Abnormalities of the PTH-vitamin D axis and bone turnover markers in children, adolescents and adults with cystic fibrosis: comparison with healthy controls

Abnormalities of calcium and vitamin D metabolism in cystic fibrosis (CF) are well documented. We tested the hypothesis that alterations in calcium metabolism are related to vitamin D deficiency, and that bone resorption is increased relative to accretion in patients with CF. Calcitropic hormones, electrolytes, osteocalcin (OC) and bone alkaline phosphatase (BAP), (markers of bone mineralisation), urinary deoxypyridinoline [total (t) Dpd, a marker of bone resorption] and lumbar spine bone mineral density (LS BMD), expressed as a z-score, were measured in 149 (81 M) CF and 141 (61 M) control children aged 5.3-10.99 years, adolescents aged 11-17.99 years and adults aged 18-55.9 years. Data were analysed by multiple regression to adjust for age. In patients, FEV1% predicted and CRP (as disease severity markers), genotype and pancreatic status (PS) were recorded. The distribution of PTH differed between groups (P<0.0001), with CF levels both below and above the control range. 25OH vitamin D (25OHD) was not different in control and CF subjects (P=0.06). Active hormonal vitamin D (1,25(OH)(2)D) was lower in the CF group (P<0.0001), not explained by 25OHD or disease severity, as was serum magnesium (P<0.0001). OC was decreased in CF adults (P=0.004), and tDpd increased in CF adolescents (P=0.003) and adults (P=0.03). The ratio of OC to tDpd (a measure of bone coupling) was similar in CF and control children, but decreased in CF adolescents (P=0.04) and adults (P=0.02), suggesting decreased overall bone accrual in CF adolescents and uncoupling of bone balance in adults. 1,25(OH)2D was weakly correlated with OC in CF children (r=0.43, P=0.01), and with tDpd in CF and control adolescents (r=0.33, P=0.05 and r=0.36, P=0.02, respectively); thus there was limited evidence of association of calcitropic hormones, which had an abnormal pattern in all age groups, with bone turnover. There was no association between calcitropic hormones or bone turnover markers and LS BMD z-score. Despite vitamin D sufficiency, abnormalities of calcium metabolism and bone turnover markers were still apparent and bone accretion was decreased relative to resorption in the CF adolescent and adult groups. These changes were not fully explained by disease severity or genotype, but are consistent with reports of decreased BMD and unique bone histomorphometry in older subjects with CF.