Background: Serum 25-hydroxyvitamin D [25(OH)D] and parathormone (PTH) have an inverse relation. The 25(OH)D threshold required to maximally suppress PTH has been used as a marker of optimal vitamin D status. Obesity is associated with lower serum levels of 25(OH)D and higher levels of PTH; however, the relation between these hormones in this setting is not well established. We aimed to assess the influence of excessive weight and weight loss after bariatric surgery on the relation between serum 25(OH)D and PTH.
Methods: Anthropometric and analytical parameters of phospho-calcium metabolism from 290 patients undergoing bariatric surgery were analyzed. The association between 25(OH)D and PTH before and 1 year after surgery was evaluated through Student's t test and simple and multivariable linear regression.
Results: At baseline, there was an inverse association between 25(OH)D and PTH plasma levels (β = - 0.571; p = 0.001). The threshold for a significant increase of PTH at baseline was a 25(OH)D level of < 10 ng/mL (p < 0.001). At 1 year after surgery (n = 194), the association between the two variables was weaker (β = - 0.291; p = 0.014). The 25(OH)D threshold for a significant increase in serum PTH levels was lower in patients who maintained obesity (n = 77) [25(OH)D 5 vs 10 ng/mL].
Conclusions: PTH suppression threshold observed in patients with morbid obesity before and after bariatric surgery is lower than that described for the general population, suggesting that the 25(OH)D plasma levels may not be representative of the body's vitamin D reserves. Other studies are required to demonstrate which optimal vitamin D levels need to be aimed for in obesity, in order to improve obese patients' bone health.
Keywords: 25-hydroxyvitamin D; Obesity; Parathormone.