Bone Abstracts

Objectives: Obese adults commonly have vitamin D deficiency and secondary hyperparathyroidism prior to bariatric surgery. Whether similar metabolic abnormalities exist in morbidly obese adolescents is not known. This study investigated the prevalence of vitamin D deficiency and evaluated the relationship between vitamin D and parathyroid hormone (PTH) in morbidly obese adolescents undergoing evaluation for bariatric surgery.

Methods: A cross-sectional study of preoperative laboratory measures from 236 adolescents evaluated for bariatric surgery between March 2006 and June 2011.

Results: Of 236 patients, 219 patients had 25-hydroxyvitamin D (25OHD) and PTH levels. Patients (76 boys and 143 girls; age, 15.9±1.2 years; 43% Caucasian; 35% Hispanic; and 15% African–American) had mean BMI of 47.6±8.1 kg/m². Serum 25OHD levels (mean 20.7±9.8 ng/ml) were deficient (<20 ng/ml) in 53%; 8% had severe deficiency (<10 ng/ml); only 18% of patients were replete (>30 ng/ml). Findings varied by race, with 82% of African-Americans, 59% of Hispanics, and 37% of Caucasians vitamin D deficient. Race was the strongest predictor of 25OHD (P<0.0002). Greater BMI was associated with lower serum 25OHD (r=−0.28, P<0.0001); this persisted after controlling for PTH (r=−0.22, P<0.002). PTH levels varied inversely with 25OHD (r=−0.24, P=0.0003). Although only eight patients (3.7%) had frankly elevated PTH levels (>55 pg/ml), 37 patients (16.9%) had PTH levels in the upper third of normal (>40 pg/ml), 89% of whom had 25OHD levels <30 pg/ml. African American race, BMI, and PTH explained 21% of the variance in 25OHD (P<0.0001).

Conclusions: The vast majority of severely obese adolescent patients presenting for bariatric surgery have sub-optimal vitamin D levels. Those at greatest risk for deficiency were African-American and those with highest BMIs. Our results also suggest that the current laboratory normal range for PTH may significantly overestimate normal values for children and adolescents and underestimate the prevalence of secondary hyperparathyroidism with its subsequent negative effects on long-term bone health. This is particularly important prior to bariatric surgery, where weight loss itself and decreased calcium and vitamin D absorption in some procedures may place these patients at further risk.