Objective: Bone mineral density is compromised in individuals with cystic fibrosis (CF); calcium is the major bone mineral. This study examined the impact of endogenous fecal calcium (V(endo)) losses on calcium balance in girls with CF. Study design V(endo) was measured in 12 girls with CF (aged 7-18 years): 7 younger, premenarcheal girls with compromised nutritional status; and 5 older, postmenarcheal girls with adequate nutritional status. V(endo) was measured as the amount of intravenously administered (42)Ca, a calcium stable isotope, in stool relative to urine over 6 days. V(endo) was compared between pre- and postmenarcheal girls by Student's t test. Actual calcium balance [absorbed calcium-(urinary calcium (V(u))+V(endo))] was compared with estimated balance (assuming V(endo)=1.6 mg/kg/day calcium) by paired t test.
Results: V(endo) was 99.3+/-42.3 mg/day. By body weight, V(endo) was highest among premenarcheal girls (3.37+/-1.09 mg/kg/day), resulting in excess losses (>1.6 mg/kg/day) of 55.0+/-45.7 mg/day. Over 1 year, this represents 20.1+/-16.7 g of unattained bone calcium or 6.7+/-4.2% of the bone calcium content of these girls.
Conclusions: V(endo) is a significant source of calcium loss in individuals with CF and may limit calcium availability for bone mineral deposition.