How does the relative activity of osteoblasts and osteoclasts change from childhood into older adulthood, and what is the primary consequence for bone mass?

• A. Osteoclasts dominate in childhood to shape bones during growth while osteoblasts take over in adulthood to maintain bone mineral homeostasis, resulting in higher bone mass after age 30.
• B. The relative activities oscillate monthly according to dietary calcium intake with no long-term trend, so aging per se does not change net bone balance unless calcium intake is below 100 mg/day.
• C. Osteoblast activity dominates throughout life while osteoclast activity is negligible after puberty, so adult bone mass remains virtually unchanged except with traumatic injury.
• D. In childhood and up to about the third decade osteoblast deposition exceeds osteoclast resorption causing net gains in bone mass; from mid-adulthood onward resorption often exceeds deposition leading to progressive bone loss.

Answer: (D)