The increasing demand of bariatric surgery requires ongoing improvement of the preoperative and post-operative care of morbidly obese patients. Outcome reporting in bariatric surgery mainly focuses on weight loss, improvement of co-morbidities and quality of life. Post-operative nutritional deficiencies are a complication of bariatric surgery, in particular after malabsorptive procedures. Deficiencies of protein, iron and vitamins B12 and D are frequently reported and may occur despite supplementation. Morbid obesity itself is associated with nutritional deficiencies as well, which become worsening after bariatric surgery. The occurrence of nutritional deficiencies has been reported in the literature but not in Taiwan. The goal of the current study is to determine the prevalence and incidence of nutritional deficiencies in diabetic patients underwent bariatric surgery and receiving standard supplementation in Taiwan