Bone density is the amount of mineral packed into a piece of bone, and it decides how much your skeleton can take before something breaks. You build it up until roughly age 30, and after that it slowly moves the other way. Athletes often assume they are safe here because they load their bones, but that is only half right.
The half that is wrong is about energy. Anyone eating too little for their training volume for years is breaking down bone while they train.
That is the part almost nobody tells men.
What counts as a normal bone density?
Bone density is usually expressed as a T-score from a DEXA scan, where around zero is average for a young adult. Blood does not measure bone density, but a few markers do show whether bone metabolism is drifting out of balance. Those markers move before the scan changes.
Loading is the strongest signal for bone. Jumping, lifting and sprinting do more than walking.
Swimming and cycling do almost nothing here, however fit they make you.
Why are endurance athletes particularly at risk?
Because a chronic energy shortfall throttles the hormonal system that maintains bone. The IOC describes this as relative energy deficiency in sport, or RED-S, where too little available energy affects bone health, hormones and immunity among other things (PMID 29773536). It explicitly affects men too.
The original consensus already widened this beyond the classic female athlete triad (PMID 24620037).
Picture a cyclist of 38 training 20 hours a week on 2,200 kilocalories a day. He is fit, he is light, and his bone is quietly paying for it.
Which blood values relate to bone?
Four markers tell you most together: vitamin D, calcium, PTH and alkaline phosphatase. On their own they say little, but the pattern between them is revealing. A low vitamin D with a high PTH, for instance, is a very different story from a low vitamin D with a normal PTH.
This is how I read that combination.
| Pattern | What it may suggest | Sensible next step |
|---|---|---|
| Vitamin D low, PTH raised, calcium normal | The body is pulling calcium from bone to keep blood calcium steady | Discuss your vitamin D status with your GP |
| Vitamin D low, PTH normal, calcium normal | A shortfall without a clear effect on bone metabolism | Food and daylight, then retest |
| Alkaline phosphatase raised, liver normal | Increased bone turnover, which can fit recovery or heavy impact | Read it alongside your training load and injury history |
| Low testosterone in a man | Has been linked to lower bone density | Energy balance and sleep first, then talk to your doctor |
| Calcium raised | Always needs medical interpretation, not self diagnosis | Contact your GP |
How can you improve bone density?
With loading, with enough energy and with enough building blocks, in that order. Resistance training and impact provide the signal, a balanced energy intake makes it possible, and calcium and vitamin D supply the material. Without the first two, a supplement does little for your bone.
The Voedingscentrum, the Dutch national nutrition centre, uses a guideline of around 1,000 milligrams of calcium a day for adults, roughly four dairy portions or the plant based equivalent.
The Gezondheidsraad, the Dutch health council, advises extra vitamin D for certain groups, and your GP can work out whether you are one of them.
What vitamin D further does for recovery and testosterone in athletes sits in vitamin D deficiency in athletes.
Does vitamin K2 really work for your bones?
Here I have to be honest: the evidence is narrower than the supplement market suggests. The best known study by Knapen and colleagues found that three years of menaquinone-7 slowed bone loss, but that research was done in healthy postmenopausal women (PMID 23525894). That is not the same group as a 42-year-old man who deadlifts.
Sellers cheerfully translate that result to everyone anyway. That is not how evidence works.
I would not advise against K2, but I would not put it above loading, energy and vitamin D either.
When should you look into this?
If you have done high volume endurance training on a tight intake for years, if you have had a stress fracture, or if your strength and recovery are fading without an obvious reason. That combination is the signal, not your age.
You will find vitamin D, calcium and alkaline phosphatase in the 360 Health blood test. The muscle side of the same story sits in sarcopenia after 40, and the full overview in healthy ageing as an athlete.
Been eating below your burn for months while your volume climbs? Fix that first. No supplement beats a structural energy deficit.
References
- Mountjoy M et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine, 2018. PMID 29773536
- Mountjoy M et al. The IOC consensus statement: beyond the Female Athlete Triad, Relative Energy Deficiency in Sport (RED-S). British Journal of Sports Medicine, 2014. PMID 24620037
- Knapen MHJ et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International, 2013. PMID 23525894
- Voedingscentrum, calcium and bone loss. Gezondheidsraad, advice on vitamin D.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
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