Impaired bone formation in male idiopathic osteoporosis: Further reduction in the presence of concomitant hypercalciuria

We present iliac bone histomorphometric data and related biochemical data from 16 nonalcoholic men (50±11 (SD) years) referred for evaluation of spontaneous skeletal and/or appendicular fractures and reduced spinal bone density. All men were eugonadal and had no known underlying disorder associated with osteopenia. For the group, mean serum chemistry values were within normal limits including immunoreactive parathyroid hormone, osteocalcin and serum 1,25-dihydroxyvitamin D [1,25(OH)₂D]. Nine men demonstrated hypercalciuria (≥0.1 mmol/kg per day) while on a constant metabolic diet of 20 mmol/day Ca. Their 24-hour urinary calcium was significantly greater than that for the remaining 7 men (7.4±1.6 vs. 5.0±0.8 mmol/day, p=0.003), as was their calciuric response to a 1 g oral calcium load (0.23±0.06 vs. 0.15±0.05 Ca/creatinine, p=0.042). Serum parameters (including parathyroid hormone and 1,25(OH)₂D) of hypercal-ciuric and normocalciuric men were not significantly different. Histomorphometric indices for cancellous bone demonstrated significant differences between the entire group of osteoporotic men and age-adjusted normal values for bone volume (11.4±4.0% vs. 23.2±4.4%), osteoid surface (5.6±3.9% vs. 12.1±4.6%), osteoblastic surface (2.0±2.3% vs. 3.9±1.9%), and mineralizing surface (1.9±2.4% vs. 5.1±2.7%);there were also significant differences in bone formation rate (total surface referent) (0.004±0.001 vs. 0.011±0.006 mm³/mm² per year). Compared with the normocalciuric group the 9 hypercalciuric men had significantly lower osteoblastic surfaces (1.6±1.9% vs. 2.5±2.6%) and mineralizing surfaces (1.4±1.5% vs. 2.7±3.2%). Cortical bone indices demonstrated a similar trend in formation parameters although these differences did not reach significance. These results suggest that idiopathic osteoporosis in men is characterized by suppressed bone formation due to reduced osteoblast proliferation and that this defect is exaggerated in hypercalciuric men as opposed to normocalciuric men. The cause for suppressed bone formation and increased intestinal absorption of calcium in some men is not known but may be the result of 1,25(OH)₂D or some previously unrecognized factor(s).