The mediating role of bariatric surgery in the metabolic relationship between parathyroid hormone and 25-hydroxyvitamin D

Vitamin D metabolism is altered in obese individuals. Our findings indicated that in patients with severe obesity, a relatively low 25(OH)D concentration was required to suppress PTH. The PTH inflection point increased following surgery-induced weight loss, highlighting 25(OH)D different regulation mechanisms in patients with obesity.

Introduction: An optimal and sufficient concentration of 25-hydroxyvitamin D (25(OH)D) has been suggested as the level required to maximally suppress intact parathyroid hormone (iPTH). We evaluated the role of surgery-induced weight loss in determining the threshold(s) of 25(OH)D required to suppress iPTH.

Methods: This study was conducted in the framework of the Tehran Obesity Treatment Study (TOTS). We prospectively analyzed 687 patients with severe obesity who participated in the TOTS and underwent bariatric surgery from March 2013 to March 2019. The patients were followed for 1 year after surgery. Anthropometric parameters and serum levels of iPTH, 25OHD, phosphorous, and calcium were measured. Nonlinear and piecewise linear regression was used to evaluate the relationship between 25(OH)D and iPTH and to determine the 25(OH)D-suppression point at which iPTH was maximally suppressed.

Results: Body mass index was 44.6 kg/m² at the baseline and decreased to 29.7 kg/m² 1 year after surgery (P < 0.05). Before the surgery, iPTH and 25(OH)D showed an exponential relationship; iPTH began to decrease rapidly at 25(OH)D concentration of 12 ng/mL, reaching maximal suppression at 30 ng/mL. However, the relationship between 25(OH)D and iPTH was non-exponential 1 year after surgery. The piecewise linear regression model revealed the 25(OH)D concentration of 21 ng/mL as the inflection point following surgery-induced weight loss.

Conclusion: In patients with severe obesity, PTH was suppressed at a relatively lower concentration of 25(OH)D; this threshold increased following surgery-induced weight loss. These findings suggest a role for bariatric surgery in regulating 25(OH)D metabolism in patients with obesity.