What causes osteoporosis?

To understand the cause of osteoporosis, we first need to know how our bones grow when we’re young and renew as we age.

As we go about our daily lives, the older, worn-out bone tissue in our bones is being broken down by specialist cells called osteoclasts and rebuilt by bone-building cells called osteoblasts. This process of renewal is called bone-remodelling.

Bone remodelling

In childhood, osteoblasts work faster, enabling the skeleton to increase in size, density and strength. Bones stop growing in length between the ages of 16 and 18 years but the total amount of bone tissue we have continues to increase slowly until our late twenties.

In younger adults, up until about the age of 35 years, there is usually a balance between the amount of bone that is removed by these specialist cells and the amount of bone that is laid down, meaning the total amount of bone tissue we have stays the same.

Bone density chart

Adapted from J Compston 1990

'Bone thinning'

After the age of about 35 years, the body starts to remove more bone than it replaces. As a result, the total amount of bone tissue starts to decrease.

This is often described as ‘bone loss’ or ‘bone thinning’. Our bones won’t look any different from the outside but inside, the cortical ‘shell’ thins and the struts that make up the inner structure become thinner and sometimes break down.

Bone thinning

Bone thinning is much more significant as we move into later life, which explains why bones become more fragile and fractures become more common in old age.

Not everyone will experience broken bones due to low bone strength. There are many other risk factors for osteoporosis besides just age. However, the older we get, the more likely osteoporosis and broken bones become.

More about osteoporosis risk factors