MON-535 Relationship between Vitamin D and PTH: The Effect of Obesity and Weight Loss after Bariatric Surgery

Obesity is associated with lower serum levels of 25-hydroxyvitamin D (25-OH-D) and higher levels of PTH. There is an inverse relationship between these parameters in the general population, however this relationship is not well established in obesity. The 25-OH-D threshold required to completely suppress PTH has been suggested as a marker of optimal vitamin D status. We evaluated the relationship between 25-OH-D and PTH in obese patients before and after bariatric surgery, and determined the influence of obesity on the threshold for suppression of PTH by 25-OH-D. This was a retrospective study of patients followed between January 2010 and June 2017. We excluded patients with CKD (GFR < 60mL/min/1.73m²), previous history of bone fractures and abnormal levels of calcium, phosphorus and magnesium. Statistical analysis was performed with t, chi-square and linear regression tests. We evaluated 290 patients, 82.4% women, mean age 41.04 ± 10.52 years, BMI 43.57±5.66 kg/m². Before surgery, we observed a significant inverse association between 25-OH-D and PTH (β=-0.644; p<0.001), which is maintained after adjustment for gender, age, BMI and season. At 25-OH-D levels > 20 ng/mL the mean PTH was 53.24±2.68 pg/mL, increasing to 66.4±2.53 pg/mL when 25-OH-D was <10 ng/ml (p <0.001). One year after bariatric surgery, the association between the variables was weaker (β=-0.313, p=0.006), being lost in patients who kept a BMI > 30 kg/m2 (β=-0.154; p=0.434), despite the association between PTH and the presence of obesity (β=5.26, p=0.043). In patients achieving a BMI <30 kg/m², PTH increased from 41.05±1.57 pg/mL to 59.83±7.09 pg/mL for 25-OH-D levels <10 ng/mL (p=0.011) and to 96.1±14.19 pg/mL for 25-OH-D <5 ng/mL (p<0.001). In patients who maintained obesity, PTH increased from 46.66±2.81 pg/mL to 112±18.41 pg/mL from 25-OH-D <5 ng/mL (p=0.001) levels. The relationship between 25-OH-D and PTH in the obese population is modified after bariatric surgery. The PTH suppression threshold, and consequently the optimal vitamin D status, appear to be altered in obese and surgically treated patients. The 25-OH-D assay in the obese population may not be representative of the body's vitamin D reserves.