Vitamin status after bariatric surgery: a randomized study of gastric bypass and duodenal switch.

Randomized trial in superobese adults aged 20-50 years with BMI 50-60 at two Scandinavian centers, assigned to laparoscopic Roux-en-Y gastric bypass or laparoscopic biliopancreatic diversion with duodenal switch. Vitamin status was assessed before surgery and at 6 weeks, 6 months, and 1 year after surgery.

Postoperative supplementation started 1 week after surgery in both groups and included daily multivitamins, iron sulfate 100 mg, calcium carbonate 1000 mg, and vitamin D3 10 μg. The gastric bypass group also received a vitamin B-12 substitute.

Standard supplementation did not fully prevent procedure-specific micronutrient deficits after bariatric surgery. Compared with gastric bypass, duodenal switch showed a steeper early thiamine decline (time × procedure interaction P = 0.013; DS change over time P = 0.001; GB P = 0.010), greater falls in vitamin A (GB P = 0.003; DS P < 0.001; interaction P = 0.002), and a larger decline in 25-hydroxyvitamin D (interaction P < 0.001). Riboflavin did not change significantly, while vitamin B-6 and vitamin C increased after surgery (both P < 0.001); vitamin B-12 was stable after duodenal switch and increased after gastric bypass. Duodenal switch also produced greater weight loss than gastric bypass (BMI loss 22.8 ± 4.7 vs 16.3 ± 4.3; 41% vs 30%), and a few duodenal switch patients developed severe low-vitamin complications, including very low vitamin A with night blindness.

This was not a supplement-versus-control trial, so the vitamin changes are strongly confounded by the two different surgical procedures and their different weight-loss effects. The active groups were small (29 duodenal switch and 31 gastric bypass participants), follow-up was limited to 1 year, and some biomarkers such as vitamin B-12 and folate were available from only one center. Supplement use was not uniform, as additional supplements were more common after duodenal switch, limiting attribution of outcomes to the standardized regimen alone.