Bone facts worth knowing: what's actually happening in your skeleton

How bone remodels, how much calcium and vitamin D you actually need, and which training builds density — with concrete numbers and clear red flags.

Bone looks like dry, dead material, but it is living tissue that breaks itself down and rebuilds every day, responds to load, and even releases hormones. If you train, your skeleton changes the same way your muscles do — just far slower and far more quietly. Here's what's genuinely interesting, and more importantly, what you can do with it.

Your skeleton is constantly rebuilt

An adult has 206 bones. A newborn has around 270 — many fuse during growth, including the skull plates, the sacrum, and the pelvis. In most people the last growth plates close between ages 16 and 21.

After that, bone doesn't sit still. Cells called osteoclasts chew out old bone, and osteoblasts lay down new bone in its place. About 10% of your skeleton is replaced every year, meaning that in roughly a decade you're carrying almost entirely new bone. Spongy bone — the vertebrae and the neck of the femur — turns over faster than dense cortical bone, which is exactly why loss shows up there first, and why the first osteoporotic fractures happen in the spine and hip.

A few numbers for perspective

Your skeleton is about 15% of body mass — around 12 kg in an 80 kg person.
The femur withstands roughly 170 MPa in compression, about four to five times more than ordinary concrete, at a fraction of the weight.
The smallest bone is the stapes in the middle ear: about 3 mm long and under 4 milligrams.
Bone marrow produces roughly 2 million red blood cells every second.
The hyoid is the only bone in the body not joined to another bone by a joint.
About 60% of bone mass is mineral (mostly calcium and phosphorus) and about 25% is collagen. Mineral gives hardness, collagen gives toughness — which is why bone splinters like dry wood rather than shattering like glass.

Bone listens to load

Bone builds where it gets compressed and bent. In professional tennis players, the cortical bone in the serving arm is up to 20% heavier than in the other arm — same person, same diet, different loading.

It works in reverse too. Astronauts in microgravity lose 1–1.5% of hip bone mass per month, and prolonged bed rest does something similar. Bone has no reason to stay expensive if nobody is loading it.

The signal that builds bone isn't duration, it's magnitude and rate of loading. Two hours of easy walking does less for density than 30 jumps.

What to actually do

Heavy compound lifts, 2× per week: squat, deadlift, press, row — 3–5 sets of 3–6 reps at roughly 80–85% of 1RM.
Impact loading: 3 sets of 10 jumps or drop-landings from 20–30 cm, 3× per week. Rest 10–15 seconds between jumps — bone responds better to separated impulses than to 100 jumps in a row.
Running progression: don't raise weekly mileage by more than 10%. Jumping ahead of that is the single most common cause of stress fractures in the shin and foot.
Balance, 5–10 minutes a day: single-leg stands of 30 seconds. After 50, the biggest fracture-saver is simply not falling.

Swimming and cycling are excellent for your heart and joints but do almost nothing for bone density — no impact, no bodyweight loading.

Food that builds bone

Calcium: 1000 mg per day for adults, 1200 mg for women over 50 and men over 70. For reference: 200 ml milk ≈ 240 mg, 30 g hard cheese ≈ 200 mg, 150 g yoghurt ≈ 200 mg, 100 g sardines with bones ≈ 380 mg, 100 g kale ≈ 150 mg.
Vitamin D: 800–1000 IU per day. In summer, 15–20 minutes of sun on arms and face covers a lot, but from October to March at northern latitudes you produce essentially none from sunlight.
Protein: 1.6–2.0 g per kg of bodyweight. A quarter of bone is collagen, and collagen is protein.
Enough calories: chronic underfeeding and a BMI under 18.5 strip bone faster than any single mineral deficiency.

What speeds up the loss

Peak bone mass arrives between 25 and 30 — after that you're defending what you built. From 40 onward you lose about 0.5–1% per year, and women lose 2–3% per year during the first 5–7 years after menopause. Smoking, more than two alcoholic drinks a day, corticosteroids taken longer than three months, and a fully sedentary day with no hard loading all push that faster.

When to see a doctor

A fracture from a fall at standing height or from trivial force — that's a warning sign at any age.
Pinpoint pain in one spot of the shin or foot that appears while running and settles at rest — stop running and get a stress fracture ruled out.
Bone pain lasting more than 2–3 weeks, waking you at night, or worsening at rest.
Losing 3 cm or more of height, or a new forward hunch.
Periods absent for 3 months or longer in women who train — that hits bone directly.
DEXA density scanning is routinely advised for women from 65 and men from 70, earlier with risk factors. A T-score of −1 to −2.5 means osteopenia, below −2.5 means osteoporosis.

The short version

Bone is living tissue — roughly 10% of your skeleton is replaced every year.
Density comes from heavy lifts (3–5 × 3–6 reps at 80–85% 1RM) and jumps (3 × 10, three times a week), not from cardio.
Target 1000–1200 mg calcium and 800–1000 IU vitamin D daily, plus 1.6–2.0 g protein per kg.
Peak bone mass hits around 25–30; after 40 you lose 0.5–1% a year, up to 3% after menopause.
A fracture from a standing-height fall, pinpoint running pain, or 3 cm of lost height means get checked, not wait.