Background: Obesity has become a public health problem worldwide and bariatric surgery is an effective approach to treat morbid obesity, diabetes mellitus, and also non-alcoholic fatty liver disease (NAFLD). However, bariatric patients often suffer from vitamin D deficiency perioperatively. Beyond its classical role in maintaining bone health, vitamin D deficiency is associated with several diseases, such as diabetes mellitus or NAFLD. Therefore, appropriate supplementation to prevent vitamin D deficiency is urgently needed. Nevertheless, there are limited data on best strategies for treating it in bariatric patients, particularly for the relatively new omega loop gastric bypass (OLGB) procedure. Methods: The main study (LOAD-study) was built upon a pre-study in evaluating vitamin D status in 50 patients who underwent OLGB (paper II). The study design of the main study was a randomized, double-blinded, controlled trial with an administration, in the first month postoperatively, of up to three oral vitamin D3 loading doses (100 000 IU; intervention group) or placebo (control group) with subsequent maintenance dose (3420 IU/day) in both groups until the 6-months visit. The primary outcome was the 25-hydroxy vitamin D concentration [25(OH)D] (paper I). Moreover, by using the baseline data, the prevalence of liver fibrosis and its associations with metabolic markers and vitamin D metabolism parameters was assessed (paper III). Additionally, there was a focus on liver fibrotic patients by examining the effect of the novel vitamin D supplementation regimen postoperatively in this sub-group. Results: The findings of the pre-study demonstrated a vitamin D deficiency prevalence of 96% preoperatively and still, after non-standardized supplementation, 80% were deficient after 12 months. Moreover, vitamin D deficiency in bariatric patients can result in secondary hyperparathyroidism (paper II) and low serum vitamin D concentrations are associated with higher stages of liver fibrosis with a prevalence of 30% (paper III). Regarding vitamin D3 supplementation, a higher increase of 25(OH)D with a lower prevalence of secondary hyperparathyroidism was observed in the intervention group compared to the control group. Moreover, no (serious) adverse events related to the study medication were found. Additionally, the loading dose regimen was more effective in patients with liver fibrosis (paper IV). Conclusions: The findings of the pre- and main study in this thesis suggest that appropriate vitamin D3 supplementation is needed due to the high prevalence of vitamin D deficiency and that low vitamin D concentrations are associated with higher stages of liver fibrosis. The high vitamin D3 loading with subsequent maintenance dose is effective and safe in achieving higher vitamin D concentrations in OLGB patients during the first six months after surgery. Noteworthy, this forced dosing regimen is more effective in liver fibrotic patients.