Bone density

Bone density, or bone mineral density (BMD), is a measure of how dense, and so how strong, your bones are. The denser the bone, the less likely it is to break. It is measured with a bone density scan, also called a DEXA scan, which uses low-dose X-rays and is more effective than ordinary X-rays at spotting low bone density (NHS).

How is bone density measured?

You lie on your back on an open table while a scanning arm passes slowly over you, usually over the hip and lower spine. It takes 10 to 20 minutes and uses far less radiation than a standard X-ray, though it is not recommended in pregnancy (NHS; NHS).

Results come as two scores. The T-score compares your bone density with a young healthy adult’s, and the World Health Organization classes it like this (NHS):

• above -1: normal
• between -1 and -2.5: mildly reduced bone density
• -2.5 or below: osteoporosis

The Z-score compares you with people of your own age, sex and ethnicity. Below -2 means your bones are less dense than expected for your age, and it is the score usually used for children and people under 30.

A scan is not a crystal ball. Someone with low bone density may never break a bone, and someone with average density may have several fractures, because age, sex and previous falls matter too. Doctors use the scan alongside a fracture risk assessment, and it is mainly offered to people over 50 with a risk of osteoporosis, or younger people with other risk factors such as smoking or a previous broken bone (NHS).

Does sitting all day affect bone density?

Bone responds to load. The general view is that long periods of disuse reduce bone mass, while mechanical loading through exercise increases it (Howe et al., 2011).

What that means for sedentary behaviour has been studied less. In a US survey of 2,117 adults aged 23 to over 90 who wore activity monitors for a week, women who sat more had lower bone density across the hip, regardless of how much moderate or vigorous activity they did, and long unbroken sitting bouts mattered more than how many bouts they had. In men, sitting time showed no link, but more moderate and vigorous activity went with higher hip bone density. Neither sex showed a link at the spine (Chastin et al., 2014). The study was cross-sectional, so it shows an association, not cause and effect. The sitting time calculator shows how those hours add up.

What helps keep bones strong?

Much of the groundwork is laid early. Lifestyle accounts for an estimated 20 to 40% of the peak bone mass an adult reaches, and the best evidence is for calcium and physical activity in late childhood and around puberty (Weaver et al., 2016).

Later in life, exercise still helps, modestly. A Cochrane review of 43 trials with 4,320 women after the menopause found a small but possibly important effect on bone density. Resistance training for the legs did best at the hip, and combined programmes did best at the spine. The trials did not show a reduction in fractures, and their reporting quality was low (Howe et al., 2011).

The NHS advises enough calcium and vitamin D, regular weight-bearing, strengthening and balance exercise, not smoking and not drinking too much alcohol (NHS). For people who already have osteoporosis, a UK consensus recommends resistance and impact exercise and avoiding postures that involve a high degree of forward bending of the spine (Brooke-Wavell et al., 2022).

When to see a doctor

Low bone density usually causes no symptoms until a bone breaks, often after a minor fall. Back pain, a curving or hunched spine and getting shorter can all be signs of small breaks in the spine, known as vertebral fractures. See a GP if back pain isn’t improving after a few weeks, your posture becomes hunched, or you are worried about your risk. Ask for an urgent GP appointment or contact 111 if pain after a recent fall or injury isn’t improving, and call 999 for severe pain after a fall (NHS).