Surgery is now considered to be the most effective treatment for reducing weight and maintaining weight loss in patients with clinically severe obesity. Although the jejuno-ileal bypass has been abandoned, the vertical banded gastroplasty (VBG) and Roux-en-Y gastric bypass (RYGB) operations are now commonly performed. A third operation, the bilio-pancreatic diversion (BPD), is performed less frequently. The RYGB and BPD procedures cause predictable selective micronutrient deficiencies that can be avoided by early supplementation. Surgical complications from all of these procedures may result in more severe forms of malnutrition. This article is intended to familiarize the nutrition support specialist with the anatomic and physiologic changes produced by these procedures, the resulting nutritional deficiencies and recommended supplementation, and the manifestations of severe malnutrition caused by complications. A case of severe malnutrition after RYGB surgery is reported for illustration.