Vitamin D and calcium
Evidence: moderate
Vitamin D deficiency is common in winter, at high latitudes and in indoor athletes, and both nutrients matter for bone. Correct a measured deficiency and meet calcium needs; do not megadose vitamin D on spec.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Vitamin D and calcium earn a place because of bone, not performance. Runners load their skeletons hard, and a bone stress injury costs months. Keeping both nutrients adequate is cheap insurance for bone, but the case is for correcting deficiency, not for routine high-dose supplementation.
Deficiency is common, and seasonal
Vitamin D is made in the skin from sunlight, so status falls in winter and at higher latitudes. A meta-analysis of athletes found a high prevalence of vitamin D inadequacy, with the greatest risk in winter, at higher latitudes and in indoor sports (Farrokhyar et al. 2015). A runner who trains outdoors in summer at a southern latitude is at lower risk than one who trains indoors through a northern winter, but neither can assume their status without a blood test.
What the nutrients do
Vitamin D and calcium are both central to bone. Vitamin D supports calcium absorption and bone turnover, and persistently low status is associated with greater susceptibility to bone stress injury (Owens et al. 2018). The role of vitamin D in muscle function and performance is often claimed but much less certain, and should not be the reason to supplement (Owens et al. 2018).
The supplementation case
Correct a measured deficiency, do not chase a high number
Test, then treat to sufficiency. Recent reviews argue against megadosing: very high bolus doses drive vitamin D catabolism and may be counterproductive rather than helpful (Owens et al. 2018). The aim is to move a deficient runner into the sufficient range, not to push status as high as possible.
The supporting trial evidence for bone comes mainly from combined supplementation. In a randomised trial of 5,201 female Navy recruits, daily 2,000 mg calcium plus 800 IU vitamin D during basic training cut stress-fracture incidence by about 20% (Lappe et al. 2008). That population was under heavy new loading rather than trained running, so the size of the effect may not carry across directly, but it is the strongest causal evidence available and points the same way.
Calcium for bone
Calcium is the building block of bone, and intake tracks injury risk. A review of young athletes found that higher calcium and dairy intake was associated with lower stress-fracture rates, with the largest reductions in female athletes taking in more than 1,500 mg per day (Tenforde et al. 2010). Food sources, dairy in particular, are the first port of call; a runner who avoids dairy needs to find calcium elsewhere and may need to supplement to hit target.
Calcium and energy availability are not interchangeable
Calcium and vitamin D help, but they cannot rescue bone in a runner who is chronically underfuelling. Low energy availability suppresses the hormones that build bone, and that has to be fixed at source. See RED-S and bone stress injuries.