Bone appears static and is not. It is continuously broken down and rebuilt throughout life, and bone density reflects the balance between those two activities rather than a fixed structure.
Two cell types work in opposition
Osteoclasts dissolve mineral and break down the protein matrix, excavating small cavities in bone.
Osteoblasts follow behind, laying down new matrix and mineralising it, and the paired process is called remodelling.
The skeleton is replaced gradually over years by this cycle, which is also how bone repairs microscopic damage and adapts its shape to load.
Peak bone mass is reached early
Building outpaces breakdown through childhood and adolescence, with the fastest accumulation during the growth spurt.
Maximum density is generally reached in the twenties, after which the balance gradually shifts the other way.
The peak matters because later loss starts from it, which is why nutrition and weight-bearing activity in youth influence the position decades later.
Oestrogen restrains the demolition side
Oestrogen acts on both cell types but its dominant effect is to limit osteoclast formation and shorten osteoclast lifespan.
With less oestrogen present, osteoclasts are more numerous and longer-lived, and they excavate faster than osteoblasts refill.
The imbalance is what produces net loss, and it explains why the rate of loss accelerates around menopause before settling to a slower ongoing rate.
Other situations that lower oestrogen, including prolonged absence of menstruation from any cause, affect bone through the same mechanism.
Density is measured, not felt
Loss of bone density produces no symptoms. There is no pain and no sensation associated with it until a fracture occurs.
It is assessed by a scan that compares mineral content against a reference population, and the result is expressed as a score rather than an absolute quantity.
Fracture risk calculators combine that score with age and other factors, because density alone does not determine how likely a bone is to break.
Load is the other signal bone responds to
Bone adapts to mechanical strain, and cells embedded within it detect loading and signal for reinforcement where it is needed.
Weight-bearing and resistance activity supply that signal, while activities that unload the skeleton do not, which is why the type of exercise matters here more than the amount of movement.
Decisions about testing and about treatment after a diagnosis depend on individual risk and belong with a doctor.