (PTH) and the depletion of bone, in order to maintain circulating calcium levels. 

The aim of this thesis was to study the effects of bariatric surgery upon calcium homeostasis. In paper I we sought to investigate whether lifestyle factors could explain the difference in bone mineral density (BMD) seen among Roux-en-Y gastric bypass (RYGBP) patients. The second paper is a population-based study that examines the postoperative trends of vitamin D and PTH among patients that have undergone either a RYGBP or a sleeve gastrectomy (SG) (the two currently most popular bariatric procedures worldwide). In paper III the purpose was to explore whether medications to treat dyspepsia had an impact upon calcium homeostasis postoperatively among RYGBP patients. And finally, the purpose of the fourth paper was to study an intramuscular injection as an alternative method for the treatment of vitamin D deficiency among patients who had undergone a biliopancreatic diversion/duodenal switch (BPD/DS) procedure. 

The results from paper I do not indicate that lifestyle factors are a cause of the reduced BMD observed after a RYGBP, therefore the procedure itself is a likely cause of reduced BMD. The analysis conducted in paper II points to the impact that bariatric procedures (specifically RYGBP and SG) have over time regarding vitamin D deficiency. The results showed both differences between procedures as well as a successive decline in vitamin D parallelled by a rise in PTH with time after surgery. In paper III our results indicate that regular PPI therapy is strongly associated with a much higher incidence of pathological PTH levels, suggesting that the combination of the procedure and the medication may have deleterious effects upon calcium homeostasis. Finally, when managing vitamin D deficiency after a BPD/DS procedure, we showed that intramuscular injections had a substantial effect upon both vitamin D levels and PTH levels.