Calcium kinetics and bone health in girls with cystic fibrosis

As the life span of individuals with cystic fibrosis (CF) increases, osteopenia and osteoporosis have emerged as important secondary consequences of the disorder. Peak bone mass is nearly entirely achieved by late puberty, making adolescence an important time of life for optimizing bone mineralization. Characterizing calcium metabolism will identify alterations in bone mineralization that may compromise bone mass in individuals with CF. Calcium absorption, losses and bone turnover were studied using calcium stable isotopes in 23 girls with CF (ages 7–18 years) who maintained regular treatment regimens (including pancreatic enzymes and ADEK vitamins). Oral (44Ca) and intravenous (42Ca) isotopes were administered with breakfast in the Pediatric Clinical Research Unit (PCRU) of The Johns Hopkins Hospital. A 24-h urine collection and blood samples (5, 15, 30, 60, 120, 240, 360, and 480 minutes post-dosing) were obtained. Spot urine samples and complete stool output were collected for 5 additional days. Dietary intake data was collected for 4 days, and bone mineral content was measured. Calcium absorption (%) (44Ca relative to 42Ca in 24-h urine collection) was similar to values observed in healthy girls, and total calcium absorption (Va) was highest during early puberty (breast Tanner stage 2–3). Urinary calcium and sodium excretion were positively associated. Endogenous fecal calcium (Vendo; n = 13) (42Ca in stool) was elevated and compromised calcium balance. Increased intestinal permeability in patients with CF may contribute to both adequate Va and elevated Vendo. Bone calcium deposition (Vo+) (evaluated by fitting a 3 compartment model to the disappearance of 42Ca in serum and urine), and retention (n = 22) were comparable among early pubertal girls with CF to values observed in healthy girls, but were low in the prepubertal (Tanner stage 1) and late pubertal (Tanner stage 4–5) girls with CF. Bone calcium deposition was positively associated with serum leptin. Bone mineral content was positively associated with body mass index and insulin-like growth factor-1. Bone health is influenced by calcium balance, nutritional status, and pubertal development in girls with CF. Findings from this study will help clinicians identify individuals with CF who are at risk for osteoporosis and will help to determine beneficial interventions to preserve bone mass.