Low vitamin D is widely assumed to cause stress fractures in athletes and recruits, and supplementation is widely given on that basis. This systematic review and meta-analysis pooled 15 studies and 4,183 participants comparing serum 25-hydroxyvitamin D in those with stress fractures against controls.
Overall, people with stress fractures had lower vitamin D — a mean difference of 5.82 nmol/L (95% CI -10.35 to -1.29). The association held in military personnel (7.33 nmol/L lower, -11.77 to -2.89) and in men (9.14 nmol/L, -14.23 to -4.06). It did not reach significance in athletes, or in women.
The most interesting result comes from the meta-regression. Baseline vitamin D status of the control group explained over half the variation between studies, and the association appeared only where controls were already vitamin D sufficient — there the difference widened to 13.53 nmol/L (-20.57 to -6.49). Where the whole population was deficient, vitamin D level did not distinguish those who fractured from those who did not.
That is a meaningful and slightly counterintuitive pattern. In a population where nearly everyone is deficient, deficiency cannot explain why some individuals fracture and others do not — it is not a discriminating variable when it is universal. This matters directly in India, where vitamin D deficiency is extremely common across the general population, including in young and physically active people. The implication is not that vitamin D is irrelevant here, but that a low level in an Indian recruit or athlete carries much less individual predictive information than the same level in a population where deficiency is unusual.
And the difference itself is modest. Around 6 nmol/L is a small gap on a scale where the sufficiency threshold sits near 50 nmol/L. Assessing vitamin D in high-risk groups is reasonable and cheap. Treating supplementation as stress fracture prevention goes beyond what these observational data support, particularly since training load, sleep, energy availability and bone geometry are stronger and more modifiable determinants.
- Check vitamin D in high-risk groups such as recruits and heavy-training athletes; it is cheap and reasonable.
- Do not present supplementation as stress fracture prevention — these are observational data.
- The association was significant in military personnel and men, not in athletes or women.
- Where population-wide deficiency is the norm, a low level carries little individual predictive value.
- Address training load, energy availability and sleep, which are stronger modifiable determinants.
The statistics, in plain English
A mean difference of 5.82 nmol/L is small relative to both the sufficiency threshold and the day-to-day variability of the assay, so this is a real but weak signal at the group level and close to useless for predicting an individual. The meta-regression finding is the most informative part: baseline vitamin D status explained 53.8% of the heterogeneity between studies, meaning the disagreement in the literature is largely explained by which populations were studied rather than by chance. Note also that these are cross-sectional comparisons — vitamin D was measured around the time of fracture in most studies, so reverse causation, where reduced activity after injury lowers sun exposure, cannot be excluded.
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