Your bones were mostly built before you turned thirty

Most women first think about their bones when a doctor mentions a scan, usually somewhere past sixty. By then the interesting decisions have already been made, mostly by a teenager who had no idea she was making them.

That is the uncomfortable shape of this topic. Bone is a savings account with a deposit window that closes early and a withdrawal period that lasts fifty years.

Bone is not the inert scaffolding it looks like

A skeleton in a museum case suggests something finished and static. Living bone is neither.

Your skeleton is continuously taken apart and rebuilt. Old bone is removed in small patches and new bone is laid down in its place, and this happens throughout your body all the time. Roughly a tenth of your skeleton is replaced in any given year, which means the bone you have now is not the bone you had a decade ago.¹

While removal and replacement stay balanced, density holds steady. Everything in this article comes down to that balance tipping.

Peak bone mass, and when it is set

Bone accumulates fastest during puberty. By the end of adolescence, most women already hold around 90 percent of the bone they will ever have. The remainder is added slowly through the twenties, and the peak lands somewhere around age thirty.^{3 9}

After that, there is no further building phase. There is maintenance, and then there is decline.

What determined that peak was largely out of your hands: genetics accounts for a substantial share. But not all of it. Nutrition during adolescence, weight-bearing activity, and whether periods were regular all shaped the final number. Girls who trained intensely enough to lose their periods, or who restricted food heavily in their teens, tend to arrive at thirty with a lower peak, and that gap does not close later.⁸

What happens around menopause

From the mid thirties, most women lose bone slowly, in the region of half a percent a year. That is unremarkable and not worth worrying about.

Then the transition arrives, and the rate changes sharply. In the year or two before the final period and for several years afterwards, loss commonly runs at two to three percent a year, and at the spine it can be higher.^{12 13}

The reason is that estrogen was restraining the removal side of the balance. Without it, old bone is taken away faster than new bone is laid down, and the deficit accumulates every single year until the rate settles again.²

Why this window matters so much

After roughly seven years, the accelerated phase ends and loss returns to a slower background rate. That sounds like relief, and it is, but it means the largest single loss of your life happens in a compressed period that most women pass through without any measurement at all.

Nothing about it is felt. There is no symptom, no ache, no signal. A woman can lose a tenth of her skeleton and notice nothing whatsoever.

Fractures are the endpoint, not density

It is easy to treat bone density as the goal because it is the thing that gets measured. It is not the thing that matters.

What matters is whether a bone breaks. Hip fracture in particular is a serious event with a long recovery and a meaningful effect on independence afterwards. Spinal fractures are quieter and more common than most people realise, often producing nothing more dramatic than gradual height loss and persistent back pain.

Density is one input into fracture risk. The others are how likely you are to fall, how strong the muscles around the joint are, and how you land. This is why a woman with unremarkable density who falls often can be at higher real risk than a woman with lower density who never falls.

Bone responds to being loaded

Here is the mechanism worth understanding, because it explains all the exercise advice that follows.

Bone adds material where it is being stressed and removes it where it is not. Put load through a bone repeatedly and it thickens in response. Remove load and it thins. This is why astronauts lose bone rapidly in weightlessness and why a limb in a cast comes out thinner than the other one.

The signal is not effort. It is force, and specifically force that is higher or more unusual than what the bone is used to. Gentle repeated activity at a load your skeleton already handles daily sends almost no signal at all.

What kind of exercise actually builds bone

Two categories do the work.

Impact. Anything where your body weight lands. Brisk walking is the entry point, but the stronger signals come from jogging, skipping, jumping, stair climbing, dancing, racquet sports and hopping. Short bursts count. A few dozen hops a day, done most days, is a real intervention.

Resistance. Loading the skeleton through muscle. Squats, deadlifts, presses, rows, lunges, and carrying heavy things. The load needs to be genuinely challenging by the last few repetitions, otherwise the bone has no reason to respond.

Two to three sessions a week of resistance work plus impact on most days is the pattern that shows up in the evidence, and it also happens to build the muscle and balance that keep you upright.¹¹

What does not count, and why

Swimming and cycling are excellent for your heart and close to useless for your bones. In both, your body weight is supported, so the skeleton never experiences the load that triggers a response.

This surprises lifelong cyclists, some of whom have notably low bone density despite being extremely fit. Cardiovascular fitness and bone strength are separate outcomes and require separate inputs. Keep the cycling. Add something that lands.

Calcium, and the ceiling on it

Calcium is the raw material. Without enough, nothing else works, which is why it dominates the conversation. The trap is assuming that more is better.

Most guidance lands at 1000 to 1200 mg a day for women over fifty. Above adequacy, extra calcium does not build extra bone, and very high supplement intakes have not demonstrated fracture benefit while carrying their own small risks.⁴

Food first is the sensible position: dairy, tinned sardines and salmon with the bones in, tofu set with calcium, fortified plant milks, almonds, and leafy greens such as kale and pak choi. Supplement the gap rather than the whole amount.

Vitamin D, and who is actually short

Vitamin D is what allows you to absorb the calcium you eat. Take one away and the other becomes theoretical.

Anyone living at a northern latitude produces essentially none from sunlight over the winter months, which is why several countries now recommend a daily supplement of around 10 micrograms, or 400 international units, from autumn to spring for the whole adult population. Higher doses are used to correct a confirmed deficiency and are not needed as a routine.^{7 5}

Darker skin, covered skin, and spending most daylight hours indoors all raise the likelihood of being genuinely short.

Protein, the underrated one

Roughly half of bone by volume is protein scaffolding, with mineral deposited into it. A calcium-rich diet with too little protein is building on nothing.

There is a second reason. Protein intake is what allows you to hold onto muscle, and muscle is what pulls on bone and what stops a stumble from becoming a fall. Older women who under-eat protein lose muscle and bone together.¹⁴

Most women fall short at breakfast and catch up unevenly at dinner. Spreading it across the day works better than loading it into one meal.

Things that quietly take bone away

Smoking is directly harmful to bone and roughly accelerates loss. Heavy alcohol interferes with the rebuilding side and increases falls, which is a double cost.

Long courses of oral steroids are one of the most significant causes of bone loss in medicine, and anyone taking them for more than three months should have the bone question raised explicitly.¹

Then there is under-eating. Persistent low energy intake, at any age, suppresses the hormonal signals that maintain bone. A woman who has been restricting for years may have bones that look a decade older than she is.

Who should be scanned, and when

Density is measured with a low-dose scan of the hip and spine, usually called DEXA. It takes ten minutes and involves less radiation than a long flight.¹⁰

It is generally offered to women over 65, and earlier to women with specific risk factors: a fragility fracture as an adult, early menopause before 45, a parent with hip fracture, long-term steroids, very low body weight, coeliac disease or another condition affecting absorption, or an eating disorder history.

If you match any of those, ask rather than wait to be offered. Guidelines describe populations; you have to raise your own case.

What the result actually means

A scan produces a T-score, which compares you to a healthy young adult. Above minus one is considered normal. Between minus one and minus 2.5 is reduced density, sometimes called osteopenia. At or below minus 2.5 is osteoporosis.

Those thresholds are administrative lines drawn across a continuous scale, not diagnoses of disease. A score of minus 2.4 is not meaningfully different from minus 2.6 in your body, only in whether treatment is offered. What the number is for is placing you in a risk calculation alongside age, weight, family history and previous fractures.

When it is not just menopause

Low bone density is not always explained by the transition, and assuming it is can miss something treatable.

An overactive thyroid speeds bone turnover. Coeliac disease and other absorption problems starve the process of raw materials, sometimes with no digestive symptoms at all. Overactive parathyroid glands pull calcium directly out of bone and show up as a raised calcium level on ordinary blood tests. Chronic kidney disease, some cancers, and certain long-term medications all have their own effect.

This matters most for a woman who is younger than expected or whose scan is worse than her history predicts. In that situation, the right response is blood tests looking for another cause rather than a supplement and a follow-up in two years.

Medication, briefly and honestly

For women at genuinely high fracture risk, medication reduces fractures, and the size of that reduction is well established.² The drugs are not a substitute for loading and nutrition; they work alongside them.

They are also not a first move for someone with mildly reduced density and no other risk factors. The threshold for offering them is deliberately based on calculated fracture risk rather than the scan number alone. If treatment is raised with you, the useful question is what your calculated ten year risk is, not what your T-score is.

What a realistic first month looks like

Week one, change nothing and find out where you stand. Roughly total your calcium on an ordinary day. Note whether you take vitamin D at all through winter. Count how many days last week involved either impact or lifting something heavy.

Week two, add one impact habit small enough to survive a bad week. Sixty seconds of hopping while the kettle boils. Stairs instead of the lift. That is a genuine signal to your skeleton and it costs nothing.

Weeks three and four, add one resistance session and keep the impact habit. If you have never lifted, start with body weight and add load once the movement is comfortable.

Then stop expecting to see anything. Bone changes over years and there is no feedback you can perceive. This is one of the few areas of health where you have to act on the mechanism rather than on how you feel.

The honest limits of this

You cannot rebuild a peak you never reached. Exercise and nutrition in midlife slow loss and improve strength and balance, and both of those genuinely reduce fractures, but no amount of squatting at fifty recovers what a restricted adolescence cost.

Nor can you tell where you stand without measurement. There is no symptom, no self-test, and no reliable proxy. Bone density is one of the small number of things in health that is entirely invisible until it fails.

Which is the argument for doing the ordinary things now, without waiting for a number to justify them. Loading your skeleton, eating enough protein and calcium, and staying steady on your feet are all worth doing on their own terms, and the bone benefit arrives whether or not anyone ever scans you.