Design: systematic review and random-effects meta-analysis with meta-regression, level 3
Population: 15 studies, 4,183 military personnel and athletes with and without stress fracture
Primary outcome: difference in serum 25-hydroxyvitamin D between stress fracture cases and controls
Effect: mean difference −5.82 nmol/L (95% CI −10.35 to −1.29); military −7.33, men −9.14; not significant in athletes or women

Fifteen studies with 4,183 participants were pooled to compare serum 25-hydroxyvitamin D between people who sustained a stress fracture and controls, in military and athletic populations.

Overall, stress fracture cases had lower levels, mean difference −5.82 nmol/L (95% CI −10.35 to −1.29). The association held in military personnel (−7.33 nmol/L, 95% CI −11.77 to −2.89) and in men (−9.14 nmol/L, 95% CI −14.23 to −4.06). It did not reach significance in athletes or in women. Meta-regression found baseline vitamin D status explained over half the heterogeneity between studies (R²=53.8%), and the association appeared only where the control group was already vitamin D sufficient (−13.53 nmol/L, 95% CI −20.57 to −6.49).

That last point is the interesting one and it is easy to read backwards. Where everybody in a cohort is deficient, a fracture case cannot be distinguished by being more deficient — the contrast disappears. So the finding is about detectability rather than about biology, and it says the difference shows up in populations where sufficiency is the norm.

For Indian practice that matters directly, because deficiency is closer to the norm here than the exception. A single level in a young recruit or player with shin pain will often be low and will not, on this evidence, identify who is at risk. The practical use is population-level: measure and correct in high-load groups before a training block, rather than measuring after a fracture to explain it.

The absent signal in women deserves emphasis rather than reassurance. Female athletes carry the energy-availability and menstrual-function risks that dominate stress fracture epidemiology in that group, and vitamin D is not where the attention should go.

Measure and correct vitamin D before a high-load training block, not after a stress fracture
In female athletes, assess energy availability, menstrual function and training load first — the vitamin D signal is absent here
Do not use a single low level to explain a stress fracture; in a deficient population it explains nothing
Record training volume escalation, footwear and surface change alongside any blood result
Treat military recruits and men as the groups where this association was actually demonstrated

Don't overread it

These are observational comparisons of levels — they cannot show that supplementing vitamin D prevents stress fractures.

The statistics, in plain English

A mean difference of −5.82 nmol/L is small — well within the assay and seasonal variation in any one person — so this is a population-level association, not a clinical cut-off. The confidence interval reaching −1.29 means the true difference could be almost nothing. The subgroup that carried the whole effect was defined by the controls' baseline status, which is a post-hoc grouping, and R²=53.8% for heterogeneity tells you the studies differ substantially in ways this factor explains.