Training
Bone under load
Bone responds to training the way muscle does — it gets stronger. It only does that if there is enough energy coming in, and the commonest reason it does not is not calcium.
What most people believe
Stress fractures come from training too much, and the nutritional side is about getting enough calcium.
What the evidence actually says
Bone is not scaffolding, it is a tissue with a budget
It is constantly being demolished and rebuilt. Loading it — running, jumping, lifting — signals it to rebuild stronger in the places taking the load, which is why weight-bearing athletes have denser bone than swimmers.
Rebuilding costs energy, and it is not an urgent cost. Faced with a shortage, a body funds the things that keep it alive today and defers the things that pay off over years. Bone remodelling is squarely in the second category, along with reproduction and immune function.
So the failure mode is not "trained too hard". It is "trained this hard on this little food", and those are different problems with different fixes. One of them gets worse if you respond by resting and eating less.
The number that actually predicts it
Energy availability is the energy left over after training, expressed against lean mass. It is the input the body is really reading, and below a threshold the hormonal consequences begin: sex hormones fall, bone turnover shifts toward breakdown, and in women periods become irregular or stop.
That is a signal, not a side effect. A runner whose periods have stopped is being told something specific about her bone, and the correct response is not to treat the symptom.
This is why the syndrome was renamed. It was the female athlete triad; it is now relative energy deficiency in sport, because it turned out to affect men too and to reach well beyond bone — into immunity, mood, cardiovascular health and the performance the whole thing was meant to protect.
Where calcium and vitamin D come in, and where they do not
They matter. Vitamin D governs how much of your calcium you absorb, and an athlete low in it can eat plenty and still not get it into bone. Athletes also lose meaningful calcium in sweat, which is why the figures used in sport sit above the general recommendation.
But they are the raw materials, not the instruction. Supplying bricks to a site with no budget to build does not produce a wall. This is the part that gets inverted in practice: the athlete with three stress fractures is handed calcium tablets and not asked what she eats.
The uncomfortable part
A meaningful share of stress fractures in endurance sport occur in athletes who are under-eating, and a meaningful share of those are under-eating on purpose, because in most endurance disciplines being lighter is faster until quite suddenly it is not.
That means the honest version of this page is partly about a psychological pattern rather than a nutritional one, and a nutrition article is the wrong instrument for it. If eating less is the strategy and the injuries keep coming, the useful conversation is with a sports physician and often a dietitian, not with a calcium supplement.
The numbers, and how firmly they are held
| What it describes | Figure | Evidence |
|---|---|---|
| Calcium for athletes in heavy training Above the general recommendation, largely for sweat losses | 1,000–1,500 mg | Probable |
| Blood vitamin D regarded as sufficient | ≥ 50 nmol/L | Established |
| Energy availability considered adequate Per kg of fat-free mass | 45 kcal/kg FFM | Established |
| Threshold below which hormonal disruption begins The number that actually predicts bone injury | < 30 kcal/kg FFM | Established |
| Athletes affected by a stress fracture at some point | ≈ 20 % | Probable |
Established means the position stands agree and you can act on it. Probable means the weight of evidence points one way with real dissent. Contested means it is genuinely unsettled, and anyone selling you certainty about it is selling something.
What follows is easier when you know what you are already eating.
What to actually do
Ask about the food before the training load
Repeated stress fractures in someone who trains sensibly point at intake, not volume. Resting and eating less makes that version worse.
Treat a missing period as bone information
It is one of the clearest signals that energy availability is too low, and it is about the skeleton as much as the cycle.
Fix vitamin D before increasing calcium
Without it you absorb a fraction of what arrives, so the order matters.
If lightness is the plan, get someone else in the room
A sports physician and a dietitian. This is the point where an article stops being the right instrument.
Sources
- IOC consensus statement — relative energy deficiency in sport (REDs)
- NIH Office of Dietary Supplements — Vitamin D
- Stress fracture epidemiology in athletes
This page is education, not medical advice. It does not diagnose anything and it is not a substitute for a clinician who knows your history. Sports nutrition in particular is a field where the evidence moves, and where a figure that suits a trained twenty-five-year-old may be wrong for you.
Every nutrient named above, measured — in the app.