The significance of physical and mechanical risk factors in the development of stress fractures is well established . The present findings indicate that genetic factors may also play a role in the development of femoral neck stress fractures. We found an interaction between the CTR C allele and the VDR C-A haplotype, and the risk of stress fractures was 3-fold higher in military conscripts lacking either one or both compared to carriers of both. In addition, the LRP5 gene haplotype A-G-G-C conferred almost a 3-fold increased risk for developing femoral neck stress fractures, and a 4-fold increased risk in combination with the VDR C-A haplotype, although these associations were mediated by low body weight and BMI and require further investigation.
Because of the important role of CTR in bone homeostasis, polymorphisms of this gene have been studied with regard to common bone parameters and disorders. Several studies report an association between the CTR polymorphism AluI and lumbar spine and femoral neck BMD in both men and women, but the reports have been somewhat contradictory. Studies in postmenopausal women revealed that the CC genotype is more common in non-osteoporotic women than the TT genotype [30, 31], and that the TT genotype is associated with lower lumbar spine and femoral neck BMD and increases the predisposition for osteoporosis [32, 33]. The opposite finding was suggested by Braga et al.  who reported that the CC genotype is associated with decreased BMD and is more common in men with hip or vertebral fractures than in control subjects . In our study, the CTR allele C together with a VDR C-A haplotype appeared to protect subjects from fractures.
Polymorphisms in CTR and VDR are associated with BMD in Spanish women . The observed interaction between a CTR minor allele and the VDR C-A haplotype and their association with stress fractures may be explained by the inhibitory effect of these proteins on parathyroid hormone production. CTR and VDR are both involved in sustaining normocalcemia by inhibiting the production of parathyroid hormone . The observed allele-haplotype interaction may have an effect on the regulatory role of the proteins and therefore on control of Ca levels.
VDR also has independent effects on bone biology and may play a role in bone pathologies such as stress fractures. VDR is essential for 1,25(OH)2D3 to induce the calcemic and phosphatemic effects that normally result in bone mineralization and remodelling . VDR genotypes increase the risk for low BMD and osteoporotic fractures [16, 38] and VDR knock-out mice develop a low bone mass phenotype with hypocalcemia, hypophosphatemia, and elevated 1,25(OH)2D3 levels . In addition, reduced serum 25(OH)D levels might predispose young men to stress fractures .
The present study indicates that the association of the LRP5 haplotype and LRP5-VDR interaction with stress fractures is mediated by low body weight and BMI, but more research is needed before any definitive conclusions can be drawn from these findings. The function of LRP5 in bone development, however, is indisputable ; mutations in LRP5 cause various bone disorders [20, 42] and polymorphisms are associated with BMD and bone mineral content in general , but also with reduced BMD and fractures . Mouse studies demonstrated that mutations in Lrp5 affect bone formation sensitivity in response to normal mechanical loading [45, 46], and thus the LRP5 haplotype A-G-G-C might affect bone sensitivity and response to mechanical loading. It is possible that bone in the lighter-weight conscripts is initially adjusted to lower load bearing and when mechanical loading sharply increases in the military service, the genetically set response might not adjust rapidly enough to react to the increased loading, thus putting lighter weight conscripts at higher risk for stress fractures. Our results support earlier findings that low weight (before and/or during military service) increases the risk of stress fractures [3, 47]. Body weight is an important predictor of BMD , and BMI and obesity have been shown in a family-based analysis to be associated with LRP5 polymorphisms , underlining the role of LRP5 also in weight regulation.
One limitation of the present study is the discrepancy in the collection periods of case and control groups. The most important physical variable was weight, because reduced weight and BMI mediate the association of the LRP5 haplotype and the LRP5-VDR interaction with femoral neck stress fractures. Interestingly, however, the same difference in weight and BMI was also observed between the cases and their healthy contemporaries, suggesting that the difference was not time-dependent. This verification is important because the mean BMI and the number of overweight conscripts has consistently increased towards the end of 20th century, whereas the physical fitness of conscripts has declined . Unfortunately, other extrinsic factors that could have changed during the 30 years scale (e.g. in nutrition or exercise) have not been examined because of the limited amount of information available. The results of the study should be interpreted with moderation and replication of the study is needed to confirm the present findings. For future studies, larger sample sizes are desirable to gain more statistical power in the analyses. In addition, functional studies on the present genetic findings are needed to elucidate the relevance of these genetic associations to femoral neck stress fractures.