Why do bones break more easily as you get older?

Trabecular bone, the spongy interior of bones, begins to decline as early as your thirties, well before hormones play a role. Between the ages of forty and eighty, women lose around 42% of the spongy bone in the spine and 29% of the spongy bone in the hip crest. The hard outer bone (cortical bone) follows later: it shrinks noticeably from around the age of fifty to sixty, and women lose approximately 31% of the cortical bone in the upper arm over that same period.

The fastest acceleration of bone loss occurs when sex hormones decline. In women after menopause, bone breakdown speeds up noticeably, because oestrogen acts as a brake on the cells that break down bone. In older men, a similar but more gradual process takes place, in which declining testosterone plays a role.

Not only the quantity of bone matters -- its quality changes too. The fibre structure of bone (collagen) forms increasingly rigid cross-links over the years, making bone less able to flex and more likely to fracture on impact. At the same time, the internal supporting struts become thinner and the outer bone layer more porous. Small cracks that bone would normally repair on its own accumulate, because that repair process slows down in older people.

Two other factors accelerate the process. First, the production of active vitamin D declines in older people, which means the gut absorbs less calcium. The body compensates by releasing more parathyroid hormone, which further drives bone breakdown. Second, older people are on average less physically active, and bone needs mechanical loading as a stimulus for building.

Ultimately, fracture risk can never be traced back to a single mechanism. More fragile, more porous bone combined with muscle loss and reduced balance means that a simple fall at an older age leads to a fracture far more often than it would at a younger age.