Relationships Between Vitamin D Status and PTH over 5 Years After Roux-en-Y Gastric Bypass: a Longitudinal Cohort Study

Abstract

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.

Keywords: Bariatric surgery; Bone turnover; Calcium; Morbid obesity; Parathyroid hormone; Vitamin D.

Fig. 1

Observed development of parathyroid hormone (PTH) and secondary hyperparathyroidism (%) from 6 months to 5 years after Roux-en-Y gastric bypass (N = 554) to the left, and for subgroups of 25-hydroxyvitamin D (S-25(OH)D) to the right

Fig. 2

Odds ratio (OR) for secondary hyperparathyroidism (SHPT) from 6 months to 5 years postoperatively in 554 Roux-en-Y gastric bypass patients by categories of serum 25-hydroxyvitamin D (S-25(OH)D, nmol/l) and ionized calcium (iCa, mmol/l) compared with reference (ref.). S-25(OH)D was adjusted for time (y): OR 1.11 (1.02–1.22), with additional adjustments for baseline S-25(OH)D and iCa as given

Fig. 3

Relative ratio (RR) for secondary hyperparathyroidism (SHPT) 5 years after Roux-en-Y gastric bypass (N = 554) by categories of serum 25-hydroxyvitamin D (S-25(OH)D or D, nmol/l) and ionized calcium (iCa, mmol/l) compared with reference (ref.)

Fig. 4

Observed development of ionized calcium by subgroups of 25-hydroxyvitamin D (S-25(OH)D), and proportions of low calcium and calcium within the lower range from 6 months to 5 years after Roux-en-Y gastric bypass (N = 552) (upper), and development of parathyroid hormone (PTH) and proportion with secondary hyperparathyroidism by subgroups of ionized calcium (lower)