So what accounts for the decrease? Researchers ruled out changes in body mass index (BMI - calculate your BMI) and use of osteoporosis medications, which leaves unanswered questions.

“The part that was unsatisfying to me was the authors didn’t go on to say what else it might be. As a rehabilitation person, we want to know what is it. Are people more active? Is it more weight bearing exercises,” says Sheila Dugan, MD, associate professor of physical medicine and rehabilitation at Rush University Medical Center in Chicago. “There’s something else, but they don’t go on to say what it might be.”

Dr. Dugan says she’d like to know if data can be further analyzed to get these answers or if future research can do that.

“This study did not look at physical activity, but that would be something I would want to see if we could go back and look at it and see if that’s what it is,” Dr. Dugan says.  

Researchers say they will continue to try and answer these questions. “We are currently looking at additional factors such as differences in diet, weight history, smoking, and other health behaviors in addition to BMI and medications,” Looker says. “We are also exploring additional ways to address the difference in the [bone scan] methods used in the two surveys to measure bone density.”  

The study says the annual cost of treating osteoporotic fractures is expected to increase by almost 50 percent from $17 billion to $25 billion between 2005 and 2025.