Introduction After bariatric surgery (BS), patients might suffer from nutrient maldigestion, malabsorption, and vitamin deficiencies. In this study, our aim was to assess pancreatic functions after BS using fecal elastase-1 assay (FE-1). Material and Methods Sixty patients (21M) undergoing BS and 20 (6M) healthy controls were included into the study. Stool samples were collected 1 year after surgery. Ten patients from one anastomosis gastric bypass (OAGB) and single anastomosis duodenal switch (SADS) groups with the lowest value of FE-1 and GIQLI scores were given pancreatic enzyme replacement therapy (PERT). After PERT, FE-1, excess weight loss (EWL), BMI, GIQLI scores, and vitamin D levels were measured. Results Vitamin D levels were detected as 19.04 (9-46.5) pg/ml, 15.1 (8.4-23.6) pg/ml, 17.8 (5-30) pg/ml, and 21.79 (11-40.3) pg/ml after sleeve gastrectomy (SG), OAGB, SADS, and control groups, respectively (p = 0.04). GIQLI scores in the first year were found to have increased in all patients (p = 0.02). FE-1 levels were found as 642.35 (566.3-711.4) mu g/g, 378.52 (183.5-561.1) mu g/g, 458.88 (252.5-593, 5) mu g/g, and 518.2 (351.6-691) mu g/g for the SG, OAGB, SADS, and control groups, respectively. There was a strong inverse correlation between EWL and FE-1 levels at the end of the first year (Spearman's rho = - 0.688,p = 0.003). After having performed PERT for patients with the lowest FE-1 levels, the levels increased to 683.39 (615.5-720) mu g/g in the OAGB and 691.5 (643.1-720) mu g/g in the SADS groups (p = 0.011). Conclusion FE-1 measurements demonstrated that many patients suffer from malabsorption after OAGB or SADS, whereas functions remain normal after SG. PERT corrects pancreatic functions without affecting weight loss and also contributes to the normal serum level of vitamin D.