A window that does not reopen
Bone is living tissue that is constantly rebuilt, but the rate at which it can be added is far higher in adolescence than later. Most adult bone mass is in place by the early twenties, and the peak reached then influences fracture risk decades on.
For a young athlete this is a rare thing: an opportunity that sport is unusually good at capturing, provided the fuel is there.
Teens
When most adult bone is laid down
Early 20s
When peak bone mass is broadly set
Decades
How long that peak keeps mattering
Why these years count
What builds bone
Swimming and cycling are excellent for fitness but provide little of the impact bone responds to. Athletes in those sports benefit from adding jumping and strength work deliberately.
- Jumping, hopping, sprinting and change of direction, short bursts of impact.
- Resistance training, progressed sensibly over months.
- Variety of directions and sports rather than one repetitive pattern.
- Consistency, a couple of sessions weekly across years rather than blocks.
What undermines it
- Not eating enough for training and growth.
- Periods that are absent or have stopped.
- Low calcium or vitamin D intake.
- Long periods off feet after injury without replacing the loading.
- Very high running volume with no strength work alongside it.
The nutrition part, kept simple
Enough total food comes first, because no amount of calcium helps if overall intake is short. Then calcium, from dairy or fortified alternatives, and vitamin D, which is difficult to get from UK food and sunlight in winter.
UK guidance is that everyone considers a daily 10 microgram vitamin D supplement in autumn and winter. For an indoor-training teenager that is worth doing.
The routine that protects bone
- Two strength sessions a week, all year, including some jumping.
- A calcium source at most meals.
- A vitamin D supplement through autumn and winter.
- Eat before and after training as a habit.
- Treat a missing period or a second bone injury as a reason to see a GP.
If there has already been a bone injury
One stress fracture is worth taking seriously; two is a clear signal to look at fuelling, cycles, vitamin D and load rather than only the injured bone. Ask the GP about bone health specifically, and expect the plan to include eating more, not only training less.