Purpose Bariatric surgery is associated with deficiencies of vitamins and minerals, and patients are routinely advised supplements postoperatively. We studied prevalence of vitamin B 12 , folate and iron deficiencies and anaemia before and after bariatric surgery over 4 years of follow-up. Materials and Methods We performed a retrospective cohort analysis of 353 people with obesity, including 257 (72.8%) women, who underwent gastric bypass (252, 71.4%) or sleeve gastrectomy (101, 28.6%) at our National Health Service bariatric centre in Northwest England. Results At baseline, mean (standard error) age was 46.0 (0.6) years, body mass index 53.1 (0.4) kg/m 2 , serum vitamin B 12 400.2 (16.4) pg/L, folate 7.7 (0.2) μg/L, iron 12.0 (0.3) μmol/L, ferritin 118.3 (8.4) μg/L and haemoglobin 137.9 (0.8) g/L. Frequency of low vitamin B 12 levels reduced from 7.5% preoperatively to 2.3% at 48 months (P < 0.038). Mean folate levels increased from baseline to 48 months by 5.3 μg/L (P < 0.001) but frequency of low folate levels increased from 4.7% preoperatively to 10.3% (P < 0.048). Ferritin levels increased from baseline to 48 months by 51.3 μg/L (P < 0.009). Frequency of low ferritin levels was greater in women (39.1%) than in men (8.9%) at baseline (P < 0.001) and throughout the study period. Haemoglobin was low in 4.6% of all patients at baseline with no significant change over the study period. Conclusion There were notable rates of haematinic insufficiencies in bariatric surgical candidates preoperatively. Our study lends further support to regular supplementation with vitamin B 12 , folic acid, and iron in people undergoing bariatric surgery.

In the original article, the units for vitamin B 12 were incorrectly identified in Tables 1 and 3. The correct units are (ng/L). The same error also appeared in the "Results" section of the Abstract and in the "Vitamin B 12 " section of the article text.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.