In patients who underwent gastric bypass surgery, estimated vitamin concentrations were either significantly increased or unchanged up to 5 years after surgery.

Purpose Secondary hyperparathyroidism (SHPT) after obesity surgery may affect bone health. Optimal vitamin D levels have not been established to prevent SHPT postoperatively. We investigated whether SHPT differed across threshold levels of serum 25-hydroxyvitamin D (S-25(OH)D) from 6 months up to 5 years after Roux-en-Y gastric bypass (RYGB). Materials and Methods We included 554 patients at follow-up 5 years postoperatively. Blood samples were analysed for S-25(OH)D, ionized calcium (iCa) and parathyroid hormone (PTH) during follow-up. Results PTH and prevalence of SHPT increased from 6 months to 5 years postoperatively, while S-25(OH)D and iCa decreased (all P < 0.001). PTH and SHPT development are related with S-25(OH)D, and PTH differed between all subgroups of S-25(OH)D. SHPT occurred less frequently across all subgroups of S-25(OH)D ≥ 50 nmol/l during follow-up: odds ratio (OR) 0.44 (95% CI 0.36-0.54) in patients with S-25(OH)D ≥ 50 nmol/l, OR 0.38 (0.30-0.49) with S-25(OH)D ≥ 75 nmol/l and OR 0.19 (0.12-0.31) with S-25(OH) D ≥ 100 nmol/l, all compared with S-25(OH)D < 50 nmol/l. At 5 years, 208/554 patients (38%) had SHPT; SHPT was found in 94/188 patients (50%) with S-25(OH)D < 50 nmol/l, in 69/222 (31%) with S-25(OH)D 50-74 nmol/l, in 40/117 (34%) with S-25(OH)D 75-99 nmol/l and in 5/27 (19%) with S-25(OH)D ≥ 100 nmol/l. An interaction existed between S-25(OH)D and iCa. Bone alkaline phosphatase remained increased with SHPT. Conclusions A significant relationship existed between S-25(OH)D and development of PTH and SHPT. The prevalence of SHPT was lower with threshold levels 25(OH)D ≥ 50 nmol/l and ≥ 75 nmol/l over the 5 years, and lowest with S-25(OH)D ≥ 100 nmol/l.

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