Vitamin D metabolism and bone mineralization in children with juvenile rheumatoid arthritis

Objective : To examine bone mineralization and bone mineral content in a cross-sectional population of children with juvenile rheumatoid arthritis (JRA). Methods : Bone mineral content was measured by single-photon absorptiometry in 44 children with JRA and 37 control children. Serum concentrations of minerals, vitamin D, parathyroid hormone, osteocalcin, bone alkaline phosphatase, and tartrate-resistant acid phosphatase, and urinary concentrations of minerals, were determined. Results : Bone mineral content was decreased in children with JRA. Significantly lower concentrations of osteocalcin (7.4±3.4 vs 12.5±2.5 μg/L) and bone alkaline phosphatase (78.8±36.4 vs 123.0±46.0 IU/L) suggested reduced bone formation; lower levels of fartrate-resistant acid phosphatase (10.3±4.1 vs 14.4±5.8 IU/L) and a lower urinary calcium/creatinine ratio (0.07±0.06 vs 0.12±0.09) suggested decreased bone resorption. The serum calcium concentration was significantly lower (9.3±1.0 vs 10.0±0.4 mg/dl), as was the parathyroid hormone concentration (19.8±8.6 vs 26.7±9.3 ng/L); 1,25-dihydroxyvitamin D values (30.1±10.5 vs 30.4±9.3 pg/ml) were normal. Conclusion : These data suggest that decreased mineralization in JRA is related to low bone turnover; parathyroid hormone and 1,25-dihydroxyvitamin D levels may be inappropriately normal for the decreased serum calcium concentration in children with JRA.