It’s well known that there’s a direct correlation between bone density and body weight. Thin people generally get lower bone density readings on DEXA scans, and as a group, people whose body mass index classifies them as “underweight” experience more hip fractures than persons of “normal” weight. But it’s still possible to have a slender build with bone density readings on the low side yet still enjoy enhanced bone quality with plenty of bone strength.
Let’s look at this in detail.
Bone density isn’t the same as bone strength
We know that bone adapts to the total weight put upon it. With every step, a heavier person shows the body the need for a certain degree of bone mass just to carry that body around, and the body in all its intelligence adjusts to that need. Lighter folks, on the other hand, do not require the same bone mass to carry them around, and nature adjusts to that also.
But when it comes to bone strength and fracture resistance, other factors aside from bone size and bone density come into play. Specifically, key factors are the amount of lean muscle mass and variations in bone quality.
For example, a healthy, small framed, thin person with good muscle mass may have the bone strength of a healthy heavier person, even though their bone density is reported to lower. Or, a small-framed person with a low bone density reading can enjoy good bone strength if quality of the bone is high. As I reported before, a case in point are Asian Americans, who as a rule have lower bone density than other ethnic groups, yet they have much lower fracture rates.
Factors that determine bone quality
Over the years, hundreds of low-weight, small-framed women have come to The Center for Better Bones because they have been told they have extremely low bone density and warned to take bone drugs if they want to avoid a fracture. Over and over I soothe their anxieties by explaining that what’s paramount is their “total load” of fracture risk factors — and not the results of a single bone density scan. I often find that these women are not living a “bone-wasting lifestyle,” have few fracture risk factors, and have no hidden secondary causes of osteoporosis — and to top it off, their bone density is stable and they have good muscle development. So it’s likely that their bone, though low in density, is high in quality and strength.
Science has yet to discover a noninvasive way to assess bone quality and thus determine skeletal strength. However, we know that a variety of important factors that help determine bone quality as you can see in the graphic below.
I should mention that it’s not just women who experience these “false positives” in bone density testing. You might recall the blog I wrote some months ago about “Richard,” a man who sought my services because his doctor told him to take bone drugs due to his low bone density. After carefully reviewing his case, I could find no risk factors to make me think he was at risk of osteoporotic fractures — he was a healthy, strong man who nevertheless was naturally small-framed and slim, the likely reason for his low bone density reading.
Richard sought a second opinion from a bone specialist who was also an academic researcher. After a thorough examination, this physician also surmised that maybe it was just his weight and body size that was the issue and reran his bone density numbers as if he were a woman… and the osteoporosis diagnosis disappeared. As a researcher, she had access to a sophisticated scanning device that provided a 3-D image of his bones, which confirmed her view that he had healthy bones. Her only follow-up suggestion was to do another bone density test next year to see whether the numbers are moving.
Pay attention to all fracture risk factors, not just bone density
When I encounter someone who has been frightened by a finding of low bone density, the one positive about the fear instilled in my clients by their doctor is that these women (and men!) are now motivated to implement all six steps of our natural Better Bones Better Program®. Anyone of any body size who strives for longevity would do well to develop a comprehensive lifestyle and nutrition program to maintain lifelong bone strength.
Bone quality references
Brown, Susan E. Helping thin women reduce bone fracture risk. Betterbones.com.
Beck, T., et al. Does obesity really make the femur stronger? BMD, geometry and fracture incidence in the Women’s Health Initiative Observational Study. J Bone Miner Res2009;24(8):1369-1379.