BACKGROUND: Bariatric surgery is widely performed to induce weight loss. OBJECTIVE: The objective was to examine changes in vitamin status after 2 bariatric surgical techniques. DESIGN: A randomized controlled trial was conducted in 2 Scandinavian hospitals. The subjects were 60 superobese patients [body mass index (BMI; in kg/m(2)): 50-60]. The surgical interventions were either laparoscopic Roux-en-Y gastric bypass or laparoscopic biliopancreatic diversion with duodenal switch. All patients received multivitamins, iron, calcium, and vitamin D supplements. Gastric bypass patients also received a vitamin B-12 substitute. The patients were examined before surgery and 6 wk, 6 mo, and 1 y after surgery. RESULTS: Of 60 surgically treated patients, 59 completed the follow-up. After surgery, duodenal switch patients had lower mean vitamin A and 25-hydroxyvitamin D concentrations and a steeper decline in thiamine concentrations than did the gastric bypass patients. Other vitamins (riboflavin, vitamin B-6, vitamin C, and vitamin E adjusted for serum lipids) did not change differently in the surgical groups, and concentrations were either stable or increased. Furthermore, duodenal switch patients had lower hemoglobin and total cholesterol concentrations and a lower BMI (mean reduction: 41% compared with 30%) than did gastric bypass patients 1 y after surgery. Additional dietary supplement use was more frequent among duodenal switch patients (55%) than among gastric bypass patients (26%). CONCLUSIONS: Compared with gastric bypass, duodenal switch may be associated with a greater risk of vitamin A and D deficiencies in the first year after surgery and of thiamine deficiency in the initial months after surgery. Patients who undergo these 2 surgical interventions may require different monitoring and supplementation regimens in the first year after surgery. This trial was registered at ClinicalTrials.gov as NCT00327912.