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Hip fractures may be one of the most devastating injuries that humans face, but they’re also less frequent than they used to be. Today Canadian researchers announce that the hip-fracture rate fell 31.8% for Canadian women and 25% for Canadian men between 1985 and 2005. (A decline has also been noted in the U.S., but over a shorter time period. That makes the U.S. numbers harder to interpret.) In every age group, it seems, the number of hip fractures per 100,000 people is dropping.
What’s driving that decline, however, is a mystery. Both men and women seem to be benefiting, which makes it unlikely that sex-specific interventions such as oral contraceptive pills or hormone replacement therapy could be the explanation. Furthermore, the authors write this week in the Journal of the American Medical Association:
There is little evidence to suggest that improvements in physical activity, calcium intake, vitamin D status, or falls prevention have affected hip fracture rates at the population level. Declining smoking rates could be associated with reductions in hip fractures because smoking is a risk factor, but it is doubtful that this would be sufficient to account for the magnitude of the change in the rates of hip fractures.
So what’s left? The authors do have one interesting theory they can’t refute: People are fatter than they used to be, and being overweight could be protective. “Greater body weight is associated with higher bone density and nonovarian aromatization of estrogen,” the authors write. Body weight also “provides padding,” they say, to the part of the thigh bone, near the hip joint, where muscles attach.