Scientists seeking to explain an apparent drop in hip fractures in this country say they’ve come up with at least one piece of the puzzle: The proportion of older Americans with osteoporosis has also declined – about 7 percent for women and 3 percent in men – over the last decade or so.

But the good news about brittle bones comes with several caveats.

Researchers say none of their theories, which include heavier body weights that may strengthen bones and more widespread use of drugs that curb bone loss, can fully explain the improvement, so they aren’t able to say what doctors or patients may be doing right.

And they caution that though the prevalence of the disease has dropped slightly, the number of people affected by bone loss has actually gone up as the population ages.

“It is still important to follow a bone-healthy lifestyle and discuss bone health with your health-care provider if you have concerns,” says Anne Looker, PhD, the study author from the National Center for Health Statistics, Centers for Disease Control and Prevention in Hyattsville, Md.

In their study, which is published in the Journal of Bone and Mineral Research, Looker and her team compared measurements of femur neck bone density in more than 6,400 adults older than the age of 50 collected from 2005 to 2006 in the National Health and Nutrition Examination Survey (NHANES) to bone density data gathered in the same survey 10 years before.

Researchers discovered that in that time the prevalence of low bone density in the neck of their femur bones dropped from 18 to 11 percent in women and from 5 to 2 percent in men.

The decline was most evident in non-Hispanic whites. Smaller changes were seen in non-Hispanic blacks and Mexican Americans.

Still, researchers point out that the number of older Americans has increased in the last decade, so while there is a decline in prevalence, more older adults actually have these two conditions than they did between 1988 and 1994.

“It does not invalidate the drop. This is because a proportion, or prevalence, can be lower, but if the total number of older adults has increased in the population, the number of people with the condition can be greater,” Looker explains.

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.