Bone versus breast density
© BioMed Central Ltd 2006
Published: 2 May 2006
The common link with oestrogen levels suggests that bone mineral density and mammographic density might also be linked. One study found weak support for this, but another study failed to provide confirmation. Overall, the relationship is very weak, if it exists at all. Other factors such as weight-bearing exercise, which have opposing impacts on these variables, may have a more dominant effect.
A unifying hypothesis would be that oestrogen levels influence both bone density and breast density, and that the effect on breast cancer risk is mediated by the effect on breast density (Figure 1). Although there may be some truth to this, it is clearly simplistic and does not provide a full explanation. For example, Cuzick and coworkers  showed that tamoxifen had a large effect on breast density in a breast cancer prevention setting, but this was not large enough to explain fully its effect on breast cancer risk.
When large cohorts are evaluated, highly significant results can arise even when the correlation between two variables is modest. In these circumstances the range of possible values for the correlation of more distant measures such as breast and bone density can be very large.
The data presented in these two reports [1, 2] and elsewhere indicate that the relationship between breast and bone density is very weak if it is present at all, and that other nonshared factors have sufficient importance to make any common effect of oestrogen levels essentially irrelevant. In these circumstances a key objective will be to determine whether bone mineral density truly predicts breast cancer risk and, if it does, to elucidate the underlying mechanism.
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